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Systematically divided into six parts, this book presents a lucid and comprehensive exposition of Clinical Cardiology. The basic concepts and procedures have been explained in a simple and logical manner and a large number of illustrations and tables have been included throughout the text to facilitate understanding of the subject. In total, there are 749 figures, 245 tables, and 675 references. The book will serve as an ideal text for postgraduate students of General Medicine, Cardiology and Pediatrics. Also, it will be an extremely useful and reliable reference source for the practising physicians.

Beginning with a clear description of basic anatomy and physiology relating to cardiovascular medicine, the book explains cardiac history taking and symptomatology. It then describes general physical examination along with arterial pulse and blood pressure. This is followed by a detailed discussion of cardiovascular examination including inspection, palpation, percussion, precordium in common heart diseases and auscultation. Thereafter, basic investigations necessary for diagnosis and management are described. These feature clinical electrocardiography which includes normal ECG, common disease conditions, drug effects, arrhythmias and prediction of coronary artery occlusion. This section also discusses radiology of the heart describing introduction, technical facts, routine reporting of x-ray chest, calcifications and other views.


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width="1206" height="1600">Clinical Examination in
Cardiology
First Edition
B.N. Vijay Raghawa Rao, MD, DM (CARDIOLOGY),
DHA, FCCP, FICC, MBA (HM)
Formerly Addl. Director, Professor & HOD Gandhi Medical
College/Gandhi Hospital, Secunderabad, Hyderabad, India
Presently Consultant Interventional Cardiologist Vijay Marie
and Yashoda Superseciality Hospitals, Hyderabad, India
E l s e v i e rTable of Contents
Cover image
Title page
Copyright
Dedication
Preface to Revised Reprint
Preface to the Earlier Edition
PART I BASIC ANATOMY AND PHYSIOLOGY
Chapter 1 Anatomy of the Heart
1 Gross Anatomy of the Heart
2 External Features of the Heart
3 The Chambers of the Heart
4 The Fibrous Skeleton of the Heart
5 The Valves of the Heart
References
Chapter 2 Lymphatic System of the Heart
1 Formation Of Right Coronary Channel
2 Formation Of Left Coronary Channel
3 Formation Of Main Supra-Cardiac Channel
4 Formation Of Right Lymphatic Duct
References
Chapter 3 Venous Drainage of the Heart
1 Coronary Sinus
2 Anterior Cardiac Veins
3 Thebesian Veins (Venae Cordis Minimi)
Reference
Chapter 4 Arterial Supply of the Heart
1 Left Coronary Artery
2 Right Coronary Artery (RCA)
3 Divergent Coronary Anatomy4 Measurements
References
Chapter 5 Nerve Supply of the Heart
1 Cardiac Plexus
2 Baroreceptors and Chemoreceptors
References
Chapter 6 The Conduction System of the Heart
1 Sinoatrial (SA) Node (Pacemaker Node of Keith–Flack Node, 1907)
2 Atrioventricular Junctional Area
3 The Bundle Branches and Terminal Purkinje Fibers
References
Chapter 7 Ultrastructure of the Myocardium
1 P Cells
2 Transitional Cells
3 Purkinje Cells
4 Amoeboid Cells
5 Contractile Or Working Myocardial Cells
References
Chapter 8 Basic Electrophysiological Principles
Sarcolemma, Intercalated Disc
Intracellular and Extracellular Ion Concentrations in Cardiac Muscle
Properties of Transmembrane Potentials
Cardiac Action Potentials
References
Chapter 9 Molecular Basis of Muscle Contraction
1) Muscular Contraction
2) Muscular Relaxation
3) The Velocity and the Amount of Tension
References
Chapter 10 The Cardiac Cycle1 Ventricular Systole
2 Ventricular Diastole
3 Atrial Systole
4 Atrial Diastole
5 The Differences in the Events Between Right and Left Sides of the Heart
6 Ecg Changes During Cardiac Cycle
References
PART 2 THE HISTORY AND SYMPTOMATOLOGY
Chapter 11 Cardinal Symptoms
Importance of History Taking
1 Chest Pain
2 Shortness of Breath (SOB)/Dyspnea
3 Palpitation
4 Fatigue
5 Syncope
References
Chapter 12 Other Symptoms
1 Hemoptysis
2 Hoarseness
3 Cyanosis
References
PART 3 GENERAL PHYSICAL EXAMINATION
Chapter 13 General Examination
1 General Build and Stature
2 Posture or Attitude
3 Gestures and Signs
4 Facial Appearance
5 Eyes
6 Nose
7 Ears8 Oral Cavity
9 Neck
10 Spine
11 Skin
12 Extremities
13 Peripheral Edema
References
Chapter 14 Arterial Pulse
1 Definition
2 Genesis of the Arterial Pulse
3 Pulse Wave Pattern
4 Examination of the Arterial Pulse
5 Characteristic Features of Pulse In Common Clinical Conditions
References
Chapter 15 Measurement of the Blood Pressure
1 Definition and Components of Arterial Blood Pressure
2 Determinants of Arterial Blood Pressure
3 Measurement of Arterial Blood Pressure
4 Hypertension
References
PART 4 JUGULAR VENOUS PULSE
Chapter 16 Introduction and Jugular Venous Pulse Waves
Introduction
Examination of Jugular Venous Pulse (JVP)
Analysis of the Jugular Venous Pulsations
Abnormalities of the Waves
References
Chapter 17 Estimation of Venous Pressure and JVP in Diseased
Conditions
1 Estimation of Venous Pressure
2 JVP in Diseased ConditionsReferences
PART 5 CARDIOVASCULAR SYSTEM EXAMINATION
Chapter 18 Inspection of the Precordium
1 Examination of the Chest
2 Examination of the Precordium
3 Cardiovascular Pulsations
References
Chapter 19 Palpation of the Precordium
1 Examination of the Chest for the Shape and Distended Vessels
2 Palpation of the Precordium for any Tenderness
3 Palpation of the Cardiovascular Pulsations, Palpable Sounds, Thrills and
Rubs
4 Tracheal Tug (Oliver’s Sign)
5 Kinetic Cardiogram
References
Chapter 20 Percussion of the Precordium and Precordial Findings in
Common Heart Diseases
Methods of Percussion
The Scheme of Percussion
Precordial Findings in Common Heart Diseases
References
Chapter 21 Cardiac Auscultation
Principles and Techniques
The Heart Sounds
The Heart Murmurs
References
PART 6A BASIC INVESTIGATIONS: CLINICAL ELECTROCARDIOGRAM
Chapter 22 Introduction and Basic Concepts
Introduction
Basic Concepts
ReferencesChapter 23 The Normal Electrocardiogram
Waves
Intervals
Segments
The Electrical Axis
Effect of Heart Position on ECG
Normal ECG Variants
References
Chapter 24 Abnormal P, T and U Waves
Abnormal P Waves
Abnormal T Waves
Abnormal U Waves
References
Chapter 25 Ventricular Hypertrophy
1 Left Ventricular Hypertrophy (LVH)
2 Right Ventricular Hypertrophy (RVH)
3 Biventricular Hypertrophy (BiVH)
References
Chapter 26 Intraventricular Conduction Defects
1 Right Bundle Branch Block (RBBB)
2 Left Bundle Branch Block
3 Fascicular Blocks
References
Chapter 27 Myocardial Infarction and Ischemia
1 Myocardial Infarction (MI)
2 Myocardial Ischemia
3 Nonspecific ST-T Changes
4 Localization of Ischemia or Infarction
5 Prediction of the Site of Coronary Artery Occlusion
6 ECG Diagnosis of MI in Bundle Branch Blocks and During RV Pacing7 Sensitivity, Specificity, and Prognostic Value of the ECG
References
Chapter 28 Pericarditis and Myocarditis
Pericarditis
Myocarditis
References
Chapter 29 Drug Effects and Electrolyte Abnormalities
Digitalis
Electrolyte Abnormalities and ECG Changes: Hyperkalemia
Hypokalemia
Hypercalcemia
Hypocalcemia
Hypermagnesemia
Hypomagnesemia
References
Chapter 30 Arrhythmias
1 Sinus Nodal Disturbances and Arrhythmias
2 Atrial Arrhythmias
3 AV Junctional Arrhythmias
4 Pre-Excitation Syndrome (PES)
5 Narrow QRS Tachycardia
6 Ventricular Arrhythmias
7 Wide QRS Tachycardia
8 Heart Block
References
PART 6B BASIC INVESTIGATIONS: RADIOLOGY OF THE HEART AND
GREAT VESSELS
Chapter 31 Introduction and Evaluation of Heart
Cardiac Imaging Technologies
Technical Facts Involved in Chest Roentgenogram
Evaluation of Chest RoentgenogramReferences
Chapter 32 The Pulmonary Vasculature
1 Normal Pulmonary Vascularity
2 Abnormal Pulmonary Vascularity
3 Pulmonary Interstitial Edema
References
Chapter 33 Cardiac Calcification
Valvular Calcification
Myocardial Calcification
Vascular Calcification
Pericardial Calcification
Tumor Calcification
References
Chapter 34 Evaluation of Extracardiac Structures and Chest X-ray in
Other Views
Evaluation of Extracardiac Structures
Evaluation of Chest X-Ray in Other Views
References
Abbreviations
IndexCopyright
Clinical Examination in Cardiology
B.N. Vijay Raghawa Rao
ELSEVIER
A division of Reed Elsevier India Private Limited
Mosby, Saunders, Churchill Livingstone, Butterworth Heinemann and Hanley & Belfus
are the Health Science imprints of Elsevier.
© 2007 Elsevier
First Edition 2007
All rights are reserved. No part of this publication may be reproduced, stored in a
retrieval system, or transmitted in any form or by any means, electronic,
mechanical, photocopying, recording or otherwise, without the prior permission of
the publisher.
ISBN-13: 978-81-312-0964-6
Medical knowledge is constantly changing. As new information becomes available,
changes in treatment, procedures, equipment and the use of drugs become
necessary. The authors, editors, contributors and the publisher have, as far as it is
possible, taken care to ensure that the information given in this text is accurate
and up-to-date. However, readers are strongly advised to con7rm that the
information, especially with regard to drug dose/usage, complies with current
legislation and standards of practice.
Published by Elsevier, a division of Reed Elsevier India Private Limited
Sri Pratap Udyog, 274, Captain Gaur Marg, Sriniwaspuri
New Delhi – 110 065, India.
Publishing Director: Sanjay K Singh
Commissioning Editor: Sonali Dasgupta
Developmental Editor: Dr Shelley Narula
Manager (Editorial Projects): Dr Radhika Menon
Production Executive: Ambrish Choudhary
Typeset by Olympus Infotech Pvt. Ltd, Chennai, India.Printed and bound at Nutech Photolithographer, New Delhi.D e d i c a t i o n
Dedicated to my parents, Shri B. Narsimha & Smt B. Laxmamma, my wife Dr
Shashikala, my daughters Dr Visha Rao & Vishala Rao and my teachers,
students & patients who constantly encouraged to write & revise this clinical
treatise.'
Preface to Revised Reprint
First Edition of Clinical Examination in Cardiology was published in 2007 by
Elsevier India Pvt. Ltd which was well received and appreciated by PG students of
Gen. Medicine, Pediatrics and Cardiology as well as by the practicing physicians,
besides being a great helpful to undergraduate students. However there were some
printing errors which were overlooked inadvertently. These errors have been
corrected and even some gures, graphs, photographs and tables have been
revised and updated in this revised reprint which will be an asset to clinical
decision making.
I am thankful to Elsevier India Pvt. for their keen interest shown in revising and
reprinting this clinical text book.
B.N. Vijay Raghawa Rao MD, DM (Cardiology), DHA,
FCCP, FICC, MBA (HM) DrPreface to the Earlier Edition
“It is man’s mission to learn to understand”
Clinical Examination in Cardiology is primarily a clinical treatise. It provides a
simple, lucid and comprehensive description of “Basic Anatomy and Physiology of
Cardiovascular Medicine, Clinical Cardiology, and Basic Bedside Investigations
(Electrocardiogram and X-ray Chest)” in a single book. It is the &rst of its kind in
the present millennium highlighting the forgotten “Clinical Cardiology, in a
scenario” where cardiovascular disease is now a global problem with enormous
economic consequences.
Besides index, this book consists of six parts with 34 chapters. Part 1 deals with
“Basic Anatomy and Physiology of Cardiovascular Medicine” with ten chapters
comprehensively described for better understanding of clinical cardiology. Part 2
follows the initial chapters which deal with “Cardiac Symptomatology” in two
chapters. Part 3 with three chapters consists of “General Physical Examination,
Arterial Pulse and Blood Pressure” described in detail. Part 4 has two chapters
describing “Jugular Venous Pulse and Jugular Venous Pressure” in detail. Part 5
follows with &ve chapters which describe cardiovascular examination–“Inspection,
Palpation, Percussion and Precordium in Common Heart Diseases, and
Auscultation”. Finally, basic investigations are described in two portions, which
are essential for comprehensive discussion of diagnosis and management of a
cardiovascular disease. This Part 6 includes, Part 6a: “Clinical
Electrocardiography” with nine chapters, which include basic concepts, normal
ECG, common disease conditions, drugs e: ects, arrhythmias and prediction of
coronary artery occlusion in a patient of acute myocardial infarction. Part 6b:
“Radiology of the Heart and Great Vessels” includes four chapters, describing
introduction, technical facts, routine reporting of an x-ray chest, calci&cations and
other views. Each chapter has adequate &gures, tables and references, which can
be used for rapid review of the material described. In total, there are 749 &gures,
245 tables, and 675 references.
