12.1 Heart Anatomy, Layers & Heart Valves

12.1 Heart Anatomy, Layers & Heart Valves

The human heart is a muscular, double-sided pump that rhythmically propels blood through the vascular circuits of the human body. Generating the hydrostatic pressure gradients required to deliver oxygen, nutrients, and hormones to peripheral tissues while transporting metabolic wastes to excretory organs, the heart operates continuously over a lifetime. Mastery of cardiac gross anatomy, tissue stratification, chamber geometry, and valvular mechanics is fundamental for clinical assessment, cardiac auscultation, and understanding hemodynamic pathophysiology.

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Anatomical Location, Orientation & Surface Landmarks

The adult human heart is approximately the size of a clenched fist, measuring roughly 12 to 14 cm in length, 9 cm in width, and possessing an average mass of 250 to 300 grams in adult females and 300 to 350 grams in adult males.

Mediastinal Position and Orientation

The heart is situated within the thoracic cavity inside the mediastinum—the central anatomical compartment situated between the bilateral pleural cavities housing the lungs. It rests superior to the central tendon of the diaphragm, anterior to the thoracic vertebral column (vertebrae T5 through T8), and posterior to the sternal body and costal cartilages of ribs 2 through 6.

Approximately two-thirds of the heart's mass lies to the left of the midsternal line, producing an asymmetric cardiac silhouette that indents the medial surface of the left lung at the cardiac notch. The heart is oriented obliquely in three-dimensional space:

• Base (Superior/Posterosuperior Surface): The broad, flattened posterior surface of the heart, measuring approximately 9 cm across. It is formed primarily by the left atrium and a portion of the right atrium. The base points posterosuperiorly toward the right shoulder and serves as the anatomical attachment site for the great vessels (ascending aorta, pulmonary trunk, superior vena cava, and pulmonary veins).
• Apex (Inferior Tip): The tapered, pointed inferior extremity formed entirely by the inferolateral tip of the left ventricle. The apex is directed anteroinferiorly toward the left hip and rests directly on the superior surface of the muscular diaphragm.

Clinical Milestone: Point of Maximal Impulse (PMI)

The cardiac apex lies deep to the fifth intercostal space along the left midclavicular line (approximately 7 to 9 cm lateral to the midsternal line). This anatomical landmark represents the Point of Maximal Impulse (PMI), where the contracting left ventricle strikes the anterior chest wall during systole. Clinicians routinely palpate and auscultate the apical pulse at this precise coordinate. Lateral displacement of the PMI (e.g., to the sixth intercostal space or anterior axillary line) serves as a classic physical finding indicative of left ventricular cardiomegaly or volume overload.

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The Pericardium: Fibrous & Serous Coverings

The heart is enclosed within a double-walled fibroserous sac designated the pericardium. The pericardium isolates the heart from neighboring thoracic viscera, allows frictionless expansion and contraction, and prevents physical overdistension.

Pericardial Coverings and Heart Wall Layers
┌────────────────────────────────────────────────────────┐
│ 1. Fibrous Pericardium (Tough, dense irregular CT outer layer) │
├────────────────────────────────────────────────────────┤
│ 2. Serous Pericardium:                                  │
│    a. Parietal Layer (Lines inner surface of fibrous sac)│
│    ────────────────────────────────────────────────── │
│    ★ Pericardial Cavity (15-50 mL serous fluid)        │
│    ────────────────────────────────────────────────── │
│    b. Visceral Layer / Epicardium (Adheres to heart)   │
├────────────────────────────────────────────────────────┤
│ 3. Myocardium (Thick cardiac muscle; functional syncytium) │
├────────────────────────────────────────────────────────┤
│ 4. Endocardium (Simple squamous endothelium lining chambers)│
└────────────────────────────────────────────────────────┘

1. Fibrous Pericardium

The fibrous pericardium is the superficial, tough, non-distensible outer layer composed of dense irregular connective tissue. It is structurally continuous with the tunica externa of the great exit vessels superiorly and anchors firmly to the central tendon of the diaphragm inferiorly and the posterior sternum via sternopericardial ligaments anteriorly. The inelastic nature of the fibrous pericardium serves three critical functions:

• It physically shields the heart from blunt trauma and adjacent thoracic infections.
• It anchors the heart within the mediastinum, maintaining its spatial orientation.
• It acts as a rigid boundary that prevents acute pathological overfilling of the cardiac chambers.

