Medical School · Year 1 · Anatomy Thorax Abdomen · includes a quiz and discussion video

Lecture 4: Heart - Internal Features and Conduction System

Unit 1.4: Human Gross Anatomy II - Thorax and Abdomen

Learning Objectives

By the end of this lecture, students will be able to:

1. Describe the internal features of each cardiac chamber
2. Identify the cardiac valves and their components
3. Explain the structure and function of the cardiac conduction system
4. Describe the cardiac skeleton and its functions
5. Correlate anatomical structures with cardiac function
6. Apply knowledge to understanding common valvular and conduction disorders

Lecture Content

I. Overview of Heart Wall

The heart wall consists of three layers that work together to produce coordinated cardiac contraction.

The epicardium forms the outermost layer, representing the visceral layer of the serous pericardium. This thin serous membrane covers the external surface of the heart and contains the coronary vessels and variable amounts of fat, particularly along the coronary sulci.

The myocardium constitutes the main bulk of the heart wall, consisting of cardiac muscle fibers arranged in complex spiral patterns. These fibers are interconnected at intercalated discs that allow rapid electrical conduction and coordinated contraction. The myocardium generates the force necessary to pump blood through the circulation.

The endocardium lines the internal surfaces of all cardiac chambers. This thin endothelial layer is continuous with the endothelium of the blood vessels entering and leaving the heart. The endocardium covers all internal structures including the valve leaflets, chordae tendineae, and papillary muscles.

The thickness of the myocardium varies according to the workload of each chamber. The left ventricle has the thickest walls, measuring ten to fifteen millimeters, reflecting the high pressures required to pump blood into the systemic circulation. The right ventricle has thinner walls of three to five millimeters because the pulmonary circulation operates at lower pressures. The atria have the thinnest walls at two to three millimeters since they serve primarily as receiving chambers with minimal pumping force required.

<image>Panel A: Epicardium as the thin outer layer with coronary vessels and adipose tissue visible along the sulci. Panel B: Myocardium as the thick middle layer with spiral muscle fiber arrangement shown in magnified inset. Panel C: Endocardium as the thin inner lining continuous with the blood vessel endothelium. Panel D: Comparison of relative wall thicknesses showing left ventricle (10-15mm), right ventricle (3-5mm), and atria (2-3mm).</image>

II. Right Atrium

The right atrium receives deoxygenated blood from the systemic circulation and delivers it to the right ventricle. It forms the right border of the heart and lies to the right of and slightly posterior to the right ventricle.

The right atrium has two distinct regions with different embryological origins. The sinus venarum forms the smooth-walled posterior part of the chamber, derived from the embryonic sinus venosus. This region receives the openings of the superior vena cava superiorly, the inferior vena cava inferiorly, and the coronary sinus between them. The atrium proper forms the anterior rough-walled part, derived from the embryonic primitive atrium. This region contains the pectinate muscles, parallel muscular ridges that run from the crista terminalis toward the atrioventricular orifice. The right auricle represents an anterior extension of this rough-walled region.

The crista terminalis is a smooth muscular ridge running vertically along the internal surface of the right atrium, separating the smooth sinus venarum from the rough pectinate muscle region. Externally, this ridge corresponds to the sulcus terminalis. The crista terminalis contains the origin of the sinoatrial node at its superior aspect where it meets the superior vena cava.

The interatrial septum forms the medial wall of the right atrium. The fossa ovalis, a shallow depression in the lower part of the septum, represents the closed foramen ovale through which blood shunted from right to left during fetal circulation. The limbus fossa ovalis, a raised ridge surrounding the fossa ovalis, represents the edge of the embryonic septum secundum.

Several valvular remnants exist within the right atrium. The valve of the inferior vena cava, also called the Eustachian valve, represents a remnant of the embryonic valve that directed oxygenated blood from the inferior vena cava through the foramen ovale during fetal life. The valve of the coronary sinus, also called the Thebesian valve, partially guards the opening of the coronary sinus.

