The Heart: Anatomy and Function

The heart is a muscular pump slightly larger than a clenched fist, weighing 250–350 g in adults, positioned in the mediastinum with roughly two-thirds of its mass to the left of the midline. It is the central engine of the cardiovascular system, beating approximately 100,000 times per day and pumping around 7,000 litres of blood every 24 hours. This guide is for educational purposes only.

## Chambers and Valves

The heart consists of four chambers: the right atrium, right ventricle, left atrium, and left ventricle. The two atria are thin-walled receiving chambers separated by the interatrial septum; the two ventricles are thick-walled pumping chambers separated by the interventricular septum. The left ventricular wall is approximately three times thicker than the right, reflecting the higher pressure it must generate to drive systemic circulation.

Four valves regulate unidirectional blood flow. The atrioventricular (AV) valves — the tricuspid (right) and mitral (left, bicuspid) — prevent backflow from ventricles to atria during systole. They are anchored by chordae tendineae to papillary muscles within the ventricular walls; these structures prevent valve prolapse during contraction. The semilunar valves — the pulmonary (right outflow) and aortic (left outflow) — prevent backflow from the great vessels into the ventricles during diastole. Each semilunar valve has three cusps. The aortic valve cusps are named left, right, and non-coronary; the left and right cusps overlie the openings of the corresponding coronary arteries.

## Coronary Circulation

The myocardium receives its blood supply from the right and left coronary arteries, which arise from the aortic sinuses (sinuses of Valsalva) just above the aortic valve cusps. The left main coronary artery divides after a short course into the left anterior descending artery (LAD) and the left circumflex artery (LCx). The LAD supplies the anterior left ventricle, interventricular septum, and apex — it is the most commonly occluded vessel in myocardial infarction and is sometimes called the "widow maker." The LCx supplies the lateral and posterior left ventricle via obtuse marginal branches. The right coronary artery (RCA) supplies the right ventricle and, in most individuals (right-dominant circulation, ~70%), gives off the posterior descending artery (PDA) supplying the inferior interventricular septum and the AV node.

Venous drainage returns to the coronary sinus (which drains into the right atrium) via the great, middle, and small cardiac veins. The anterior cardiac veins drain the right ventricular free wall directly into the right atrium.

## Conduction System

The cardiac conduction system generates and coordinates the electrical impulses that trigger each heartbeat. The sinoatrial (SA) node — the heart's intrinsic pacemaker — is located at the junction of the superior vena cava and the right atrium and fires at 60–100 bpm. The impulse spreads across both atria via internodal pathways, causing atrial contraction. It then reaches the atrioventricular (AV) node, located in the floor of the right atrium at the apex of the triangle of Koch; the AV node introduces a delay of approximately 0.1 seconds, allowing ventricular filling to complete before ventricular contraction begins.

From the AV node the impulse travels along the bundle of His through the fibrous skeleton of the heart (which electrically insulates the atria from the ventricles) into the right and left bundle branches. The left bundle branch further divides into anterior and posterior fascicles. The bundle branches terminate in the Purkinje fibre network, which rapidly distributes the impulse to ventricular myocardium, producing the coordinated contraction of both ventricles. The intrinsic rate of the AV node is 40–60 bpm; the Purkinje system can sustain a ventricular escape rhythm of 20–40 bpm if higher pacemakers fail.

## The Cardiac Cycle

The cardiac cycle encompasses all events associated with one heartbeat. Systole refers to ventricular contraction and ejection; diastole refers to ventricular relaxation and filling. At a heart rate of 75 bpm, one cycle lasts approximately 0.8 seconds (systole ~0.3 s, diastole ~0.5 s).

During isovolumetric contraction, all valves are closed and ventricular pressure rises rapidly. When left ventricular pressure exceeds aortic pressure (~80 mmHg), the aortic valve opens and ejection begins. The stroke volume (volume ejected per beat) is approximately 70 mL at rest; the ejection fraction (stroke volume / end-diastolic volume) is normally 55–70%. During isovolumetric relaxation, all valves close again; when left ventricular pressure falls below left atrial pressure, the mitral valve opens and passive filling begins. Late diastole, atrial contraction adds a further ~20% to ventricular filling ("atrial kick"), particularly important when heart rate is elevated and diastolic filling time is shortened.

## Auscultation Points

Heart sounds are auscultated at specific surface landmarks that are not directly overlying the valves but rather where sound is transmitted towards the listener's stethoscope. The aortic area lies in the right second intercostal space, parasternal. The pulmonary area is at the left second intercostal space, parasternal. The tricuspid area is at the left lower sternal border (fourth/fifth intercostal space). The mitral area is at the cardiac apex — the point of maximal impulse (PMI), normally in the left fifth intercostal space, midclavicular line.

The first heart sound (S1) results from mitral and tricuspid valve closure at the onset of systole. The second heart sound (S2) results from aortic and pulmonary valve closure at the onset of diastole; physiological splitting of S2 (wider on inspiration) reflects the increased right heart filling and delayed pulmonary valve closure during inspiration. A third heart sound (S3) in early diastole indicates rapid ventricular filling and is pathological in adults over 40, suggesting heart failure. A fourth heart sound (S4) in late diastole is associated with a non-compliant ventricle and occurs during atrial contraction.

## Common Pathologies

Coronary artery disease (CAD) — atherosclerotic narrowing of coronary arteries — is the leading cause of death worldwide. Stable angina presents as exertional chest pain relieved by rest; unstable angina and NSTEMI involve plaque rupture with partial occlusion; STEMI involves complete occlusion. Valvular disease includes aortic stenosis (most common valvular disease in developed countries, characteristically systolic murmur radiating to the neck), mitral regurgitation (holosystolic murmur at apex), and mitral stenosis (often rheumatic in origin, producing a mid-diastolic murmur). Heart failure (reduced or preserved ejection fraction) results from any condition impairing the heart's pumping capacity, leading to congestion, oedema, and reduced cardiac output.