2. Serous Pericardium

Deep to the fibrous pericardium lies the serous pericardium, a delicate, closed two-layered serous membrane formed by simple squamous mesothelium resting upon thin loose connective tissue:

• Parietal Layer: Lines the internal surface of the fibrous pericardium. At the base of the heart where the great vessels emerge, the parietal layer reflects back over the external surface of the heart.
• Visceral Layer (Epicardium): Adheres intimately to the outer surface of the ventricular and atrial myocardium, forming the outermost histological layer of the heart wall.
• Pericardial Cavity: The slit-like potential space situated between the parietal and visceral serous layers. It normally contains approximately 15 to 50 mL of clear, lubricating serous pericardial fluid secreted by mesothelial cells. This fluid film virtually eliminates friction between the continuously contracting heart wall and the rigid pericardial sac.

Clinical Correlations: Pericarditis & Cardiac Tamponade

• Pericarditis: Inflammation of the pericardial layers, frequently resulting from viral infections, autoimmune disorders, or post-myocardial infarction (Dressler syndrome). The inflamed, roughened serous layers rub against each other during each heartbeat, producing a distinctive high-pitched scratching sound auscultated through a stethoscope termed a pericardial friction rub, accompanied by sharp pleuritic retrosternal chest pain.
• Cardiac Tamponade: Because the outer fibrous pericardium is rigid and non-distensible, acute accumulation of excess fluid, pus, or blood (hemopericardium from cardiac rupture or penetrating trauma) within the pericardial cavity increases intrapericardial pressure dramatically. This elevated external pressure compresses the thin-walled atria and ventricles, severely restricting diastolic filling. Consequently, end-diastolic volume drops, stroke volume and cardiac output plummet, and arterial blood pressure collapses. Hallmark clinical findings include Beck's triad (muffled heart sounds, jugular venous distension, and systemic hypotension). Emergency treatment requires pericardiocentesis—inserting a wide-bore needle into the pericardial space to aspirate fluid and relieve chamber compression.

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The Heart Wall: Three Distinct Histological Layers

The heart wall consists of three structurally and functionally distinct anatomical layers: the superficial epicardium, the intermediate myocardium, and the deep endocardium.

1. Epicardium (Visceral Serous Pericardium)

The epicardium constitutes the outermost tunic of the heart wall. It is composed of a superficial monolayer of simple squamous mesothelial cells overlying a subepicardial layer of loose areolar connective tissue and variable amounts of adipose tissue. The epicardial fat deposits cushion the coronary blood vessels, cardiac lymphatic channels, and autonomic nerve plexuses that traverse the surface of the heart within the coronary and interventricular sulci.

2. Myocardium (Contractile Muscular Core)

The myocardium forms the middle layer and constitutes the vast majority (over 90%) of the heart wall mass. It is composed of specialized cardiac muscle tissue arranged in intricate, spiraling, figure-eight bundles anchored into the fibrous skeleton of the heart. When these muscle bundles contract, they exert a wringing or twisting motion that squeezes blood upward from the apex toward the great base arteries, maximizing ejection efficiency.

Histologically, cardiac myocytes are uninucleated (or occasionally binucleated), short, branched, striated cells joined end-to-end at specialized junctional complexes termed intercalated discs. Intercalated discs contain two critical structural components:

• Desmosomes (Maculae Adherentes): Strong intercellular mechanical anchors that bind intermediate filaments of adjacent myocytes, preventing cells from pulling apart during vigorous systolic contractions.
• Gap Junctions (Nexus): Transmembrane protein channels (formed of connexons) that provide low-resistance hydrophilic pathways for the direct diffusion of ions between neighboring cells. Depolarization of one myocyte triggers the instantaneous electrical excitation of adjacent myocytes, causing the entire atrial or ventricular myocardium to contract as a single coordinated physiological unit termed a functional syncytium.