This book is primarily focused for postgraduate students of “General Medicine,
Cardiology and Paediatrics”. However, it will also be useful for the undergraduate
students for better understanding of clinical cardiology, which is a part of general
medicine. It may also prove useful to those who wish to broaden their knowledge
of clinical cardiology and will aid in their day-to-day practice of cardiology.
Besides my teaching experience of undergraduate and postgraduate medical
students, I have also used standard textbooks and journals of Cardiovascular
Medicine as references in compiling this clinical entity.I am thankful to my postgraduates, Dr Pramod, Dr Rajkiran and Dr Narender
for providing beautiful photographs. I am indebted to my patients at my clinic,
Remedy Superspeciality Hospital and Gandhi Medical College & Hospital for their
immense cooperation.
My special thanks to Mr Sanjay Singh and Dr Shelley Narula of Elsevier India
Pvt. Ltd. for their constant encouragement and keen interest shown in completing
this clinical treatise.
B.N. Vijay Raghawa Rao, MD, DM (Cardiology), DHA,
FCCP, FICC, MBA (HM) DrPART I BASIC ANATOMY AND
PHYSIOLOGYChapter 1
Anatomy of the Heart
1. Gross Anatomy of the Heart
2. External Features of the Heart
i. The Sulci of the Heart
ii. The Surfaces of the Heart
iii. The Borders of the Heart
3. The Chambers of the Heart
i. Right Atrium
ii. Right Ventricle
iii. Left Atrium
iv. Left Ventricle
4. The Fibrous Skeleton of the Heart
i. Components and Attachments
ii. Extensions
iii. The Bundle of His
5. The Valves of the Heart
General Description
i. Mitral Valve
ii. Tricuspid Valve
iii. Semilunar (SL) Valves
References
1 Gross Anatomy of the Heart
The heart is a conical, hollow muscular organ situated in the middle mediastinum behind the sternum and costal
rd th thcartilages of the 3 , 4 & 5 ribs. It lies obliquely so that 2/3 of the heart is to the left of the midline. The heart
rests upon the diaphragm and is tilted forward and to the left so that the apex is anterior to the rest of the heart.
The size and weight of the heart may vary depending upon the age, sex, body length, epicardial fat, and
general nutrition. The average human adult heart measures about 12 cm × 9 cm and weighs about
1325 ± 75 g in males and 275 ± 75 g in females (see Table 1.1). It is described as follows:
1. External features
2. Chambers of the heart
3. The fibrous skeleton of the heart
4. The valves of the heart
Table 1.1 Gross anatomy of the heart
Features Description
1. Shape Conical hollow muscular organ
2. Location Middle mediastinum behind the sternum and costal cartilages of 3rd–5th ribs
3. Average size 12 cm × 9 cm
4. Average weight 325 ± 75 g in males275 ± 75 g in females
2 External Features of the Heart
The heart has four chambers, right and left atria, and right and left ventricles, which are separated from each
other by sulci and consist of surfaces and borders. The atria lie above and behind the ventricles (see Table 1.2).
Table 1.2 External features of the heart
Features Description
1. Chambers RA, RV, LA and LV
2. Sulci Right and left coronary sulci, interatrial groove and anterior and posterior interventricular
sulci
3. Surfaces Diaphragmatic or inferior, anterior or sternocostal and left surfaces
4. Borders Right border formed by: SVC and RA Left border formed by: LV and left auricle Inferior
border formed by: RV
RA: right atrium, RV: right ventricle, LA: left atrium, LV: left ventricle, SVC: superior vena cava
i The Sulci of the Heart
The atria are separated from the ventricles externally by the coronary (atrioventricular) sulci and from each other
by an interatrial groove, which is faintly visible posteriorly and hidden by the aorta and pulmonary trunk
anteriorly.
The two ventricles are separated from each other by the interventricular sulci (grooves), which descend from
the coronary sulcus toward the apex:
• The anterior interventricular sulcus contains the left anterior descending coronary artery which courses over the
muscular ventricular septum between the right and left ventricles to the apex (see Figs 1.1 and 1.2).
• The posterior interventricular sulcus which is situated on the diaphragmatic surface of the heart is the pathway
for the posterior descending coronary artery, which is usually the terminal branch of the right coronary artery
or less frequently of the left circumflex artery.
Fig. 1.1 External features of the heart—anterior view (diagrammatic).=
Fig. 1.2 External features of the heart—anterior view.
The right coronary artery travels in the right coronary sulcus between the right atrium and right ventricle until it
descends on the posterior surface of the heart while the left circum ex artery runs in the left coronary sulcus
between the left atrium and left ventricle.
The crux of the heart is the area on the posterior basal surface:
• Where the coronary sulcus meets the posterior interventricular sulcus.
• The coronary artery which crosses the crux (usually the right coronary artery) gives a small branch to the
nearby AV node and
• Internally, the atrial septum joins the ventricular septum at this junction.
ii The Surfaces of the Heart
• The area below the crux is known as the diaphragmatic or inferior surface of the heart. The diaphragmatic
rd rdsurface of the heart is formed in its left 2/3 by the left ventricle and its right 1/3 by the right ventricle.
• The anterior or sternocostal surface of the heart is formed mainly by the right atrium and right ventricle and
partly by the left ventricle and left auricle.
• The left surface of the heart is mostly formed by the left ventricle and at the upper end by the left auricle (see
Figs 1.3 and Fig. 1.4).Fig. 1.3 External features of the heart—posterior view (diagrammatic).
Fig. 1.4 External features of the heart—posterior view.
iii The Borders of the Heart
• The right border is more or less vertical which is formed by superior vena cava and the right atrium.
• The left border is oblique and curved, mainly formed by the left ventricle and partly by the left auricle.
• The inferior border is nearly horizontal and is formed mainly by the right ventricle.
3 The Chambers of the Heart
i Right Atrium (RA)
It receives venous blood from whole of the body via the superior vena cava (SVC) at its upper end and inferior
vena cava (IVC) at its lower end and pumps it into the right ventricle through the right atrioventricular(tricuspid) valve during the ventricular diastole.
a) External Features
• It is a somewhat quadrilateral chamber situated behind and to the right side of the right ventricle, externally
separated by the right coronary sulcus.
• A hollow conical muscular projection, the right auricle (right atrial appendage) arises from the antero-superior
part of the right atrium and extends upwards and to the left of the ascending aorta.
• Along the right border of the right atrium, there is a shallow vertical groove, the sulcus terminalis which extends
between the oriAces of the superior vena cava and inferior vena cava which is produced by an internal
muscular ridge called the crista terminalis. The upper part of the sulcus contains the SA node at the lateral
margin of the junction of the superior vena cava with right atrium and the atrial appendage (see Table 1.5).
b) Internal Features
The right atrial wall measures about 2 mm in thickness. The interior of the right atrium is broadly divided into
three parts (see Figs 1.5, 1.6 and Table 1.3):
• The smooth posterior part or sinus venarum
• The rough anterior part or pectinate part (atrium proper) and
• The septal wall.
Fig. 1.5 Interior of right atrium.Fig. 1.6 Interior of the right atrium (diagrammatic).
Table 1.3 Development of the right heart
Fetal structures Portion developed
1. Right horn of sinus venosus Sinus venarum
2. Primitive atrial chamber Atrium proper
3. Septum primum and septum secundum Atrial septal wall
4. Primitive ventricle Inflow tract and body of right ventricle
5. Right part of the bulbus cordis Infundibulum of right ventricle
(1) The smooth posterior part or sinus venarum
• Developmentally, this portion is derived from the right horn of the sinus venosus.
• Superior vena cava opens at the upper end, inferior vena cava opens at the lower end and the coronary sinus
opens between the opening of the inferior vena cava and the right atrioventricular (AV) orifice.
• The oriAce of the superior vena cava has no valve while the oriAce of the inferior vena cava is guarded by a
rudimentary valve, the Eustachian valve. The Thebesian valve guards the oriAce of the coronary sinus (see
Table 1.4).
• The caval oriAces vary in shape and size depending upon the phase of respiration, the cardiac cycle and
contraction or relaxation of the surrounding muscle bands which play a role in promoting venous return and
preventing atrial reflux.
Table 1.4 Openings into the right atrium
1. Superior vena cava It has no valves
2. Inferior vena cava It has rudimentary Eustachian valve
3. Coronary sinus It has Thebesian valve
Table 1.5 Nodes of the heart
Node Location
1. SA node In sulcus terminalis at the lateral margin of the junction
of SVC with right atrium=
2. AV node In the triangle of Koch, anterior and medial to the
coronary sinus just above the septal tricuspid leaflet
(2) The rough anterior part or atrium proper
• Developmentally, this portion is derived from the primitive atrial chamber.
• Crista terminalis, remnant of the upper part of the right venous valve is a smooth muscle ridge which extends
from the upper part of the atrial septum between the orifices of SVC and IVC.
• A series of transverse muscular ridges called muscular pectinati arise from the crista terminalis, run forwards
and downwards towards the TV oriAce, giving the appearance of the teeth of a comb. The muscles are
interconnected to form a reticular network in the auricle.
(3) The septal wall
• Developmentally, it is derived from the septum primum and septum secundum.
• An oval depression above and to the left of the opening of the inferior vena cava called fossa ovalis is formed
by the septum primum. A sickle shaped sharp margin that surrounds the upper, anterior, and posterior margins
of the fossa ovalis is the limbus fossa ovalis, developed from the lower free margin of septum secundum. The
anterior limb of the limbus is continuous with the left horn of the valve of inferior vena cava. The remains of
the foramen ovale is a small slit like valvular opening (patent foramen ovale, PFO) between the upper part of
the fossa and the limbus which is normally occluded after birth and is occasionally present (see Table 1.6).
• The triangle of Koch, a triangular area bounded in front by the base of the septal lea et of tricuspid valve,
behind by the antero-medial margin of the opening of coronary sinus and above by the tendon of Todaro,
which is a subendocardial ridge extending dorsally from the central Abrous body to the left horn of the valve of
inferior vena cava. The AV node is located in this triangle anterior and medial to the coronary sinus, just above
the septal leaflet of the tricuspid valve.
• The torus aorticus is a slight bulge in the antero-superior part of the septum and is caused by the bulging of
the right posterior aortic (non-coronary) cusp and the right coronary cusp of the aortic root. The proximity of
the aortic root to the right atrium permits an aneurysm of the sinus of Valsalva to rupture into the right atrium.
• The proximal right coronary artery is in immediate vicinity of the septum as it enters the coronary sulcus.
Table 1.6 Description of the inter-atrial septum
Features Description
1. Fossa ovalis Oval depression above and to the left of the opening of the superior vena cava
2. Limbus fossa ovalis Sickle shaped sharp margin that surrounds upper, anterior and posterior margins of
the fossa ovalis
3. PFO (normally occluded Small slit like valvular opening between upper part of fossa ovalis and limbus
after birth)
4. Torus aorticus Bulge in antero-superior part of the septum due to bulging of the right posterior and
right coronary cusps of aorta
ii Right Ventricle (RV)
It is a triangular or a crescent shaped chamber, which receives the venous blood from the right atrium during
ventricular diastole and pumps it into the pulmonary circulation during ventricular systole (see Table 1.7).
Table 1.7 Description of right heart
Features Description
1. Shape of the right atrium (RA) Quadrilateral chamber
2. Verticle groove on the RA Sulcus terminalis: contains SA node
3. Wall thickness of the RA 2 mm
4. Internal portions of RA (i) Smooth sinus venarum
(ii) Rough pectinate (atrium proper)
(iii) Atrial septal wall5. Shape of the right ventricle (RV) Triangle or crescent
6. RV wall thickness 4–5 mm
7. Portions of RV (i) Rough inflow tract
(ii) Smooth infundibulum
(iii) Body of the RV
a) External Features
• The right ventricle is normally the most anterior cardiac chamber, lying directly beneath the sternum. It is
partially below, in front of, and medial to the right atrium but anterior and to the right of left ventricle.
rd rd• It forms 2/3 of the sterno-costal surface, most of the inferior surface and 1/3 of the diaphragmatic surface
of the heart.
b) Internal Features
2The right ventricular wall measures 4–5 mm in thickness, thinner than that of left ventricle in the ratio of 1:3.
The interior of the right ventricle consists of three portions (see Figs 1.7 and 1.8):
• The rough inflow tract
• The smooth outflow tract or infundibulum and
• The apical trabecular portion or body of the RV.