<image>Panel A: Sinus venarum as the smooth posterior wall with SVC, IVC, and coronary sinus openings. Panel B: Pectinate muscles as parallel ridges in the anterior portion and right auricle with crista terminalis separating regions. Panel C: Interatrial septum with fossa ovalis depression and limbus as the surrounding ridge. Panel D: Eustachian valve at the IVC opening, Thebesian valve at the coronary sinus, and tricuspid valve orifice inferiorly.</image>

III. Right Ventricle

The right ventricle receives blood from the right atrium and pumps it to the pulmonary circulation for oxygenation. It forms most of the anterior surface of the heart and contributes to the inferior border.

The right ventricle has two functional regions. The inflow tract receives blood from the tricuspid valve and has rough walls covered with trabeculae carneae, muscular ridges and columns that prevent suction of the walls during contraction. The outflow tract, also called the conus arteriosus or infundibulum, leads to the pulmonary valve and has smooth walls. The supraventricular crest, a muscular ridge, separates these two regions.

The trabeculae carneae exhibit three morphological forms. Ridges are attached along their entire length to the ventricular wall. Bridges are attached at both ends but free in the middle, spanning across the ventricular cavity. Papillary muscles project into the cavity with only their bases attached to the wall, their apices giving rise to the chordae tendineae.

The septomarginal trabecula, also called the moderator band, is a specialized muscular band that crosses from the interventricular septum to the base of the anterior papillary muscle. This structure carries the right bundle branch of the cardiac conduction system, ensuring coordinated contraction of the papillary muscle with the rest of the ventricle.

The right ventricle contains three papillary muscles corresponding to the three cusps of the tricuspid valve. The anterior papillary muscle is the largest and arises from the anterior wall. The posterior papillary muscle arises from the inferior wall. The septal papillary muscle, often small or replaced by chordae arising directly from the septum, arises from the interventricular septum. Each papillary muscle sends chordae tendineae to two adjacent cusps of the tricuspid valve.

<image>Panel A: Inflow tract with rough walls containing trabeculae carneae in three forms (ridges, bridges, and papillary muscles) receiving blood from the tricuspid valve. Panel B: Outflow tract (conus arteriosus) with smooth walls leading to the pulmonary valve separated by the supraventricular crest. Panel C: Septomarginal trabecula (moderator band) crossing from the interventricular septum to the anterior papillary muscle carrying the right bundle branch. Panel D: Three papillary muscles (anterior, posterior, septal) with chordae tendineae attaching to the tricuspid valve cusps.</image>

IV. Left Atrium

The left atrium receives oxygenated blood from the pulmonary circulation and delivers it to the left ventricle. It forms most of the posterior surface or base of the heart.

The left atrium is predominantly smooth-walled, a characteristic reflecting its embryological derivation primarily from the absorbed pulmonary veins rather than the primitive atrium. The only rough-walled portion is the left auricle, which contains pectinate muscles similar to the right auricle.

Four pulmonary veins enter the left atrium, two from each lung. The right pulmonary veins enter the right side of the posterior wall, while the left pulmonary veins enter the left side. None of these openings possess valves, which has implications for the regurgitation of blood into the pulmonary veins during atrial fibrillation.

The interatrial septum viewed from the left atrium shows the valve of the foramen ovale, a flap-like structure representing the embryonic septum primum. This valve normally fuses with the septum secundum during the first year of life. In approximately twenty-five percent of adults, a probe-patent foramen ovale persists, where the valve fails to fuse completely but remains functionally closed by the higher left atrial pressure pressing the valve against the septum.

<image>Panel A: Smooth-walled main chamber of the left atrium derived from absorbed pulmonary veins. Panel B: Four pulmonary vein openings on the posterior wall with two right and two left entries. Panel C: Rough-walled left auricle projecting anteriorly with pectinate muscles similar to the right auricle. Panel D: Interatrial septum with valve of foramen ovale and mitral valve orifice leading to the left ventricle with probe-patent foramen ovale inset.</image>

V. Left Ventricle

The left ventricle receives blood from the left atrium and pumps it into the systemic circulation via the aorta. It forms the apex and left border of the heart, as well as most of the inferior surface.