Cardiac myocytes are exceptionally rich in mitochondria, which occupy roughly 25% to 35% of cell volume (far more than in most skeletal muscle fibers). This intense mitochondrial density reflects an absolute, non-negotiable reliance on aerobic cellular respiration to generate adenosine triphosphate (ATP), rendering cardiac tissue exquisitely vulnerable to hypoxic or ischemic injury.

3. Endocardium (Inner Endothelial Lining)

The endocardium is the glistening white innermost layer that lines the internal lumina of all four cardiac chambers. It consists of a delicate monolayer of simple squamous endothelium resting upon a thin subendothelial lamina of dense irregular connective tissue rich in elastic fibers. The endocardium is structurally continuous with the tunica intima (endothelial lining) of the great blood vessels entering and exiting the heart (the venae cavae, aorta, and pulmonary trunk) and completely folds over and covers the structural collagenous cores of all four cardiac valves. Its polished, frictionless surface prevents platelet adherence, turbulent eddy currents, and pathological intravascular thrombosis.

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The Four Heart Chambers

Internally, the heart is partitioned into four distinct muscular cavities: two superior receiving chambers (the right and left atria) and two inferior pumping chambers (the right and left ventricles).

The Atria: Superior Receiving Chambers

The atria are relatively small, thin-walled chambers situated superior to the coronary sulcus (atrioventricular groove). Because their primary mechanical role is to receive returning venous blood and propel it down into the adjacent relaxed ventricles across open atrioventricular valves, they require minimal muscular contractile power.

• Auricles: Each atrium features a wrinkled, ear-shaped pouch-like flap called an auricle extending anteriorly over the great exit arteries. Auricles slightly increase total atrial chamber capacity during states of elevated venous return.
• Right Atrium: Receives deoxygenated systemic venous return and myocardial venous drainage via three primary vessels:
  1. Superior Vena Cava (SVC): Returns blood from regions superior to the diaphragm (head, neck, upper limbs, thoracic wall).
  2. Inferior Vena Cava (IVC): Returns blood from regions inferior to the diaphragm (abdominopelvic viscera, pelvis, lower extremities).
  3. Coronary Sinus: Returns venous blood from the coronary circulation draining the myocardium itself. Internally, the posterior wall of the right atrium is completely smooth (the sinus venarum), whereas the anterior muscular wall is lined by parallel, comb-like muscular ridges termed pectinate muscles. The smooth and rough portions are separated by a vertical ridge called the crista terminalis. The medial wall is formed by the interatrial septum, which displays a shallow oval depression termed the fossa ovalis—the remnant of the fetal foramen ovale that shunted blood from the right atrium directly to the left atrium in utero.
• Left Atrium: Receives freshly oxygenated blood returning from the pulmonary circulation via four pulmonary veins (two right pulmonary veins and two left pulmonary veins). Internally, the luminal surface of the left atrium is entirely smooth, with pectinate muscles restricted exclusively to the interior of its small left auricle. The left atrium forms the bulk of the heart's posterior base.

The Ventricles: Inferior Pumping Chambers

The ventricles make up the majority of the heart's mass and volume. As the primary muscular pumps, they generate high hydrostatic pressures to propel blood through extensive vascular beds.