Fig. 1.7 Interior of the right ventricleFig. 1.8 Interior of the right ventricle (diagrammatic)
(1) The rough inflow tract
• Develops from the primitive ventricle of the heart tube, and consists of:
(i) Tricuspid valve and
(ii) The trabecular muscles of the anterior and inferior walls which direct the blood anteriorly, inferiorly, and
to the left at an angle of 60° to the outflow tract.
• The trabecular carnae (muscles) are arranged into ridges, bridges and pillars (papillary muscles). The
septomarginal trabecula is a muscular ridge extending from the ventricular septum to the base of the anterior
papillary muscle. It contains the right branch of the AV bundle and is presumed to prevent the over-distension
of the right ventricle, hence also called as the moderator band.
• The supraventricular crest, a muscular ridge situated between the tricuspid and pulmonary oriAces separates
the inflow and outflow tracts.
(2) The smooth outflow tract or infundibulum
• Develops from the right part of the bulbus cordis, forms the superior portion of the right ventricle and gives rise
to the pulmonary trunk.
• The apex of the conical shaped infundibulum has pulmonary orifice guarded by three semilunar cusps.
• The blood entering the infundibulum is ejected superiorly and posteriorly into the pulmonary trunk.
(3) The apical trabecular portion or body of the RV:
It is also derived from the primitive ventricle of the heart tube, and is much coarser than that of the left ventricle.
iii Left Atrium (LA)
rdIt is a quadrangular chamber situated posteriorly, behind and to the left of the right atrium, and forms 2/3 of
the base of the heart.
• The left atrium receives the oxygenated blood from the pulmonary veins and serves as a reservoir during left=
ventricular systole and pumps the blood into the left ventricle through the left atrio-ventricular (mitral) oriAce
during the left ventricular diastole.
• The posterior part is derived from the incorporation of the single pulmonary vein while the anterior part
including the left auricle is developed from the left half of the primitive atrium (see Table 1.8).
Table 1.8 Development of the left heart
Fetal structures Portion developed
1. Incorporated pulmonary veins Posterior portion of left atrium (LA)
2. Left half of primitive atrium (i) Anterior portion of LA and
(ii) Left auricle
3. Left part of bulbus cordis Outflow of left ventricle (LV)
4. Left part of primitive ventricle (i) Free wall, apex of LV
(ii) Inflow tract of LV
a) External Features
• The anterior wall of the left atrium is formed by the interatrial septum while the posterior surface of the left
atrium forms the anterior wall of the oblique sinus of the pericardium.
• Its appendage, the left auricle projects anteriorly to overlap the infundibulum of the right ventricle.
b) Internal Features
• The wall of the left atrium is 3 mm thick, slightly thicker than that of the right atrium. Most of the wall is
smooth and only a network of muscular pectinati is present within the left auricle.
• Two pulmonary veins open into the left atrium on each side of the posterior wall. There are no true valves at
the junction of the pulmonary veins and the left atrium, but the ‘sleeves of the atrial muscle extend from the
left atrial wall around the pulmonary veins for 1 or 2 cm and may exert a partial sphincter-like in uence,
tending to lessen the reflux during atrial systole or mitral regurgitation.
• The atrial septum is also smooth and shows the fossa lunata corresponding to the fossa ovalis of the right
atrium.
iv Left Ventricle (LV)
The left ventricle is roughly bullet-shaped with blunt tip directed anteriorly, inferiorly and to the left where it
forms the apex of the heart with the lower ventricular septum.
The left ventricle receives blood from the left atrium during ventricular diastole and ejects blood into the
systemic circulation during ventricular systole.
a) External Features
The left ventricle forms:
• the apex (with lower ventricular septum)
rd• 1/3 of the sterno-costal surface
• most of the left surface and
rd• 2/3 of the inferior surface of the heart.
The left ventricle is posterior and to left of the right ventricle and inferior, anterior and to the left of the left
atrium.
b) Internal Features
The left ventricular chamber is approximately an ellipsoidal in shape and its walls measure 8–15 mm thick.
However the tip of the LV apex is often thin, measuring 2 mm or less. The left ventricle consists of (see Figs 1.9):
• The ventricular septum
• The free wall of the left ventricle• The inflow tract
• The outflow tract
• The apical portion of the left ventricle.
Fig. 1.9 Interior of the left ventricle (diagrammatic).
(1) Ventricular septum
• The medial wall of the left ventricle is the ventricular septum.
• It is roughly triangular in shape, with the base of the triangle at the level of the aortic cusps.
• It is entirely muscular but a small portion located superiorly just below the right coronary and posterior
rdcoronary cusps, is membranous (see Table 1.9). The upper 1/3 of the septum is smooth endocardium while
rdthe remaining 2/3 of the septum and remaining LV walls are ridged by interlacing muscles, the trabeculae
carneae.
Table 1.9 Development of ventricular septum
Portion of the ventricular
Developed fromseptum
1. Muscular portion Trabeculae and medial walls between the primitive LV and RV, appose and fuse
together to form incomplete muscular septum at 3 mm stage
2. Membraneous portion Lower edge of conal septum and inferior endocardial cushion at 6 mm stage
(2) The free wall of the left ventricle:
The ridged trabeculae carneae excluding the ventricular septum is the free wall of the left ventricle (see Fig.
1.10).=
=
=
=
=
=
Fig. 1.10 Interior of the left heart with LV free wall and mitral valve removed.
(3) The inflow tract
• The anteromedial lea et of the mitral valve (MV) extending from the top of the posteromedial septum to the
anterolateral ventricular wall separates the LV cavity into an inflow tract and an outflow tract.
• The funnel shaped in ow tract is developed from the left part of the primitive ventricle and consists of mitral
oriAce with its mitral valve apparatus, which directs the atrial blood inferiorly, anteriorly and to the left
towards the LV apex.
(4) The outflow tract
• The conical smooth walled out ow tract is situated above, in front of and slightly to the right of the mitral
orifice.
• It is developed from the left part of the bulbus cordis.
• It is surrounded by the inferior surface of the anteromedial mitral lea et, the ventricular septum and the left
ventricular free wall.
3• It orients the blood ow from the LV apex to the right and superiorly at an angle of 90° to the in ow tract
ejecting the blood into the ascending aorta through the aortic orifice during ventricular systole.
• The summit of the aortic vestibule is occupied by the aortic annulus guarded by three semilunar cusps.
(5) The apical portion of the left ventricle is characterized by Ane trabeculations, also developed from the
primitive ventricle (see Table 1.10).
Table 1.10 Description of the left heart
Features Description
1. Shape of the left atrium (LA) Quadrangular chamber
2. LA wall thickness 3 mm
3. Portions of LA (i) Auricle: has musculi pectinati
(ii) Smooth walled body
(iii) Smooth interatrial septum with fossa lunata
4. Shape of the left ventricle (LV) Bullet shaped with ellipsoidal chamber
5. LV wall thickness 8–15 mm, while apex is 2 mm thick
6. Portions of LV (i) Triangular ventricular septum
(ii) Free wall is ridged with trabeculae carnae
(iii) Funnel shaped inflow
(iv) Smooth conical shaped out flow4 The Fibrous Skeleton of the Heart
i Components and Attachments
• The Abrous skeleton of the heart is made up of four fibrous rings (mitral, tricuspid, pulmonary and aortic) (see
Fig. 1.11 and Table 1.11) at the bases of both ventricles around the mitral, tricusid, pulmonary and aortic
orifices, which provide the attachment to:
(i) Atrial and ventricular musculature
(ii) Valves of the heart and
(iii) Roots of the aorta and pulmonary trunk.
• The pulmonary ring lies above, in front of and slightly to the left of the aortic ring. Both the rings are set at right
angles to each other and connected by a fibrous septum known as tendon of infundibulum.
• The medial aspects of mitral and tricuspid rings are fused by the central Abrous body known as trigonum
fibrous dextrum or right fibrous trigone.
• The left margin of the trigone connects aortic and mitral rings, which is named as trigonum 9brosum
sinistrum or left fibrous trigone.
• The right and left Abrous trigones which partially encircle the mitral and tricuspid oriAces are the mitral and
tricuspid annuli that give attachment to the mitral and tricuspid valves, atrial and ventricular muscle.
Fig. 1.11 Fibrous skeleton of the heart (diagrammatic).
Table 1.11 Fibrous skeleton of the heart
Features Description
1. Four fibrous rings (i) Pulmonary
(ii) Aortic
(iii) Mitral
(iv) Tricuspid (iii + iv form central fibrous body)
2. Two trigones Right fibrous trigone
Left fibrous trigone
ii Extensions
• An important extension of the Abrous skeleton is the membranous ventricular septum, which extends inferiorly
and anteriorly from the right Abrous trigone. It is located at the summit of the muscular septum, and provides
support for the right coronary and noncoronary aortic cusps.
• A portion of the membranous septum extends slightly above the tricuspid valve, forming a small portion of the
medial wall of the right atrium.iii The Bundle of His
It penetrates the central Abrous body and travels along the inferior margin of the membranous ventricular
septum. At the crest of the muscular septum, above the level of junction of the right coronary and noncoronary
(posterior) aortic cusps, the His bundle divides into a left bundle branch and a right bundle branch. The right
fibrous trigone is sometimes calcified in old age while this a constant feature in sheep’s heart.
5 The Valves of the Heart
There are two pairs of valves in the heart (see Fig. 1.12).
• A pair of atrioventricular valves: mitral and tricuspid and
• A pair of semilunar valves: aorta and pulmonary.
Fig. 1.12 Transverse section through the ventricles showing the valves of the heart (diagrammatic).
General Description
• They maintain unidirectional flow of the blood and prevent its regurgitation in the opposite direction.
• The anatomy of the atrioventricular (AV) valves is more complex than that of the semilunar (SL) valves.
• Each cardiac valve has a central spongiosa (collagenous core) and a peripheral Abrosa. Both the sides of the
Abrosa are covered by a loose Abroelastic tissue usually containing mucopolysaccharides and the entire valve is
covered by endothelium.
• The loose Abroelastic tissue on the atrial aspect of the AV valves is known as atrialis, ventricular surface of all
four valves (AV & SL), the ventricularis, and aortic and pulmonary surfaces of SL valves are known as
4arterialis (see Table 1.12).
• The endothelium and loose connective tissue of the AV valves are continuous with the atrial and ventricular
endothelium and those of the SL valves are continuous with the aortic and pulmonary intima.
rd• Smooth striated cardiac muscle may extend onto the proximal 1/3 of the atrialis in the AV valves and often
rd 4–8contain blood vessels. The distal 2/3 of AV valves and both SL valves are avascular.
Table 1.12 Valve leaflet surfaces
Surface Description
1. Atrialis Atrial surface of atrioventricular valve leaflets
2. Ventricularis Ventricular surface of all leaflets
3. Arterialis Arterial surface of the semilunar valve leaflets
i Mitral Valve
Mitral valve develops primarily from (see Table 1.13):=
9• LV muscle wall: predominantly by delamination of the muscular ventricular wall, hence valve cusps initially
10are thick and fleshy.
• Endocardial cushions:
(i) anterior mitral leaflet → from superior and inferior endocardial cushions
(ii) posterior mitral leaflet → from left lateral endocardial cushion.
Table 1.13 Development of the valves
Part Developed from
1. Mitral
Anterior mitral leaflet Superior and inferior endocardial cushions
Posterior mitral leaflet Left lateral endocardial cushion
2. Tricuspid
Anterior leaflet Right lateral and dextro dorsal endocardial cushions
Posterior leaflet Right lateral endocardial cushion
Septal leaflet Inferior endocardial cushion
3. Semilunar valves Truncus arteriosus
Truncal and intercalated valve cushions
The mitral valve consists of six major anatomic components: annulus, lea ets, chordae tendinae, papillary
muscles, posterior left atrial wall and left ventricular free wall (see Fig. 1.13 and Table 1.14).
Fig. 1.13 Mitral valve complex (diagrammatic).
Table 1.14 Structure of the mitral valve
Features Description
1. Annulus Saddle shaped; 4–6 cm2 in size; fibrous anteromedial portion and muscular
posterolateral portion
2. Leaflets Sail shaped AML
C shaped scalloped PML
3. Papillary muscles Anterolateral PM at 4° clock position=
=
=
=
=
Posteromedial PM at 7° clock position
4. Chordea tendinae 120 in number
Cuspal: primary, secondary and tertiary
Commissural: anterolateral and posteromedial
a) Mitral Annulus
• Shape of the mitral annulus is catenoid or saddle shaped in the embryo as well as in the adults.
2• The size of the annulus is 4–6 cm (corresponds to mitral valve area–MVA) and the circumference of the valve
11–13(not really that of the annulus) is 8–10.5 cm with a mean of 9.4 cm.
rd• Anteromedial portion is formed from the Abrous trigone and collagen Abers which encircle 1/2 or 2/3 of the
annulus while rest of the annulus (posterolateral portion) is devoid of Abrous tissue and formed by the
myocardium of the LV and left atrium.
• The decrease in annular size during ventricular systole is due to contraction of this posterolateral portion.
b) Leaflets
Mitral valve has two lea ets: anterior mitral lea et (AML) and posterior mitral lea et (PML). However, it can
14have two small minor commissural cusps which are normally incomplete.