The left ventricular myocardium is significantly thicker than the right ventricle, measuring two to three times thicker to generate the higher pressures required for systemic circulation. In cross-section, the left ventricle appears circular while the right ventricle wraps around it in a crescent shape.

The internal surface of the left ventricle contains finer trabeculae carneae than the right ventricle. The walls are covered with these muscular ridges except in the outflow region, the aortic vestibule, which has smooth walls leading to the aortic valve.

The left ventricle contains only two papillary muscles, corresponding to the two cusps of the mitral valve. The anterior papillary muscle is larger and arises from the anterolateral wall. The posterior papillary muscle arises from the posteroinferior wall. Each papillary muscle sends chordae tendineae to both cusps of the mitral valve, not just the adjacent cusp. This arrangement prevents prolapse of either cusp during ventricular systole when the papillary muscles contract.

The interventricular septum separates the left ventricle from the right ventricle. The muscular part comprises over ninety percent of the septum and is thick and strong. The membranous part forms a small superior portion of the septum, measuring less than one millimeter thick. The membranous septum lies just below the aortic valve and is the most common site of ventricular septal defects.

<image>Panel A: Left ventricular thick walls (2-3 times RV thickness) with circular cross-section compared to crescent-shaped RV. Panel B: Fine trabeculae carneae on the walls with smooth aortic vestibule leading to the aortic valve. Panel C: Two papillary muscles (anterior larger, posterior smaller) with chordae tendineae extending to both cusps of the mitral valve. Panel D: Interventricular septum with muscular and membranous portions indicated and the membranous area marked as the common VSD site.</image>

VI. Cardiac Valves - Atrioventricular Valves

The atrioventricular valves prevent regurgitation of blood from the ventricles to the atria during ventricular systole. Each valve consists of cusps or leaflets, an annulus, chordae tendineae, and papillary muscles.

The tricuspid valve guards the right atrioventricular orifice and has three cusps named for their positions: anterior, posterior, and septal. The valve annulus is a fibrous ring that provides structural support and attachment for the cusps. The chordae tendineae are fibrous cords that connect the free edges and ventricular surfaces of the cusps to the papillary muscles. The three papillary muscles contract during ventricular systole, pulling on the chordae to prevent the cusps from everting into the atrium.

The mitral valve, also called the bicuspid valve, guards the left atrioventricular orifice and has two cusps: the anterior cusp, which is larger and lies between the mitral and aortic orifices, and the posterior cusp, which is smaller and lies posterolaterally. The anterior cusp separates the inflow and outflow tracts of the left ventricle. Two papillary muscles send chordae to both cusps, ensuring coordinated support.

The mechanism of atrioventricular valve function involves precise coordination. During ventricular diastole, the valves open passively as blood flows from atria to ventricles. During ventricular systole, rising pressure pushes the cusps toward closure while simultaneously causing the papillary muscles to contract. The papillary muscle contraction maintains tension on the chordae tendineae, preventing the cusps from prolapsing into the atrium despite the high ventricular pressure.

<image>Panel A: Tricuspid valve from ventricular aspect with three cusps (anterior, posterior, septal) and chordae tendineae connecting to three papillary muscles. Panel B: Mitral valve with two cusps (anterior larger, posterior smaller) and chordae tendineae connecting to two papillary muscles. Panel C: Valve mechanism during diastole showing open cusps allowing blood flow from atria to ventricles. Panel D: Valve mechanism during systole with closed cusps and papillary muscle contraction maintaining chordae tension to prevent prolapse.</image>

VII. Cardiac Valves - Semilunar Valves

The semilunar valves prevent regurgitation of blood from the great arteries back into the ventricles during ventricular diastole. These valves have a different structure than the atrioventricular valves, lacking chordae tendineae and papillary muscles.