• Right Ventricle: Forms most of the anterior surface of the heart. It receives deoxygenated blood from the right atrium through the tricuspid valve and forcefully ejects it across the pulmonary semilunar valve into the pulmonary trunk toward the pulmonary capillary beds of the lungs.
  • Internal Architecture: Its internal luminal walls are sculpted with irregular, crisscrossing muscular ridges called trabeculae carneae. Projecting into the lumen from the ventricular walls are cone-shaped papillary muscles, which anchor the free margins of the atrioventricular valve cusps via glistening, collagenous tendon-like cords termed chordae tendineae ("heart strings"). A specialized muscular band called the moderator band (septomarginal trabecula) extends from the interventricular septum to the anterior papillary muscle, transmitting electrical conduction impulses rapidly to synchronize papillary muscle contraction.
• Left Ventricle: Forms the cardiac apex and the posteroinferior surface. It receives oxygenated blood from the left atrium through the bicuspid (mitral) valve and forcefully propels it across the aortic semilunar valve into the ascending aorta to supply the systemic circulation of the entire body.
  • Marked Myocardial Hypertrophy: The myocardium of the left ventricle is approximately three times thicker (10 to 15 mm) than that of the right ventricle (3 to 5 mm). This substantial anatomical difference directly mirrors their contrasting hemodynamic workloads: the right ventricle pumps blood into the short, low-resistance pulmonary circuit against a peak systolic pressure of only ~20 to 25 mmHg, whereas the left ventricle must overcome the immense peripheral vascular resistance of the entire systemic arterial tree against a normal peak systolic pressure of ~120 mmHg.
  • Cavity Geometry: In transverse cross-section, the left ventricular lumen is circular and thick-walled, acting as a high-pressure cylinder. When it contracts, it bulges into and compresses the adjacent right ventricular cavity, which appears flattened into a crescent shape around the left ventricle. The thick muscular partition separating the two ventricular cavities is the interventricular septum.

Comprehensive Chamber Comparison Table

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The Four Heart Valves: Ensuring Unidirectional Flow

Blood must flow through the heart in a strictly unidirectional circuit. Unidirectional flow is enforced by four cardiac valves that open and close passively in response to changing hydrostatic pressure gradients across their leaflets.

1. Atrioventricular (AV) Valves

The atrioventricular (AV) valves are positioned at the junctions between the atria and ventricles. Their primary physiological purpose is to prevent the backflow (regurgitation) of blood into the atria during ventricular contraction (systole).

• Tricuspid Valve (Right AV Valve): Positioned between the right atrium and right ventricle; composed of three flexible, triangular connective tissue cusps (anterior, posterior, and septal).
• Bicuspid Valve (Mitral Valve / Left AV Valve): Positioned between the left atrium and left ventricle; composed of two heavy, triangular cusps (anterior and posterior). Its resemblance to a bishop's two-sided miter hat gives rise to the clinical name "mitral valve."
• Operating Mechanism and Anchoring Apparatus:
  • During ventricular diastole (filling), atrial pressure exceeds ventricular pressure. The AV valve cusps hang loosely down into the relaxing ventricular cavities, allowing blood to flow freely from atria into ventricles.
  • During ventricular systole (contraction), ventricular pressure rises sharply above atrial pressure. The upward surge of blood forces the edges of the AV valve cusps together, sealing the atrioventricular orifices.
  • The Crucial Role of Chordae Tendineae and Papillary Muscles: The free margins and ventricular surfaces of the AV valve cusps are tethered to ventricular papillary muscles by strong collagenous cords called chordae tendineae. Simultaneously with ventricular contraction, the papillary muscles contract, exerting strong downward tension on the chordae tendineae. This mechanical anchoring does not open or close the valves; rather, it acts like the guy wires of a parachute, preventing the valve cusps from everting or prolapsing backward into the low-pressure atria under the immense systolic pressure. Rupture of a papillary muscle (e.g., following an acute myocardial infarction) leads to catastrophic acute mitral regurgitation, pulmonary edema, and cardiogenic shock.

2. Semilunar (SL) Valves

The semilunar (SL) valves are located at the base of the two large arterial trunks emerging from the ventricles. Their primary physiological purpose is to prevent the backflow of blood from the great arteries into the ventricles during ventricular relaxation (diastole).