(i) AML
rd• AML is sail shaped and is attached in a hinge like to 1/3 of the anteromedial portion of the annulus.
• It is directly continuous with 1/2 of the non-coronary (posterior) cusp and most of the left coronary cusp
of aorta.
• It forms the semicircular posterior border of the LV outflow tract.
(ii) PML
rd• PML is ‘C’ shaped, hinges on the posterolateral 2/3 of the annulus.
• It is longer at its base (6 cm of circumference) and shorter in its basal to apical length (1.2 cm) than
15AML (3 cm of circumference and 2.3 cm of basal to apical length) however both have similar
11,12surface area.
16• The surface area of both lea ets is about 2½ times that of the oriAce area, while the cross sectional
area of both leaflets is 20% more than the mitral orifice.
• PML is sub-divided by medial and lateral clefts into three scallops-posteromedial, middle and
anterolateral portions with middle being the largest (1.3 cm width compared to 1.0 cm for other
15two).
(iii) Surface of the leaflets
• The atrial surface of the cusps is generally smooth except near the free edge where chordae tendinea are
attached. Slightly away from the free edge on the atrial surface are Ane nodules called the noduli
Albini.
• The ventricular surface of the cusps is irregular due to insertion of the chordae tendinae.
(iv) Commissures: The leaflets are connected to each other at junctions called commissures (SL valve
commissures are ‘spaces’), anterolateral and posteromedial commissures.
c) Papillary Muscles (PM)
There are two papillary muscles: anterolateral and posteromedial, located below the commissures projecting from
14the trabeculae carneae. Interpapillary muscle distance is relatively constant. Each PM has six heads.
(i) Anterolateral PM: It is situated at 4° clock position and is supplied by a diagonal branch of left anterior
descending) artery and obtuse marginal of left circumflex artery.
(ii) Posteromedial PM: It is situated at 7° clock position and is supplied by posterior descending artery of right
coronary artery in 85%, left circumference artery in 7% and by both in 8% (co-dominant) of individuals.
d) Chordae Tendinae (CT)
These are complex network of exible cord like structures primarily made of collagen. They are of two types:cuspal (leaflet) chords and commissural chords.
There are about 12 chordae attached to each of the six heads of each PM. These chordae divide about three
17times before their ultimate attachment. In total, there are about 120 chordae attached to both mitral leaflets.
(i) Cuspal (leaflet) chords are classified into three groups: primary, secondary and tertiary.
• Primary group originate near the PM apices, divide into a number of Aner strands that insert at the
extreme edge of the cusps. These chordae prevent the cusp inversion into the left atrium during
ventricular systole.
• Secondary group of chordae also originate near the PM apices, are thicker in diameter, but less in
number as compared to the Arst group and tend to insert on the ventricular surface of the cusps. They
serve to anchor the valve.
• Tertiary group of chordae originate from the ventricular wall, may actively contain muscle and are
14,18attached to the ventricular aspect of posterior leaflet. These chords are specific to the PML.
(ii) Commissural chords arise from the anterolateral and posteromedial PM and branch in a fan-like manner
to be inserted on to both commissures.
e) Mitral Valve Closure
The closure of mitral valve involves a complex interplay of active and passive processes. It consists of three
phases: initial leaflet phase, annular phase and ventriculogenic phase.
(i) Initial leaflet phase: At the end of the rapid filling phase, the leaflets gradually move passively towards
the closed position due to vortex currents generated under their ventricular surfaces.
(ii) Annular phase: With the onset of atrial contraction, annular contraction begins which continues
throughout the ventricular systole. The annular contraction is an important phase in MV closure and
causes 20–40% decrease in annular orifice. Non-homogenous structure of the annulus produces an
eccentric narrowing of the orifice during annular contraction.
(iii) Ventriculogenic phase: With isometric contraction, the contraction of intra-valvular muscle fibers
occurs and leaflets become concave in shape which opposes leaflet eversion during ventricular systole.
The large sail shaped AML swings and is engulfed by Gusset like C shaped PML causing closure of mitral
valve. Leaflets are further sealed together by the opposing effect of intraluminal pressures. Papillary
muscles and chordae maintain isometric tension and thereby stabilizes the leaflets during ventricular
systole.
ii Tricuspid Valve
The tricuspid valve develops from: (see Table 1.13)
(i) RV muscle wall → by delamination of muscle wall
(ii) endocardial cushion:
• Anterior leaflet from right lateral and dextro-dorsal endocardial cushions
• Posterior leaflet from right lateral endocardial cushion and
• Septal leaflet from inferior endocardial cushion.
The tricuspid valve like mitral valve also consists of six major anatomic components: right atrial wall, annulus,
leaflets, papillary muscles, chordae tendineae and right ventricular free wall (see Fig. 1.14 and Table 1.15).=
=
Fig. 1.14 Tricuspid valve complex (diagrammatic).
Table 1.15 Structure of the tricuspid valve
Features Description
1. Annulus Circumferential in shape, 5–8 cm2 in size-anterior, posterior and septal portions
2. Leaflets Largest anterior, smallest medial or septal and scalloped posterior
3. Papillary muscles Anterior (largest), posterior (usually multiple) and septal (may be rudimentary)
4. Chordae tendinae Five types: fan shaped, rough, deep, basal and free edged (deep and free edged are
unique to TV)
a) Tricuspid Annulus
• It is nearly circumferential, larger than mitral annulus but lies at a lower level than the mitral annulus.
2 2• The size of the tricuspid annulus is 5–8 cm with a mean of 7 cm (tricuspid valve area-TVA) and
circumference of 11.4 cm ± 1.1 in males and 10.8 ± 1.3 in females.
• The posterior lea et makes up the largest portion of the annulus (7.5 cm), followed by the anterior (3.7 cm),
19and septal (3.6 cm) leaflets.
b) Leaflets
Tricuspid valve has three leaflets: anterior, posterior and septal.
19• Anterior leaflet is the largest with a width of 2.2 cm.
19• Septal (medial) leaflet is the smallest with a width of 1.5 cm.
19• The posterior leaflet measures 2.0 cm in width and may have 1–3 scallops produced by small clefts.
There are three commissures:
• Anteroposterior with a size of 1.1 cm
• Posteroseptal with 0.8 cm size and
• Anteroseptal commissure with a size of 0.5 cm.
c) Papillary Muscles (PM)
TV has three papillary muscles: anterior, septal (medial) and posterior.
• The anterior PM is the largest, located below the anteroposterior commissure originating from the moderator
band as well as from the anterolateral ventricular wall.
• The posterior PM lies beneath the posteroseptal commissure attached to the posterior wall of the RV and receive
chordae from posterior and septal leaflets. It is usually multiple.
• The septal PM is small originating from the wall of the infundibulum. It has extensive attachments to the
ventricular septum and receives chordae from the anterior and septal lea ets. At times, septal PM is
rudimentary, absent, double or multiple.=
=
d) Chordae Tendineae
It may arise from the papillary muscles or from the muscle of the posterior or septal walls of the RV.
On an average, there are 25 chordae inserted into the tricuspid valve:
• 7 chordae are inserted into the anterior leaflet
• 6 into the posterior leaflet
• 9 into the septal leaflet and
• 3 into the commissures.
19TV has 5 types of chordae tendinea:
• Fan shaped
• Rough
• Deep
• Basal and
• Free-edge.
19The deep and free-edge are unique to the tricuspid valve. Deep chordae provide a second arcade for lea et
attachment, while free-edge are single and inserted into the leaflet’s free edge.
Fan shaped chordae are inserted into the three commissures while basal chordae are the shortest and measure
an average of 0.6 cm. Rough and deep chordae may be as long as 2.2 cm.
iii Semilunar (SL) Valves
• Semilunar valves are derived from:
(i) the truncus arteriosus and
(ii) truncal and intercalated valve cushions.
• The aortic and pulmonary valves are called semilunar valves because their cusps are semilunar in shape.
• They are situated at the summit of the out ow tract of their corresponding ventricle, the pulmonary valve is
anterior, superior and slightly to the left of the aortic valve.
• Each semilunar valve consists of an annulus, three equal-sized semicircular cusps, three equal spaced
commissures and three sinuses of Valsalva (see Table 1.16).
Table 1.16 Structure of the semilunar valves
Features Description
1. Annulus PV: 7–9 cm in circumference
AV: 2.5 cm2 in area
2. Leaflets AV: 2 anterior: right and left coronary cusps
1 posterior or noncoronary cusp
PV: 2 posterior: right and left cusps
1 anterior cusp
3. Sinuses of Valsalva AV: 2 anterior: right and left
1 posterior
PV: 2 posterior: right and left
1 anterior
PV: pulmonary valve; AV: aortic valve.
a) Annulus
Unlike aortic valve the pulmonary valve has no discrete annulus or Abrous ring. The apex of the infundibulum
presents the pulmonary oriAce which is circular and guarded by three semilunar cusps. The pulmonary valve
annulus is about 1.5 cm above the level of the aortic valve annulus, but its circumference is similar: 7–9 cm. The
2average size of the aortic annulus is 2.5 cm .b) Leaflets (Cusps)
• Each cusp is attached by its semicircular border (lower edge) to the wall of the aorta or pulmonary trunk while
the upper free edges project into the lumen.
5• The aortic and pulmonary valves are similar in configuration except that the aortic cusps are slightly thicker.
• The three aortic valve cusps: two anterior-right and left coronary cusps; one posterior or noncoronary cusp,
while the pulmonary valve has two posterior cusps-right and left and one anterior cusp (see Figs 1.15 and
1.16).
• The aortic left and noncoronary cusps are continuous with AML of the mitral valve. The free margin of each
cusp contains a central Abrous nodule on its ventricular surface called noduli Arantii which marks the contact
sites of closure. From each side of the nodule, a thin smooth margin (lunule) extends to the base of the cusp,
which is less prominent in the pulmonary valve (see Fig. 1.17).
• There may be variation in the cusps and commissural sizes and positions of the sinuses of Valsalva which result
in asymmetric lines of closure and may accelerate ‘wear and tear’ (aging) of the valve structure especially of
that of the aortic valve. Because of less systolic pressure in RV, these acquired senile (aging) changes in
pulmonary valve do not occur.
Fig. 1.15 Structure of the aortic valve (diagrammatic)—N: noncoronary cusp, R: right anterior cusp, L: left
anterior cusp, AML: anterior mitral leaflet.
Fig. 1.16 Structure of the pulmonary valve (diagrammatic)—R: right posterior cusp, L: left posterior cusp, A:
anterior cusp.=
=
=
Fig. 1.17 Structure of the aortic valve (diagrammatic).
c) Commissures
• Each semilunar valve has equally spaced three commissures i.e. the small space between the attachments of the
adjacent cusps (vs. AV valves).
• The circumference connecting these points is termed as the sinotubular junction, which separates the sinuses
of Valsalva from the adjacent tubular portion of the vessel. In aorta, a distinct hump or line marks this junction
which was originally described by Leonardo da Vinci as the ‘supra-aortic ridge’.
• The circumference is measured at this sinotubular junction with echocardiography and at necropsy.
• While the lowermost portion of aorta (at the junction of the aortic valve with the ventricular septum) which is
referred as the aortic ring, is measured by the surgeons to determine the size of the aortic prosthetic valve.
d) Sinuses of Valsalva
A pouch-like dilatation above each cusp is known as sinus of Valsalva. The aortic right and left sinuses of
Valsalva give rise to right and left coronary artery respectively.
The aortic sinuses of Valsalva have close relation with right and left sided chambers. Hence, rupture of the
right and non-coronary sinuses of Valsalva may communicate with right sided chambers (out ow tract RV and
RA) while rupture of left sinus of Valsalva communicates with left sided chambers (LA or LV out ow tract). With
age,
• The aortic cusps thicken
• Nodules thicken and enlarges
• Sinuses of Valsalva calcify and dilate and
• Lunules develop fenestrations.
With age, the pulmonary valve cusps also thicken slightly but rest of the changes are less prominent.
e) SL Valve Closure
During ventricular systole, the cusps are passively thrust upward away from the center of the lumen. During
ventricular diastole, the cusps fall passively into the lumen of the vessel as they support the column of blood
above while the nodules meet in the center which contributes to the support of the lea ets, thus preventing the
regurgitation of blood.
References
1. Edwards WD. Applied anatomy of the heart. In: Brandenburg RO, Fuster V, Giuliani ER, eds., et al. Cardiology:
Fundamentals and Practice. Chicago: Year Book; 1987:47–109.
2. Prakash R. Determination of right ventricular wall thickness in systole and diastole. Electrocardiographic and
necroscopy correlation in 32 patients. Br Heart J. 1978;40:1257–1261.
3. Walmsley R, Watson H. Clinical Anatomy of the Heart. New York: Churchill Livingstone; 1978. 1–22.
4. Gross L, Kugel MA. Topographical anatomy and history of the valves in the human heart. Am J Pathol.
1931;7:445–474.
5. Waller BF. Morphological aspects of valvular heart disease. Part I. Curr Probl Cardiol. 1984;9(7):1–66.
6. Clarke JA. An X-ray microscopic study of the blood supply to the valves of the human heart. Br Heart J.1965;27:420–423.