The pulmonary valve lies at the junction of the right ventricle and pulmonary trunk. It has three semilunar cusps named according to their position: anterior, right, and left. Each cusp is pocket-shaped, with the opening of the pocket facing the pulmonary trunk.

The aortic valve lies at the junction of the left ventricle and ascending aorta. It also has three semilunar cusps, named for their relationship to the coronary arteries: the right coronary cusp lies above the right aortic sinus from which the right coronary artery arises, the left coronary cusp lies above the left aortic sinus from which the left coronary artery arises, and the non-coronary or posterior cusp lies above the posterior aortic sinus which gives rise to no coronary artery.

Each semilunar cusp has specific structural features. The nodule, also called the nodule of Arantius, is a small fibrous thickening at the center of the free edge of each cusp. The lunulae are the thin crescentic portions of the free edge on either side of the nodule. When the valve closes, the nodules of adjacent cusps meet in the center while the lunulae overlap, ensuring a competent seal. The sinuses of Valsalva are the dilations of the arterial wall behind each cusp, which fill with blood during diastole and push the cusps together.

Semilunar valve function depends on passive hemodynamic forces. During ventricular systole, the high pressure in the ventricle pushes blood through the open valve. As the ventricle relaxes in diastole, the elastic recoil of the arterial wall pushes blood back toward the ventricle, filling the valve pockets and pushing the cusps together to close the valve. The coronary arteries fill during this diastolic phase when the aortic valve is closed and blood enters the coronary ostia in the aortic sinuses.

<image>Panel A: Pulmonary valve viewed from above with three cusps (anterior, right, left) from the right ventricular outflow. Panel B: Aortic valve with three cusps (right coronary, left coronary, non-coronary) with coronary ostia visible in the right and left sinuses. Panel C: Detailed cusp structure showing the nodule at the center of the free edge, lunulae as thin portions on either side, and sinuses of Valsalva. Panel D: Valve function diagram showing cusps open during systole and closed during diastole with blood filling the pockets.</image>

VIII. Cardiac Skeleton

The cardiac skeleton is a framework of dense fibrous connective tissue that provides structural support for the heart and plays a critical role in coordinating cardiac function.

The components of the cardiac skeleton include the four fibrous rings, called annuli fibrosi, surrounding each valve orifice. These rings provide attachment sites for the valve leaflets and for the surrounding myocardium. The right fibrous trigone is a large mass of fibrous tissue located between the aortic, mitral, and tricuspid valve rings, representing the strongest part of the cardiac skeleton. The left fibrous trigone lies between the aortic and mitral valve rings. The membranous part of the interventricular septum connects the fibrous skeleton to the ventricular septum.

The cardiac skeleton serves three essential functions. First, it provides structural support and attachment points for the cardiac valves, maintaining the shape of the valve annuli and ensuring proper valve function. Second, it provides attachment for the atrial and ventricular myocardium, which arise from and insert into the fibrous skeleton. Third, and critically for cardiac electrophysiology, it provides electrical insulation between the atrial and ventricular myocardium. The fibrous tissue does not conduct electrical impulses, forcing the cardiac action potential to travel from atria to ventricles only through the specialized conduction tissue of the atrioventricular bundle.

This electrical insulation has important implications. Without the cardiac skeleton, electrical impulses could spread directly from atrial myocardium to ventricular myocardium, causing uncoordinated contraction. The cardiac skeleton ensures that the only electrical pathway from atria to ventricles is through the AV node and bundle of His, allowing the delay necessary for atrial contraction to complete before ventricular contraction begins.

<image>Panel A: Four fibrous rings (annuli fibrosi) surrounding the tricuspid, mitral, aortic, and pulmonary valve orifices viewed from above. Panel B: Right fibrous trigone as a large fibrous mass between three valve rings and left fibrous trigone between aortic and mitral rings. Panel C: Membranous interventricular septum connecting the cardiac skeleton to the ventricular septum with atrial and ventricular myocardium attachment sites. Panel D: Electrical insulation function illustrated showing blocked direct impulse transmission and the single pathway through the AV bundle.</image>

IX. Cardiac Conduction System

The cardiac conduction system consists of specialized cardiac muscle cells that generate and conduct electrical impulses more rapidly than ordinary myocardium, coordinating the sequence of cardiac contraction.