• Pulmonary Semilunar Valve: Situated at the junction between the right ventricle and the origin of the pulmonary trunk; consists of three pocket-like, crescent-shaped cusps.
• Aortic Semilunar Valve: Situated at the junction between the left ventricle and the origin of the ascending aorta; consists of three heavy pocket-like, crescent-shaped cusps.
• Operating Mechanism:
  • During ventricular systole, ventricular pressure rapidly exceeds the pressure in the pulmonary trunk (~10 mmHg) and aorta (~80 mmHg). The high pressure forces the semilunar valve cusps flat against the arterial walls, opening the valves and allowing blood to be forcefully ejected into the great arteries.
  • During ventricular diastole, the ventricles relax and intraventricular pressure plummets below arterial pressure. Blood in the aorta and pulmonary trunk momentarily flows backward toward the lower-pressure heart. This reversing blood fills the crescent-shaped pockets of the cusps, causing them to balloon outward and meet in the center of the vessel, tightly sealing the arterial orifices.
  • Key Structural Distinction: Semilunar valves do not possess chordae tendineae or papillary muscles. Their cup-like geometrical architecture and the mechanical support provided by their fibrous rings are fully sufficient to withstand diastolic arterial pressure without everting.

Comprehensive Heart Valves Comparison Table

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Coronary Circulation: The Myocardial Blood Supply

Although the cardiac chambers are continuously bathed in blood, the myocardium is far too thick to receive adequate oxygen and nutrients via direct luminal diffusion. The heart possesses its own dedicated vascular network: the coronary circulation.

Coronary Arterial Tree Architecture
Ascending Aorta (Base)
├── Left Coronary Artery (LCA)
│   ├── Anterior Interventricular Artery (Left Anterior Descending / LAD) ["Widow Maker"]
│   └── Circumflex Artery
└── Right Coronary Artery (RCA)
    ├── Right Marginal Artery
    └── Posterior Interventricular Artery

Coronary Arterial Supply

The right and left coronary arteries arise from the base of the ascending aorta immediately superior to the aortic semilunar valve cusps (within the aortic sinuses of Valsalva):

1. Left Coronary Artery (LCA): Passes to the left beneath the pulmonary trunk and divides into two major branches:
   • Anterior Interventricular Artery (Left Anterior Descending [LAD]): Traverses the anterior interventricular sulcus toward the cardiac apex. It supplies the anterior walls of both ventricles and the anterior two-thirds of the interventricular septum. Clinically designated the "widow maker", occlusion of the LAD leads to extensive anterior myocardial infarction and high mortality.
   • Circumflex Artery: Encircles the heart to the left within the coronary sulcus. It supplies the left atrium and the posterior wall of the left ventricle.
2. Right Coronary Artery (RCA): Courses to the right within the coronary sulcus, giving off two major branches:
   • Right Marginal Artery: Runs along the inferior lateral border of the heart to supply the myocardium of the lateral right ventricle.
   • Posterior Interventricular Artery: Follows the posterior interventricular sulcus toward the apex, supplying the posterior ventricular walls and posterior third of the interventricular septum.

Clinical Nuance: Diastolic Myocardial Perfusion

Unlike all other systemic vascular beds—which receive peak arterial perfusion during ventricular systole—the myocardium receives the vast majority of its arterial blood supply during ventricular diastole. During ventricular systole, the vigorous contraction of the ventricular myocardium compresses intramural coronary vessels (especially in the subendocardium), sharply reducing left ventricular coronary flow. During diastole, the myocardium relaxes, the aortic valve snaps shut, and the elastic recoil of the aorta forces blood backward into the aortic sinuses, driving rich perfusion through the coronary arteries.

Coronary Venous Drainage

After coursing through myocardial capillary beds, deoxygenated blood is collected by several cardiac veins:

• Great Cardiac Vein: Occupies the anterior interventricular sulcus alongside the LAD artery.
• Middle Cardiac Vein: Runs in the posterior interventricular sulcus alongside the posterior interventricular artery.
• Small Cardiac Vein: Runs along the right inferior margin of the heart. All of these major coronary veins empty into the Coronary Sinus—a broad, thin-walled venous vessel lying in the posterior coronary sulcus. The coronary sinus discharges deoxygenated myocardial blood directly into the cavity of the right atrium.