7. Duran CM, Gunning AJ. The vascularization of the heart valve: A comparative study. Cardiovasc Res.
1968;2(3):290–296.
8. Montiel MM. Muscular apparatus of the mitral valve in man and its involvement in left sided cardiac
hypertrophy. Am J Cardiol. 1970;26(4):341–344.
9. Dox X, Corone P. Embriologie cardiaque: Malformations (1). In: Embryologie Cardioaque-Editions Techniques Paris.
Chirurgicale: Encyclopedie Medico; 1992. 1–20
10. Streeter GL. Developmental horizons in human embryos: Description of age groups XI. 13–20 somites and age
group XII. 21–29 somites. Contrib embryol. 1942;30:211–245.
11. Perloff JK, Roberts WC. The mitral apparatus: Functional anatomy of mitral regurgitation. Circulation.
1972;46(2):227–239.
12. Waller BF, Morrow AG, Maron BJ, Del Negro AA, Kent KM, McGrath FJ, et al. Etiology of clinically isolated,
severe, chronic, pure mitral regurgitation: Analysis of 97 patients over 30 years of age having mitral valve
replacement. Am Heart J. 1982;104(2 Pt 1):276–288.
13. Roberts WC. Morphologic features of normal and abnormal mitral valve. Am J Cardiol. 1983;51(6):1005–1028.
14. Netter FH. CIBA collection of medical illustration: Heart vol 5. Summit NJ: CIBA pharmaceutcal; 1987. 9–112.
15. Ranganathan N, Lam JHC, Wigle ED, Silver MD. Morphology of human mitral valve: II. The valve leaflets.
Circulation. 1970;41:459–467.
16. Brock RC. The surgical and pathological anatomy of the mitral valve. Br Heart J. 1952;14(4):489–513.
17. Constant J. In: Bedside Cardiology. 3rd Boston, Mass: Little, Brown and Company; 1985. pp. 38–39.
18. Carbello BA. Mitral valve disease. Curr Probl Cardiol. 1993;18(7):423–478.
19. Silver MD, Lam JHC, Ranganathan N, Wigle ED. Morphology of human tricuspid valve. Circulation.
1971;43(3):333–348.
Chapter 2
Lymphatic System of the Heart
1. Formation Of Right Coronary Channel
2. Formation Of Left Coronary Channel
3. Formation Of Main Supra-Cardiac Channel
4. Formation Of Right Lymphatic Duct
References
The lymphatic drainage of the heart ows from subendocardial vessels to an extensive capillary plexus lying
1,2throughout the subepicardium. These capillaries converge in collecting lymphatic channels which run
alongside the coronary vessels which form the right lymphatic duct (see Figs 2.1 and 2.2).Fig. 2.1 Lymphatic drainage of the heart.Fig. 2.2 Lymphatic drainage of the heart (diagrammatic)—PVT: posterior interventricular trunk, AVT: anterior
interventricular trunk, OMT: obtuse marginal trunk.
There are two main lymphatic channels:
• Right coronary channel (RCC) and
• Left coronary channel (LCC).
1 Formation Of Right Coronary Channel
The posterior interventricular trunk (PVT) runs along with the posterior descending artery (PDA) in the
posterior interventricular sulcus up to the crus of the heart, and then encircles around to the right from
posterior to anterior in the right coronary sulcus to become the right coronary channel.
2 Formation Of Left Coronary Channel
The two major LV channels are: anterior interventricular trunk and obtuse marginal trunk.
• The anterior interventricular trunk (AVT) ascends from apex to base along with left anterior descending
artery (LAD) in the anterior interventricular sulcus.
• The obtuse marginal trunk (OMT) runs alongside the left circumflex artery in left coronary sulcus.
• Near the base of the pulmonary artery; the two LV channels, anterior interventricular trunk and obtuse
marginal trunk join together to form left coronary channel.
3 Formation Of Main Supra-Cardiac Channel
The right coronary channel unites with the left coronary channel to become main supra-cardiac channel
(MSC), which passes upward beneath the left atrial appendage,behind the pulmonary artery to enter a
pretracheal lymph node (cardiac lymph node; CLN) between the arch of aorta and the pulmonary artery.
4 Formation Of Right Lymphatic Duct
From the CLN, the right lymphatic duct (RLD) arises which runs cephalad in the mediastinum to drain into
the junction of the internal jugular vein (IJV) and right subclavian vein (Sci v).
The thoracic duct, the largest lymphatic vessel of the body, extends from the upper part of the abdomen to
the lower part of the neck crossing the posterior and superior mediastinum and drains into the junction of theleft subclavian and left internal jugular veins or left brachiocephalic vein.
On the right side of the thorax, there are three main lymphatic ducts:
• Right jugular lymphatic duct drains into the right jugular vein.
• Right subclavian lymphatic duct drains into right subclavian vein and
• Right mediastinal lymphatic duct drains into right brachiocephalic vein.
Occasionally, the right jugular and right subclavian lymphatic ducts unite to form right lymphatic duct.
The lymph vessels like veins contain valves to prevent backward flow.
References
1. Feola M, Merklin R, Cho S, Brockman SK. The terminal pathway of the lymphatic system of the human
heart. Ann Thorax Surg. 1977;22:531–536.
2. Miller AJ. Lymphatics of the Heart. New York: Raven press; 1982.Chapter 3
Venous Drainage of the Heart
1. Coronary Sinus
i) Great Cardiac Vein
ii) Oblique Vein of Left Atrium (or Oblique Vein of Marshall)
iii) Posterior Vein of the LV
iv) Middle Cardiac Vein (or Posterior Interventricular Vein)
v) Small Cardiac Vein
2. Anterior Cardiac Veins
3. Thebesian Veins (Venae Cordis Minimi)
Reference
1There are three venous drainage systems:
• Coronary sinus
• Anterior cardiac veins and
• Thebesian veins.
About 60% of the venous blood of the heart drains into the right atrium via the coronary sinus and remaining
40% drains into the di- erent chambers of the heart via anterior cardiac veins and Thebesian veins (see Fig. 3.1
and Table 3.1).
Fig. 3.1 Venous drainage of the heart (diagrammatic).
Table 3.1 Venous drainage of the heart
Coronary sinus Anterior cardiac veins Thebesian veins
1. Drains: LCA territory Drains: RCA territory Drains: Both the territories, but
primarily RCA territory2. Branches: 5 in number Branches: 1–4 Branches: Small and numerous
Great cardiac vein
Oblique vein of left atrium
Posterior vein of LV
Middle cardiac vein
Small cardiac vein
Coronary sinus conveys the greater part of the blood from the left coronary artery territory while the anterior
cardiac veins drain most of the blood from the right coronary artery territory (see Fig. 3.2).
Fig. 3.2 Venous drainage of the heart (diagrammatic)—OV of LV: obtuse vein of left ventricle, PV of LV:
posterior vein of left ventricle, LMV: left marginal vein, RMV: right marginal vein.
1 Coronary Sinus
• Coronary sinus is about 2–3 cm long situated in the posterior part of the atrioven-tricular groove (coronary
sulcus) near the crux of the heart.
• It begins in the left part of the AV groove where it receives the great cardiac vein and oblique vein of left
atrium at its left end, then passes downwards and to the right along the posterior part of the AV groove
receiving posterior vein of left ventricle in its middle part and middle cardiac vein and small cardiac vein at
its right end. Finally, the coronary sinus opens into the inferoposteromedial aspect of the right atrium
between the orifice of the IVC and septal leaflet. The AV node lies just above its opening.
• A crescent shaped rudimentary valve, the Thebesian valve guards the opening of the coronary sinus. All the
tributaries of coronary sinus are provided with valves except the oblique vein of left atrium.
• It is developed from the left horn and body of the sinus venosus. The Thebesian valve is derived from the
lower part or the venous valve.
i) Great Cardiac Vein
• It begins near the apex of the heart as anterior interventricular vein in the anterior interventricular sulcus,
runs upwards along with left anterior descending artery, and turns leftward near the bifurcation of the leftcoronary artery to circle posteriorly under the left atrium in the left AV sulcus to become great cardiac vein.
• It receives the left marginal vein near its termination in the coronary sinus.
ii) Oblique Vein of Left Atrium (or Oblique Vein of Marshall)
• It drains the posterior surface of the left atrium, runs on the posterior surface of the left atrium and terminates
in the left of the coronary sinus.
• It develops from the left common cardinal vein (duct of Cuvier) which may sometimes form a large left
superior vena cava.
iii) Posterior Vein of the LV
• It drains the inferior surface of the left ventricle, runs on the diaphragmatic surface of the LV and ends in the
middle of the coronary sinus on the left side of the middle cardiac vein.
iv) Middle Cardiac Vein (or Posterior Interventricular Vein)
• It arises near the posterior aspect of the cardiac apex and ascends in the posterior inter-ventricular sulcus
along with the posterior descending artery and terminates into the right end of the coronary sinus or may
sometimes directly drain into the right atrium.
v) Small Cardiac Vein
• It lies in the right coronary sulcus accompanying the right coronary artery, receives the right marginal vein
draining the RV and RA and terminates in the right end of the coronary sinus or sometimes directly into the
right atrium.
2 Anterior Cardiac Veins
There are 2–4 anterior cardiac veins which drain the anterior RV wall, run superiorly on the anterior wall of
the RV and cross the right AV sulcus to terminate directly into the anterior part of the right atrium.
3 Thebesian Veins (Venae Cordis Minimi)
These are small numerous veins draining the myocardium directly into the cardiac chambers, primarily into
the right atrium and right ventricle (see Fig. 3.2).
References
1. James TN. In: Anatomy of the Coronary Arteries. New York: Hoeber Medical Division. Harper & Row; 1961:1–
77.Chapter 4
Arterial Supply of the Heart
1. Left Coronary Artery
i) Left Anterior Descending Artery (LAD)
ii) Left Circumflex Artery (LC or LCx)
iii) Ramus Intermedius
2. Right Coronary Artery (RCA)
3. Divergent Coronary Anatomy
4. Measurements
i) Length
ii) Luminal Diameter
References
The heart is supplied by two coronary arteries: left coronary artery and right coronary artery (see Table
4.1)
Table 4.1 Coronary artery (CA) distribution
CA branch Distribution
1. Diagonal branches Anterolateral portion of LV
2. Septal branches Anterosuperior 2/3rd of vent. septum
3. LA branch Left atrium
4. OM Lateral free wall of LV
5. Conus branch RVOT
6. SA nodal branch SA node (in 50–60%)
7. AV nodal branch AV node (in 85%)
8. PDA Posteroinferior 1/3rd of vent. septum
9. PLV branch Posterolateral LV (85%)
1 Left Coronary Artery
The left main coronary artery (LM or LMCA) arises from left anterior coronary sinus, passes behind the
pulmonary trunk (RVOT), runs forwards and to the left between the pulmonary trunk and left auricle where it
1bifurcates into left anterior descending (LAD) and left circumflex (LC or LCx) branches (see Fig. 4.1).Fig. 4.1 Origin of the coronary arteries.
i) Left Anterior Descending Artery (LAD)
It runs in the anterior interventricular groove towards the apex. LAD gives 2–6 diagonal (D) and 3–5 septal (S)
branches.
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rd• The septal branches originate from the LAD at a right angle and supply anterosuperior 2/3 portion of the
ventricular septum.
LAD terminates:
• Beyond the ventricular apex (type-III) along the diaphragmatic aspect in 78%.
• At the apex (type II) or before the apex (type-I) in 22%.
Angiographically LAD is divided into three portions: Proximal, Mid and Distal (see Figs 4.2, 4.3 and 4.4).
• Proximal LAD is the portion from its origin from the LMCA to its first diagonal (D1) branch.
• Mid LAD is the portion between first diagonal (D1) and second diagonal (D2) branches.
• Distal LAD is the portion of the LAD beyond (D2) branch.
Fig. 4.2 Coronary angiogram of LCA-RAO caudal.Fig. 4.3 Coronary angiogram of LCA-LAO cranial.
Fig. 4.4 Coronary angiogram of LCA-AP cranial.
ii) Left Circumflex Artery (LC or LCx)
After its origin from the LMCA, LCx travels in the left AV groove (left coronary sulcus) and in the posterior part
of AV groove; it anastomoses with the branches of right coronary artery (RCA).
In right dominant coronary circulation (85%), LCx gives 1–2 left atrial branches which supply the left atrium
and 1–3 obtuse marginal branches (OM) that supply lateral free wall of the LV.
In left dominant coronary circulation (8%) in addition to left atrial and OM branches, it gives rise to
posterolateral left ventricular (PLV) branch, posterior descending artery (PDA), AV nodal artery and sinus
nodal artery (in 40–50%).
Similarly, LCx is angiographically divided into three portions: Proximal, Mid, and Distal (see Figs 4.2, 4.3
and 4.4).
• Proximal LCx is the portion from its origin from the LMCA to the origin of the first obtuse marginal branch.
• Mid LCx is the portion of LCx between the first OM and second OM branches.