The sinoatrial node, commonly called the SA node, serves as the primary pacemaker of the heart. Located at the junction of the superior vena cava and the right atrium in the superior aspect of the crista terminalis, the SA node generates spontaneous action potentials at a rate of sixty to one hundred beats per minute. The SA nodal artery, arising from the right coronary artery in approximately fifty-five percent of individuals and from the left circumflex in forty-five percent, supplies this structure.

The internodal pathways conduct the impulse through the atrial walls from the SA node to the AV node. Three pathways are described: the anterior internodal tract, the middle internodal tract or Wenckebach bundle, and the posterior internodal tract or Thorel bundle. Interatrial conduction spreads through Bachmann's bundle to the left atrium.

The atrioventricular node, or AV node, lies in the interatrial septum just above the opening of the coronary sinus, in a region called the triangle of Koch. The AV node delays the cardiac impulse by approximately one hundred milliseconds, allowing atrial contraction to complete before ventricular contraction begins. The intrinsic rate of the AV node is forty to sixty beats per minute, serving as a backup pacemaker if the SA node fails. The AV nodal artery, arising from the right coronary artery in approximately eighty percent of individuals, supplies this structure.

The bundle of His, or atrioventricular bundle, is the only electrical connection between the atrial and ventricular myocardium, penetrating through the cardiac skeleton at the right fibrous trigone. This bundle runs in the membranous part of the interventricular septum before dividing into the bundle branches.

The right bundle branch travels in the septomarginal trabecula or moderator band to reach the anterior papillary muscle, then spreads through the right ventricular wall via Purkinje fibers.

The left bundle branch penetrates the interventricular septum and divides into anterior and posterior fascicles. The anterior fascicle supplies the anterior and superior portions of the left ventricle. The posterior fascicle, which has a dual blood supply making it more resistant to ischemic damage, supplies the posterior and inferior portions.

The Purkinje fibers are the terminal ramifications of the bundle branches, spreading throughout the ventricular subendocardium. These fibers conduct impulses rapidly to the ventricular myocardium, producing coordinated ventricular contraction from apex to base. The intrinsic rate of Purkinje fibers is twenty to forty beats per minute.

<image>Panel A: SA node at the SVC-right atrium junction in the crista terminalis with intrinsic rate (60-100 bpm) and internodal pathways spreading through the atria. Panel B: AV node in the triangle of Koch above the coronary sinus opening with delay function (40-60 bpm intrinsic rate) and bundle of His penetrating through the right fibrous trigone. Panel C: Right bundle branch traveling in the moderator band to the right ventricular wall and left bundle branch dividing into anterior and posterior fascicles. Panel D: Purkinje fiber network spreading through ventricular walls (20-40 bpm intrinsic rate) with blood supply from coronary arteries indicated.</image>

X. Blood Supply to Conduction System

The conduction system receives its blood supply from the coronary arteries, with most structures supplied by the right coronary artery in the majority of individuals.

The SA node receives its blood supply from the SA nodal artery, which arises from the right coronary artery in approximately fifty-five percent of individuals and from the left circumflex artery in approximately forty-five percent.

The AV node receives its blood supply from the AV nodal artery, which arises from the right coronary artery in approximately eighty percent of individuals, typically as the artery courses around the crux of the heart. In twenty percent, the left circumflex artery supplies the AV node.

The bundle of His receives dual blood supply from the AV nodal artery and from septal perforating branches of the left anterior descending artery.

The right bundle branch receives blood supply from septal perforating branches of the left anterior descending artery.

The left anterior fascicle receives blood supply primarily from septal perforating branches of the left anterior descending artery, making it vulnerable to isolated injury in anterior myocardial infarction.