• Distal LCx is the portion of the LCx beyond second OM branch.
iii) Ramus Intermedius
In some patients, a large intermedius or ramus medianus branch may originate directly from the LMCA,
bisecting the angle between the LAD and LCx, so that there is a tri-furcation of the LMCA.
2 Right Coronary Artery (RCA)
It arises from the right coronary sinus which is lower is position than that of the left coronary artery passes
forwards and to the right between the pulmonary trunk and right auricle, then runs downwards in the right
anterior coronary sulcus, winds round the inferior border of the heart to reach the diaphragmatic surface of the
heart. Here, it passes upwards and to the left in the right posterior coronary sulcus and reaches the crux of the
heart and terminates by anastomosing with the branches of the LCx.
In general, it gives rise to:
• Conus artery• SA nodal branch
• Right atrial branches
• RV and acute marginal branches which supply the free wall of the right ventricle.
(i) The conus artery: It is usually the Brst branch of RCA, which supplies the infundibulum of the RV
(RVOT), but in 40–50% it may originate from a separate ostium in the right coronary sinus (third coronary
artery).
(ii) SA nodal artery: In 50–60%, SA nodal artery originates from the RCA, runs along the anterior right
atrium to the superior vena cava, which it encircles in a clockwise or anticlockwise before it penetrates the SA
node.
However, when the SA nodal artery arises from the LCx (in 40–50%), it crosses behind the aorta and in front
2of the left atrium to reach the superior vena cava and penetrating the SA node.
(iii) In right dominant coronary circulation (in 85%), RCA gives rise to AV nodal artery, PDA which
rdsupplies the postero-inferior 1/3 of the ventricular septum and one or more posterolateral LV branches (PLV),
which supply the posterolateral portion of LV.
(iv) Co-dominant coronary circulation or balance system occurs in 7% where PDA and PLV may originate
from both RCA and LCx.
(v) In type-I LAD (where it terminates before the apex), the larger and longer PDA from RCA supplies the
ventricular apex also, which is then described as Super dominant RCA.
(vi) Angiographically, RCA is divided into three parts: Proximal, Mid, and Distal (see Figs 4.5 and 4.6).
• Proximal RCA is the portion of the RCA from its origin from the right anterior coronary sinus to the origin of
its RV branch.
• Mid RCA is the portion of RCA between its RV branch and its PDA branch.
• Distal RCA includes PDA and PLV branches.
Fig. 4.5 Coronary angiogram of RCA-LAO view.
Fig. 4.6 Coronary angiogram of RCA-RAO view.
3 Divergent Coronary Anatomy3In 1–2%, there may be divergent coronary anatomic features. These include:
• Anomalous origin of LCx from RCA
• Separate ostia of LAD and LCx or
• Separate ostia of RCA and its conus branch
• Occassionally, the right or left coronary ostium arises 1 cm or more (usually 2.5 cm) above the sinotubular
4junction. This ostial dislocation is termed as a high-takeoff coronary artery.
4 Measurements
i) Length
1(i) The LMCA usually ranges from 1–25 mm in length and it is termed as short LMCA if it is 1 cm
(ii) The LAD measures 10–13 cm in length
(iii) Non-dominant LCx is 6–8 cm in length, while
(iv) RCA is 12–14 cm in length before it gives rise to PDA (see Table 4.2).
Table 4.2 Coronary artery: Length and luminal diameters
Coronary artery Length (mm) Luminal diameter (mm)
1. LMCA 1–25 2–5.5 (4)
2. LAD 100–130 2–5.0 (3.6)
3. LCx 60–80 1.5–5.5 (3)
4. RCA 120–140 1.5–5.5 (3.2)
ii) Luminal Diameter
The luminal diameter of LMCA is 2.0–5.5 mm (mean of 4 mm), LAD: 2.0–5.0 mm (mean of 3.6 mm) and
1LCx: 1.5–5.5 mm (mean of 3 mm). RCA has a luminal diameter of 1.5–5.5 mm (mean of 3.2 mm).
LAD and LCx generally taper in diameter as they extend from the LMCA, while RCA maintains a fairly
constant diameter till it gives rise to PDA.
References
1. Baroldi G. Diseases of the coronary arteries. In: Silver MD, ed. Cardiovascular Pathology: I. New York:
Churchill Livingstone; 1983:317–391.
2. Anderson KR, Ho SY, Anderson RH. Location and vascular supply of sinus node in human heart. Br Heart J.
1979;41:28–32.
3. Click RL., et al. Anomalous coronary arteries, location, degree of atherosclerosis and effect on survival— A
report from Coronary Artery Surgery study. J Am Coll Cardiol. 1989;13:531.
4. Spring DJ, Thomsen JH. Severe atherosclerosis in the “single coronary artery”—Report of a previously
undescribed pattern. Am J Cardiol. 1973;31(5):662–665.Chapter 5
Nerve Supply of the Heart
1. Cardiac Plexus
i) Superficial Cardiac Plexus
ii) Deep Cardiac Plexus
iii) Both Sympathetic and Parasympathetic Fibers
2. Baroreceptors and Chemoreceptors
i) Baroreceptors
ii) Chemoreceptors
References
1,2The nerve supply of the heart is derived from (see Tables 5.1 and 5.2):
1. The cardiac plexus formed by the sympathetic and parasympathetic (vagal) fibers (see Figs 5.1 and 5.2) and
2. Baroreceptors and chemoreceptors.Fig. 5.1 Cardiac plexus (diagrammatic).Fig. 5.2 Nerve supply of the heart.
Table 5.1 Nerve supply of the heart
Cardiac plexus Branches to
1. Superficial cardiac plexus • RCA (through coronary plexus)
(below the aortic arch) • Left anterior pulmonary plexus
• Deep cardiac plexus
2. Deep cardiac plexus (behind • Both atria
the aortic arch) • Both coronary arteries (through coronary plexus)
• Right and left anterior pulmonary plexus superficial cardiac plexuTable 5.2 Peculiarities of nerve supply to the heart
Nerve supply Features
1. Sympathetic innervation More at the base than at the apex of the heart
2. Vagal activity Greater in posterior ventricular myocardium
3. Right sympathetic and vagus nerves Affect SA node > AV node
4. Left sympathetic and vagus nerves Affect AV node > SA node
1 Cardiac Plexus
i) Superficial Cardiac Plexus
It is situated below the arch of aorta and in front of the right pulmonary artery.
It is formed by:
(i) The superior cardiac branch of superior cervical ganglion of the left sympathetic chain and
(ii) The superior cardiac branches (from superior and inferior cervical nerves) of the left vagus nerve.
The superficial cardiac plexus gives branches to
(i) The deep cardiac plexus
(ii) Right coronary artery (coronary plexus) and
(iii) Left anterior pulmonary plexus.
ii) Deep Cardiac Plexus
It is situated in front of the bifurcation of the trachea and behind the arch of aorta.
It is formed by:
(i) The cardiac branches of superior, middle and inferior cervical ganglia (except the cardiac branch arising
from superior cervical ganglion of left sympathetic chain, which forms super3cial cardiac plexus) and upper
4 or 5 thoracic ganglia of right and left sympathetic chain.
The inferior cervical ganglion and first thoracic ganglion are fused together to form a Stellate ganglion.
(ii) Inferior (thoracic) cardiac branches of vagus and/or recurrent laryngeal nerves of both sides except
superior cardiac branches of left vagus (which form superficial cardiac plexus).
The deep cardiac plexus gives branches to:
• superficial cardiac plexus
• both atria
• both coronary arteries (coronary plexus) and
• right and left anterior pulmonary plexus.
iii) Both Sympathetic and Parasympathetic Fibers
These in5uence the SA node, AV node, both the atrial and ventricular myocardium, although vagal 3bers are
3sparse to the ventricles.
(i) Sympathetic stimulation to the heart is largely mediated by the release of norepinephrine and
parasympathetic stimulation by acetylcholine.
(ii) The sympathetic innervation is more at the base than at the apex of the heart, while the vagal activity is
greater in the posterior ventricular myocardium which accounts for the vagomimetic e6ect of the inferior
myocardial infarction.
(iii) The right sympathetic and vagus a6ect the SA node more than the AV node while the left sympathetic
and vagus affect the AV node more than the SA node.
(iv) Hence, stimulation of right stellate ganglion causes sinus tachycardia with less a6ect on AV nodal
conduction. Whereas stimulation of left stellate ganglion shortens the AV nodal conduction time and
refractory period and produces the shift in sinus pacemaker to an ectopic site.
(v) Also, stimulation of right (cervical) vagus slows sinus node discharge rate (producing bradycardia) whilethe stimulation of left vagus prolongs the AV nodal conduction time and refractoriness.
In general, right sympathetic chain shortens the refractoriness primarily of the anterior portion of the ventricles
while the left sympathetic chain primarily affects the posterior surface of the ventricles.
2 Baroreceptors and Chemoreceptors
4The cardiovascular regulatory mechanisms includes (see Table 5.3 and Fig. 5.4):
( a ) Chemical Regulatory Mechanisms through circulating vasodilators (bradykinin) and circulatory
vasoconstrictors (epinephrine, nor-epinephrine, angiotensin II and vasopressin) and
(b) Neural Regulatory Mechanisms which consist of:
• Sympathetic and parasympathetic systems through superficial and deep cardiac plexus and
• Medullary vasomotor and cardiac vagal centers.
The ventrolateral region of medulla (Pressor area) exerts excitatory e6ects (increase) on sympathetic
activity while the medial and caudal parts of fourth ventricle in medulla (Depressor area) cause decrease
of sympathetic activity.
Cardiac vagal center consists of inhibitory vagal 3bers originating from the neurons of vagal nuclei
located in the medulla (dorso-motor nucleus, nucleus of tractus solitarius and nucleus ambiguous) to
converge on:
• The sympathetic pre-ganglionic neurons of spinal cord to decrease the sympathetic activity and
• The heart to decrease the heart rate and force of the cardiac contraction. The vasomotor and cardiac
vagal centers are in5uenced by the a6erent 3bers from the baroreceptor and chemoreceptors (see Fig.
5.3).
Table 5.3 Baroreceptors and chemoreceptors
Receptors Location
Baroreceptors
1. Arterial baroreceptors (pressure Carotid sinus
receptors) Aortic arch
Root of subclavian artery
Pulmonary trunk
2. Cardiac baroreceptors Atriocaval receptors (RA)
i. Volume receptors Pulmonary venoatrial receptors (LA)
ii. Pressure receptors Atrial: RA, LA, interatrial septum
Ventricular: LV, interventricular septum (Bezold-Jarisch reflex)
Chemoreceptors
1. Carotid bodies Common carotid artery bifurcation
2. Aortic bodies Around aortic archFig. 5.3 Vasomotor center (diagrammatic).Fig. 5.4 Baroreceptors ( ) and chemoreceptors (xx).
i) Baroreceptors
Since they are sensitive to stretch, they are also called as mechanoreceptors. All are innervated by vagus nerve
except the carotid sinus baroreceptors which are supplied by carotid sinus nerve, a branch of glossopharyngeal
(IX cranial) nerve. Broadly, there are two types of baroreceptors: arterial and cardiac.
(a) Arterial Baroreceptors
• These are located in the walls of the blood vessels mainly in the adventitial layer.
• The a6erent signals are mainly carried through vagus to the vasomotor center and the cardio-vagal center in
medulla.
• The arterial baroreceptors are situated at:
1. Carotid sinus (dilated initial part of internal carotid)
2. Aortic arch
3. Root of subclavian artery
4. Junction of thyroid artery with common carotid artery and
5. Pulmonary trunk near its division.
• Increased aortic pressure causes re5ex inhibition of vasomotor center (VMC) and stimulation of cardiac vagal
center (CVC), thereby decreasing the heart rate and systemic vascular resistance, while lowering the aortic
pressure results in stimulation of VMC and inhibition of CVC, thereby increasing the heart rate and systemic
vascular resistance.
• The range of operation of these baroreceptors is between 60–200 mmHg of mean blood pressure.
(b) Cardiac Baroreceptors
These are located in the walls of the heart i.e. subendocardial in distribution.
The cardiac baroreceptors are:
1. Atrio-caval receptors located at the junction of right atrium with inferior vena cava and superior vena cava.
2. Pulmonary veno-atrial receptors located at the junction of pulmonary vein with left atrium. Both [1] and [2]
are volume receptors i.e. increase blood volume causes distension of the atrial walls producing re5ex
tachycardia (Bainbridge reflex) and moderate diuresis due to the release of atrial natriuretic peptide (ANP).
3. Atrial receptors scattered throughout the atria and interatrial septum. Increase in atrial pressure increases
their impulse activity (however their discharge is sparse and irregular) resulting in re5ex vasodilatation
especially in the renal vessels.
4. Ventricular receptors are scattered throughout the left ventricle and ventricular septum. They are stimulated
(their discharges are also sparse and irregular) by the injection of veratridine, serotonin or nicotine into
coronary artery (especially left) or pulmonary artery or by partial occlusion of aorta or coronary sinus,
resulting in profound bradycardia and hypotension due to re5ex sympathetic inhibition (Bezold-Jarisch
reflex).
ii) Chemoreceptors
Chemoreceptors are sensitive to the changes in the blood chemistry. Their main function is to keep the alveolar
pCO at a normal level of 40 mmHg and also to maintain arterial pO , pCO and pH.2 2 2
The important chemoreceptors are: Carotid bodies and Aortic bodies
(i) Carotid bodies: These are located near common carotid artery bifurcation and are innervated by carotid
sinus nerve, a branch of glossopharyngeal nerve.
(ii) Aortic bodies: These are scattered around aortic arch and are innervated by aortic nerve, a branch of
vagus nerve.
A6erent 3bers from these chemoreceptors ascend to relay in the nucleus of tractus solitarius of medulla.
Hypoxia, hypercapnia and acidemia stimulate these receptors which activates the vasomotor center and
respiratory neurons in the medulla producing pressure e ects with an increase in the rate and depth of
respiration.
References
1. Mitchell GAG. Cardiovascular Innervation. Baltimore: Williams & Wilkins; 1956.
2. Janes RD, Brandys JC, Hopkins DA, Johnstone DE, Murphy DA, Armour JA. Anatomy of human extrinsic
cardiac nerves and ganglia. Am J Cardiol. 1986;157(4):299–309.
3. Randall WC. Nervous Control Cardiovascular Function. New York: Oxford University Press; 1982.
4. Jain AK. Cardiovascular regulatory mechanisms. Text book of Physiology: I. New Delhi: Avichal Publishing Co;
2001. 324–328Chapter 6
The Conduction System of the Heart
1. Sinoatrial (SA) Node (Pacemaker Node of Keith–Flack Node, 1907)
P Cells (Pale/Nodal Cells)
Transitional Cells (T cells)
Internal Atrial Myocardium
2. Atrioventricular Junctional Area
Transitional Cell Zone
Atrioventricular (AV) Node (Tarawa, 1906)
The Bundle of His (AV Bundle, Common Bundle)
3. The Bundle Branches and Terminal Purkinje Fibers
The Left Bundle Branch (LBB)
The Right Bundle Branch (RBB)
The Terminal Purkinje Fibers
References
The conduction system of the heart consists of three major parts (see Fig. 6.1 and Table 6.1):
1. Sinoatrial (SA) node
2. AV junctional area and
3. The bundle branches and terminal Purkinje fibers.
Fig. 6.1 Conduction system of the heart.
Table 6.1 Conduction system of the heart
Structures Location
1. SA node Lateral junction of SVC and RA
2. AV node At the apex of Koch’s triangle3. The bundle of His Chord-like distal continuation of AV node
4. Left bundle branch (LBB) Forms a cascade down the left ventricular septal surface
5. Right bundle branch (RBB) Beneath the non-coronary aortic cusp Direct continuation of His bundle along
the right side of the ventricular septum
6. The terminal Purkinje fibers On the endocardial surface of both ventricles
1 Sinoatrial (SA) Node (Pacemaker Node of Keith–Flack, 1907)
SA node is spindle shaped, 10–12 mm long and usually 1 mm thick situated in the sub-epicardium (less than 1 mm from
the epicardial surface) at the lateral junction of the superior vena cava and right atrium.
Blood supply to the SA node is by the sinus nodal artery, which arises from proximal RCA in 55–60%, LCx in 40–45%
1and from both in 11%.
Histologically, the sinus node has:
• P cells
• Transitional cells and
• Atrial muscle cells.
P Cells (Pale/Nodal Cells)
• P cells are ovoid, small (5–10 μm in greatest diameter) and resemble the primitive myocardial cells.
• The nuclei are of normal size, but double or multiple nuclei in a single cell are not observed.
• There are fewer mitochondria in a P cell compared to the normal contractile cells.
• Myofibrils are few in number and rarely attached to the sarcolemma.
• The P cells are the source of normal impulse formation in the SA node. However, intercellular contact is direct plasma
membrane to plasma membrane, a factor responsible for slow conduction within the sinus node.
Transitional Cells (T cells)
• T cells are intermediate between P cells and atrial myocardial cells (elongated than P cells but shorter and narrower
2than atrial myocardial cells).
• They are located at the margins of the sinus node, where nodal cells become contiguous with atrial myocardium.
• T cells provide a ‘functional pathway’ for the distribution of sinus impulses formed in the P cells to the rest of the atrial
myocardium.
• The Abrous tissue and fat increase with advancing age and hence the Abrous tissue is predominant in adult sinus node
3as compared to that of an infant.
Internal Atrial Myocardium
4Certain population of atrial myocardial cells have diBerent electrophysiological properties, and James and Sherf
supported the concept of three specific internodal tracts between SA and AV nodes:
• Anterior internodal tract
• Middle internodal tract and
• Posterior internodal tract.
i) The Anterior Internodal Tract (Bachmann–James)
It leaves the antero-superior part of the SA node and curves around in front of the superior vena cava, divides into two
bundles of Abers one entering the left atrium while the other coursing over the anterior portion of the interatrial septum
and descending obliquely behind the root of the aorta to enter the anterosuperior margin of the AV node.
ii) The Middle Internodal Tract (Wenckebach)
It leaves the postero-inferior margin of the SA node, curves behind the SVC and course along the posterior margin of the
interatrial septum to enter the superior margin of the AV node.
iii) The Posterior Internodal Tract (Thoral)
It leaves the postero-inferior margin of the SA node and follows the course of crista terminalis and eustachian ridge to
enter the posterior margin of the AV node.
5The middle and posterior internodal tracts may also extend the Abers from the right atrium to the left atrium. All the
three tracts anastomose with each other above the AV node and have transitional cells and common atrial myocardial6cells. However, presently the evidence does not support the presence of specialized internodal tracts and preferential
internodal conduction in some parts of the atrium may be due to atrial myocardial Aber orientation, size and geometry
2,7,8rather than specialized tracts between the nodes.
2 Atrioventricular Junctional Area
It consists of three distinct areas:
• Transitional cell zone
• AV node and
• His bundle.
Transitional Cell Zone
9,10It is the ‘outer layer’ of the AV node which connects the right atrial myocardium with the AV node. This area is
sub11divided into three main groups: Anterior or superior, Middle and Posterior or inferior.
Atrioventricular (AV) Node (Tarawa, 1906)
• AV node is a small ovoid structure (smaller than SA node) measuring 1 × 3 × 5 mm, lies just beneath the right atrial
posterior epicardium, anterior to the ostium of coronary sinus and directly above the insertion of the septal leaHet of
tricuspid valve.
• It is located at the apex of the triangle of Koch formed above by the tendon of Todaro, in front by the base of the
12septal leaflet of tricuspid valve and base by the ostium of the coronary sinus.
• The tendon of Todaro originates from the central fibrous body and passes posteriorly through the atrial septum.
• The compact AV node becomes the penetrating bundle of His at the apex of the triangle of Koch and passes through the
membranous ventricular septum below the point of attachment of the tendon of Todaro to the central fibrous body.
The blood supply is through the AV nodal artery which is a branch of RCA in 85–90% and LCx in 10–15%.
Histologically, it has four types of cells: P cells, transistional cells, atrial myocardial cells and Purkinje cells.
Electrophysiologically, AV node is divided into three regions:
• AN region
• N region and
• NH region.
AN region corresponds to the transitional cell groups in the upper posterior portion of the AV node.
N region is a small enclosed region where transistional cells merge with mid nodal cells.
NH region is the anterior portion of the bundle of lower nodal cells.
The dead end pathways consist of groups of cells that form an apparent electrophysiological cul de sac that does not
contribute to overall conduction in the AV node.
The Bundle of His (AV Bundle, Common Bundle)
It is a chord like distal continuation of the AV node measuring 20 mm in length and up to 2 mm in diameter. It is
anatomically subdivided into three portions:
• Proximal non-penetrating
• Middle penetrating and
• Distal branching portion.
Proximal non-penetrating portion is distal to AV node.
Middle penetrating portion is the tunneled segment within the Abrous tissue of the central body and the membranous
septum.
Distal branching portion
i) His bundle bifurcates at the crest of the muscular septum into right and left bundles, immediately distal to the
membranous ventricular septum.
ii) The blood supply is from both LAD and PDA and hence this portion of the conduction system is less subject to the
13ischemic damage.
iii) Histologically, His bundle primarily consists of Purkinje cells.
3 The Bundle Branches and Terminal Purkinje FibersThe Left Bundle Branch (LBB)
It forms a cascade down the left ventricular septal surface beneath the noncoronary aortic cusp. The left bundle radiates
14in a fanlike fashion with two major divisions: thin anterosuperior and thick posteroinferior fascicles. However,
15 16,17Tarawa and recent studies indicated a trifascicular division of the left bundle branch.
The Right Bundle Branch (RBB)
It is the direct continuation of the His bundle positioned along the right side of the ventricular septum. The right bundle
branch becomes a subendocardial structure in the middle lower thirds of the ventricular septum and remains unbranched
to the apex of the right ventricle (see Fig. 6.2).
Fig. 6.2 Bundle branches and Purkinje fibers.
Blood Supply
The left bundle branch receives blood supply from both LAD and RCA (see Table 6.2).
Table 6.2 Blood supply of the bundle branches
Structures Arteries
1. LBB Both LAD and RCA
50% by septal branch of LAD and AVnodal branch of RCAi. Anterosuperior fascicleii
50% only by septal branch of LADii. Posteroinferior fascicle
50% only by the AV nodal branch of RCA2. RBB50% by both AV nodal branch of RCA andseptal branch of LAD
Similar to that of anterosuperior fascicle of LBB
The anterosuperior fascicle is supplied by the septal branches of LAD and AV nodal branch of RCA in 50%; and only
by the septal branches of LAD in the other 50%.
The posteroinferior fascicle is supplied by the AV nodal branch of RCA in 50% and by both the AV nodal branch of
RCA and the septal branch of LAD in the other 50%.
The blood supply of right bundle branch is similar to that of the anterosuperior fascicle of the left bundle branch.
Histologically, the bundle branches (LBB and RBB) mainly consist of Purkinje cells intermingled with ordinary
contractile myocardial cells.
The Terminal Purkinje Fibers
These Abers connect with the ends of the bundle branches to form interweaving networks on the endocardial surface of
both ventricles. However, Purkinje Abers tend to be concentrated at tips of the papillary muscles rather than at the base
18of the ventricles. They are more resistance to ischemia than the common myocardial fibres.
References
1. Vieweg MVR, Alpert JS, Hagan DS. Origin of the sinoatrial node and atrioventricular node arteries in right, mixed and
left inferior emphasis systems. Cathet Cardiovasc Diag. 1975;1(4):361–373.
2. Davies MJ, Anderson RH, Becker AE. The Conduction system of the Heart. Butterworth. 1983:1–200.
3. Lev M. Aging changes in the human sinoatrial node. J Gerontol. 1952;9(1):1–9.
4. James TN, Sherf L. Specialized tissues and preferential conduction in the atria of the heart. Am J Cardiol.
1971;28(4):414–427.
5. James TN. Anatomy of the conduction system of the heart. In: Hurst JW, Logue RB, Rackley CE, Sonnennblick EH,
Wallace AG, eds., et al. The Heart. 5th New York: McGraw-Hill; 1982:26–56.
6. Becker AE, Bouman LN, Janse MK, Anderson RH. Functional anatomy of the cardiac conduction system. In: Harrison
DC, ed. Cardiac Arrhythmias. Boston, Hall: A Decade of Progress; 1981:1–18.
7. Anderson KR, Ho SY, Anderson RH. The location and vascular supply of the sinus node in the human heart. Br Heart J.
1979;41(1):28–32.
8. Zipes DP. Genesis of cardiac arrhythmias: Electrophysiological consideration. In: Braunwald E, ed. Heart Disease: A
ndText book of Cardiovascular Medicine. 2 Saunders: Philadelphia; 1982:581–620.
9. Tarawa S. Das Reizeitungs system des Saugetierkerzens. Gustav Fischer: Jena; 1906.
10. Anderson AH, Becker AE, Brechenmacher C, Davies MJ, Rossi L. The human atrioventricular junctional area: A
morphological study of the AV node and bundle. Eur J Cardiol. 1975;3(1):11–25.
11. James TN. The connecting pathways between the sinus node and AV node and human heart. Am Heart J. 1963;66:498–
508.
12. Franco PM. Recherches sur les faiseaux de connxion auriculaires dans condition normales et pathologies. Arch Mal Coer.
1951;22:287–292.
13. Trautwein W, Uchizono K. Electron microscopic and electrophysiologic study of the rabbit heart. Z Zellforsch Mikrosk
Anat. 1963;61:96–109.
14. Rosenbaum MB, Elizari JO. The Hemiblocks Tampa FL. Tampa Tracings; 1970.
nd15. Rossi L. Histopathology of Cardiac Arrhythmias. 2 Milan: Casa Editrice Ambrosiana; 1970:1–75.
16. Massing GK, James TN. Anatomical configuration of the His bundle and bundle branches n the human heart.
Circulation. 1976;53:609–621.