The left posterior fascicle has dual blood supply from both the left anterior descending artery and the posterior descending artery, making it more resistant to ischemic damage.

These vascular relationships explain the conduction abnormalities that may accompany myocardial infarction in different coronary artery territories. Right coronary artery occlusion may cause sinus bradycardia or AV block due to ischemia of the SA and AV nodes. Left anterior descending artery occlusion may cause right bundle branch block or left anterior fascicular block.

XI. Clinical Correlations

Valvular heart disease manifests as either stenosis, narrowing of the valve orifice that impedes forward flow, or regurgitation, incomplete closure that allows backward flow.

Aortic stenosis is the most common valvular disease in elderly patients, typically resulting from calcification of a previously normal or congenitally bicuspid valve. The left ventricle hypertrophies to overcome the increased resistance, eventually leading to heart failure. Patients present with the classic triad of angina, syncope, and dyspnea.

Mitral regurgitation is the most common valvular abnormality overall. Causes include mitral valve prolapse, rheumatic heart disease, infective endocarditis, ischemic papillary muscle dysfunction, and annular dilation from left ventricular enlargement. Regurgitation causes volume overload of the left ventricle and atrium.

Mitral stenosis is most commonly caused by rheumatic heart disease, which causes fibrosis and fusion of the valve leaflets. The elevated left atrial pressure transmits backward to the pulmonary circulation, causing pulmonary congestion and eventually right heart failure.

Conduction system disorders produce characteristic electrocardiographic patterns. First-degree AV block manifests as a prolonged PR interval, indicating delayed conduction through the AV node. Second-degree AV block type I, or Mobitz I or Wenckebach, shows progressive PR prolongation until a beat is dropped, typically indicating AV nodal disease. Second-degree AV block type II, or Mobitz II, shows sudden dropped beats without PR prolongation, indicating disease below the AV node with higher risk of progression to complete block. Third-degree or complete heart block shows complete AV dissociation with the atria and ventricles beating independently.

Bundle branch blocks produce widened QRS complexes with characteristic patterns. Right bundle branch block shows an RSR' pattern in V1. Left bundle branch block shows a broad R wave in leads I and V6 with no septal Q wave.

Ventricular septal defects are the most common congenital heart defect, usually occurring in the membranous portion of the septum. The defect creates a left-to-right shunt because left ventricular pressure exceeds right ventricular pressure. Small defects may close spontaneously; large defects require surgical repair.

Infective endocarditis involves infection of the valve leaflets, producing vegetations composed of bacteria, fibrin, and platelets. Previously abnormal valves are at increased risk. Intravenous drug users commonly develop tricuspid valve endocarditis from bacteria introduced through non-sterile injection.

<image>Panel A: Valvular disease showing stenotic calcified aortic valve and regurgitant mitral valve with prolapsed leaflet. Panel B: ECG strips demonstrating AV blocks including first-degree (prolonged PR), second-degree type I (Wenckebach), second-degree type II, and complete heart block. Panel C: Bundle branch block patterns with RBBB pattern in V1 and LBBB pattern in V6 with widened QRS. Panel D: VSD in the membranous septum with left-to-right shunt and endocarditis showing vegetation on valve leaflet with embolization risk.</image>

Summary

• The right atrium has smooth sinus venarum and rough pectinate muscles separated by the crista terminalis; the fossa ovalis marks the closed foramen ovale
• The right ventricle has the moderator band carrying the right bundle branch; the conus arteriosus is the smooth outflow tract
• The left atrium is mostly smooth-walled and receives four pulmonary veins
• The left ventricle has the thickest wall and two papillary muscles supporting the mitral valve
• Atrioventricular valves have chordae tendineae and papillary muscles; semilunar valves do not
• The cardiac skeleton provides structural support and electrical insulation between atria and ventricles
• The conduction system progresses from SA node to AV node to bundle of His to bundle branches to Purkinje fibers
• The AV bundle is the only electrical connection between atrial and ventricular myocardium

Key Terms

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