17. Demoulin JC, Kulbertus HE. Histopathological examination of concept of left hemiblocks. Br Heart J. 1972;34:807–814.
18. Becker AE, Bowman LN, Janse MJ, Anderson RH. Functional anatomy of the cardiac conduction system. In: Harrison
DC, ed. Cardiac Arrhythmias. Boston: Hall: A Decade of Progress; 1981:–324.Chapter 7
Ultrastructure of the Myocardium
1. P Cells
2. Transitional Cells
3. Purkinje Cells
4. Amoeboid Cells
5. Contractile Or Working Myocardial Cells
a) Sarcolemma
b) Intercalated Discs
c) Sarcotubular System
d) Diadic Cleft
e) Contractile Proteins
References
The ultrastructure of the myocardium consists of (see Table 7.1 and Fig. 7.1):
Table 7.1 Ultra-structure of the myocardium
Cells Location
1. P cells SA node
2. Transistional cells Margins of SA node
3. Purkinje cells Margins of SA node, inter nodal tracts, adjacent to AV node, in the His bundle, LBB and
RBB
4. Amoeboid cells Eustachian ridge
5. Contractile cells Atrial and ventricular myocardiumFig. 7.1 Ultra-structure of the myocardial fibers.
1 P Cells
See chapter 6.
2 Transitional Cells
See chapter 6.
3 Purkinje Cells
These cells are broader and shorter than contractile myocardial cells measuring 20–50 m in length and 10–30 m in cross
section. They are found in the margins of SA node, internodal tracts, adjacent to the AV node, in the His bundle and in its
bundle branches.
4 Amoeboid Cells
The conduction system of the heart also consists of elongated, triangular, oval or non-geometric shaped amoeboid cells in the
Eustachian ridge, which have multilobular nuclei, many mitochondria, myofibrils and granules (which give dark appearance to
these cells) with pseudopodic prolongations that fill the spaces between the neighboring cells.
They may act as an auxiliary pacemaker or may be a source of atrial natriuretic peptide (ANP).
5 Contractile or Working Myocardial Cells
• They are 10–20 μm in diameter and 50–100 μm in length surrounded by a cell membrane, known as sarcolemma.
• The sarcoplasm (cytoplasm) has many mitochondria and centrally located elongated nucleus and many (50–60) myo; brils
which are inserted in the region of intercalated disc (see Fig. 7.2 and Table 7.2).
• Each myo; bril is striated, 1–2 m in diameter and lie parallel to one another and composed of a series of sarcomeres, the
basic contractile units.
• The cell membranes of some adjacent cells form close margins called intercalated discs or junctions.
• The contractile myocardial cells are similar whether they are from the atrial or ventricular myocardium. However, the
1,2contractile myocardial cells which contain an intricate sarcotubular system of tubules, vesicles and cisternae are absent or
rare in atrial myocardium.
• The myo; brils are made up of the contractile proteins (which give striated appearance) while each intercalated disc consists
of three types of specialized junctions.Fig. 7. 2 Branching and anastomosing myocardial ; bers with faint striations. The ; bers are made up of cells with centrally
placed nucleus. Cells are separated from one another by intercalated discs (arrows).
Table 7.2 Contractile myocardial cell
Features Description
1. Size 10–20 μm in diameter 50–100 μm in length
2. Sarcolemma Bi-layered phospholipid with channels
3. Intercalated discs Macula adherens, fascia adherens, gap junction
4. Sarcotubular system T system, longitudinal sarcoplasmic reticulum
5. Diadic cleft 10000 diadic clefts/cell; 11 L-type Ca2+ channels and 100 feet/cleft
6. Contractile proteins Myosin, actin, titin, tropomyosin, troponin T, C and I
a) Sarcolemma (Sarco = Flesh; Lemma = Thin Husk)
The cell membrane consists of a bi-layer boundary of phospholipid molecules (see Fig. 7.3).
Fig. 7.3 Bi-layered cell membrane showing opening (channel).The tail end of the phospholipid molecule is non-polar and hydrophobic pointing toward the center of the cell membrane.
The head end is polar and hydrophilic pointing toward the outer and inner layers of the cell membrane (see Fig. 7.4).
Fig. 7.4 Tail and head of a phospholipid molecule of the bi-layered cell membrane.
At rest, the resistance to ions (both cations and anions) is greater across the cell membrane than in the cytoplasm especially
in the non polar hydrophobic layer. The cell membrane has openings called c h a n n e l s that span the cell membrane and serve as
conduits through which ions move. These channels are broadly made up of two types of protein complexes:
i) Type I Intrinsic Membrane Protein (Voltage Operated Channels)
These protein molecules protrude through the entire cell membrane and have major part outside the cytoplasm i.e. they are
anchored to the inner layer of the cell membrane e.g. Na, K, Ca channels, Na-K pump.
ii) Type II Intrinsic Membrane Protein (Ligand Receptors or Receptor Operated Channels)
These protein molecules have major part in the cytoplasm with a very small fraction penetrate:
• Only the outer layer of the cell membrane and serve as receptor sites for neurotrans-mitters and hormones.
• The inner layer of the cell membrane and serve as receptors for adenylate cyclase.
b) Intercalated Discs
Three types of specialized junctions make up each intercalated disc:
• Macula adherens or desmosome (see Fig. 7.5)
• Fascia adherens and
• Nexus or gap junctions (see Fig. 7.6).Fig. 7.5 Intercalated discs—Desmosome.Fig. 7.6 Intercalated disc—Gap junction.
i) Desmosome and Fascia Adherens
They form the areas of strong adhesions between the cells and may provide a linkage for the transfer of mechanical energy
from one cell to next cell.
ii) The Gap Junctions
The cells are separated only by about 10–20 A and are connected by a series of hexagonally packed subunit bridges but are in
functional contact with each other. They provide:
(i) A low resistance electrical coupling between adjacent cells that permit the movement of ions and other small molecules and
(ii) A biochemical coupling that permit cell to cell movement of ATP and other high energy phosphates.
• The gap junctions permit the conduction velocity faster in the direction of the long axis of the ; ber than transversely.
However, the conduction delay or block occurs more commonly in the longitudinal direction.
• The gap junctions permit a multicellular structure like heart to function electrically like an orderly, synchronized and
interconnected unit and are also responsible partly for the anisotropic type of conduction in the myocardium.
• Acidosis increases and alkalosis decreases gap junctional resistance. An increased gap junctional resistance slows the rate of
3action potential propagation which could lead to conduction delay or block.
• C o n n e x i n s are the proteins that form the intercellular channels of gap junctions.
c) Sarcotubular System
It is a highly specialized system of internal conduction of depolarization within the muscle ; ber. It is made up of T-system and
longitudinal sarcoplasmic reticulum (see Fig. 7.7).Fig. 7.7 Sarcotubular system; ECF: extracellular fluid.
i) T-system or Transverse Tubular System
• The cell membrane invaginates to form the transverse tubular system.
• The transverse tubules are arranged perpendicular to the long axis of the cell but branch longitudinally and can directly
4connect with other transverse tubules.
5• At the area of Z band/line the T tubules give off a cistern-like structure, the intermediary vesicle.
ii) Longitudinal Sarcoplasmic Reticulum
• Longitudinal sarcoplasmic reticulum is a plexiform labyrinth of vesicles and tubules oriented parallel to the myofibrils.
2+• At Z band, these have local dilatations, lateral sacs or terminal cisternae which are rich in glycogen and Ca .
• So, a triad of an intermediary vesicle and two lateral sacs is formed at Z line which is known as sub-sarcolemma cisternae
(cisternae = baskets, Latin) and their function is to release calcium from the calcium release channels (ryanodine receptors)
in the longitudinal sarcoplasmic reticulum to initiate the myocardial contraction.
• The impulse rapidly spreads down the transverse tubules and intermediary vesicles to stimulate the lateral sacs (Junctional
6sarcoplasmic reticulum [JSR]) to release calcium into the diadic cleft for the initiation of myofibrillar contraction.
• There is one triad per sarcomere (two in skeletal muscle). Hence, the sarcotubular system plays an important role both in the
7 5electrical impulse conduction and electromechanical coupling.
d) Diadic Cleft
• Diadic cleft is a cleft-like space between the lateral sac of sarcoplasmic reticulum i.e. JSR and T tubular sarcolemmal
6membrane (It is triadic in skeletal muscle) (see Fig. 7.8).
• The narrow space or cleft between the sarcolemma and JSR is bridged by structures called the ‘feet’ which serve as the sites
for calcium release to the diadic cleft space.
• The calcium pump in the longitudinal SR plays an important role in the delivery of calcium to the JSR at the diadic cleft.
• There are 11 L-type calcium channels and 100 feet within the cleft, one cleft per 2 half sarcomere and about 10000 diadic
clefts in each cell.
• There is a preferential localization of Na/Ca exchangers in the sarcolemma at the cleft through which mainly calcium Luxes
out of the cell, while the calcium from the extracellular space enters the diadic cleft via i) L-type Ca channels in the
sarcolemma and ii) the release of calcium from the JSR via the feet into the cleft (i.e. calcium induced calcium release
[CICR]) by the stimulation from the intermediary vesicles of T system.Fig. 7.8 Diadic cleft; JSR: junctional sarcoplasmic reticulum, T tubule: transverse tubule.
e) Contractile Proteins
The sarcomere, the basic contractile unit between two Z lines is made up of major and minor proteins organized into thick and
thin filaments (A and I bands respectively) which give a pattern of dark and light bands under light microscope (see Fig. 7.9).
• The A band (highly refractile material i.e. Anisotropic) is the wide dark area between two peripherally located light band, the
I band (lower refractile material i.e. isotropic).
• The A band is about 1.5 m in length and consists of both thick myosin and thin actin ; laments arranged in a hexagonal
pattern with six actin filaments surrounding each myosin filament.
• The I band is about 1.0 m in length and consists of only thin actin ; laments attached to the Z line (Zwischenscheibe i.e.
between disc).
• The H zone (after the discoverer Hansen) is a lighter band in the center of the A band and consists of only myosin filaments.
• In the center of the H zone is a thin dark line, the M line to which the thick myosin ; laments are attached. The M line is
particularly pronounced during muscle contraction.Fig. 7.9 Bands in myofibril.
The major contractile proteins are:
• Myosin thick filament
• Titin
• Actin thin filament
• Tropomyosin and
• Troponin T, C and I.
While the minor contractile proteins are:
• Alpha actinin• c-protein and
• Nebulin.
i) Myosin
It consists of a short bilobular head and a long tail or shaft (see Fig. 7.10).
(i) The short compact bilobular head is 30 nm in length and 4 nm in diameter. It has two types of chains (myosin heavy chain
and myosin light chain) and two important sites.
There are three chains around the base of each myosin head:
• One myosin heavy chain (MHC) (i.e. 2 MHC/bilobed head), the motor of contraction and
• Two myosin light chains (MLC) (i.e. 4 MLC/bilobed head) which perhaps inhibit the contractile process by interaction with
8actin.
The two important sites are:
• Actin binding site, where myosin comes in contact with actin and
• ATPase site that hydrolysis (breaks) ATP
(ii) The long tail or shaft is 100 nm in length and 2 nm in diameter. It carries the load during contraction.
Fig. 7.10 Myosin bilobed head and tail; Actin: actin binding site, ATPa: ATPase site.
ii) Titin (Connectin)
9It is the largest, extraordinarily long, flexible and slender myofibrillar protein.
• It is 0.6–1.2 μm in length extending from Z line to just short of the M line.
• It has two distinct segments:
(i) Inextensible segment and
(ii) Extensible segment.
10• Titin protein has two main functions:
(i) Inextensible portion tethers the myosin molecule to the Z line.
(ii) Extensible portion stretches as the sarcomere lengthens.
iii) Actin
• The thin actin ; laments are composed of two actin units, which interwine in a helical pattern, both being carried on a heavier
backbone, the tropomyosin molecule. (see Fig. 7.11)
• The thin actin filament is anchored to the Z line by alpha actinin and stretches from the Z line to the edge of the H zone.
• It is 4–5 nm in diameter.Fig. 7.11 Actin and Tropomyosin–troponin complex.
iv) Tropomysin-Troponin Complex
• Tropomyosin is a continuous coil of long ; laments located in the groove between the two chains (double helix) of actin (see
Fig. 7.12).
• It covers the binding site of actin where the myosin head comes in contact with the actin i.e. it prevents the interaction
between actin and myosin filaments.
• Located at regular intervals of 38.5 nm along the tropomyosin molecules are three small globular regulatory proteins, the
troponins.
• The three regulatory troponins are:
(i) Troponin T: binds other troponins to tropomyosin.
(ii) Troponin I: inhibits the interaction of myosin with actin.
2+(iii) Troponin C: contains binding sites for Ca that initiates the muscle contraction.
Fig. 7.12 Actin, tropomyosin and troponin.
v) Myosin isoforms
There are three myosin isoforms depending upon the electrophoretic mobility: V , V and V .1 2 3
• Each myosin isoform consists of two distinct types of MHC genes.
(i) V1 consists of αα MHCs
(ii) V3 consists of ββ MHCs and
(iii) V2 consists of αβ MHCs.
• Each myosin isoform is produced by different gene located on different chromosome (V on chromosome 3, V on chromsome1 3
14).