The Human Heart — Chambers, Valves & Blood Flow, Visualised

Introduction: The Engine That Never Rests

The human heart is a hollow, muscular organ weighing approximately 250–350 g in adults — roughly the size of a closed fist. It beats around 100,000 times per day, 40 million times per year, and over 3 billion times across an average lifetime — without ever taking a break. Located in the middle mediastinum within the pericardial sac, it is positioned obliquely: approximately two-thirds to the left of the median plane, with its apex pointing anteriorly, inferiorly, and to the left (roughly at the level of the 5th intercostal space, midclavicular line — the site for auscultating the mitral valve and palpating the apex beat).

The heart pumps approximately 5 litres of blood per minute at rest (cardiac output = heart rate × stroke volume). During intense exercise, cardiac output can rise to 20–25 litres per minute in trained athletes. Over a lifetime, the heart pumps enough blood to fill approximately 1 million barrels.

Gross Anatomy: Surfaces, Borders & Relations

The heart has four surfaces and four borders, each with clinical significance for imaging and examination:

• Sternocostal (Anterior) Surface: Formed mainly by the right ventricle. Lies deep to the sternum and costal cartilages of ribs 3–5.
• Diaphragmatic (Inferior) Surface: Formed mainly by the left ventricle, partly by the right ventricle. Rests on the central tendon of the diaphragm.
• Pulmonary (Left) Surface: Formed mainly by the left ventricle. Creates the cardiac impression on the left lung.
• Base (Posterior Surface): Formed mainly by the left atrium. Faces posteriorly — this is why the oesophagus (lying directly behind) is compressed by an enlarged left atrium in mitral stenosis.

The Four Chambers

The heart is divided longitudinally by the interventricular and interatrial septa into four chambers:

Right Atrium

Receives deoxygenated blood from the systemic circulation via the superior vena cava (SVC), inferior vena cava (IVC), and coronary sinus. Its internal surface is smooth posteriorly (sinus venarum, derived from the embryological sinus venosus) and rough anteriorly (pectinate muscles, derived from the primitive atrium). The fossa ovalis — a smooth oval depression in the interatrial septum — is the remnant of the foetal foramen ovale, which normally closes at birth. A patent foramen ovale (PFO) persists in approximately 25–30% of adults and is a potential pathway for paradoxical embolism.

Right Ventricle

Receives blood from the right atrium via the tricuspid valve (right atrioventricular valve) and pumps it to the lungs via the pulmonary trunk through the pulmonary valve. Its walls are relatively thin (approximately 3–5 mm) because it pumps against the low-resistance pulmonary circulation (pulmonary pressure: ~25/8 mmHg). In pulmonary hypertension, RV wall thickness increases in response to elevated afterload — eventually leading to cor pulmonale and right heart failure.

Internal features: muscular ridges called trabeculae carneae, papillary muscles (anterior, posterior, septal), and chordae tendineae connecting papillary muscles to the tricuspid valve cusps.

Left Atrium

Receives freshly oxygenated blood from the lungs via the four pulmonary veins (two from each lung). It is the most posterior cardiac chamber — hence its intimate relationship with the oesophagus. The left atrial appendage (LAA) is a pouch of the left atrium frequently involved in atrial fibrillation (AF): in AF, loss of coordinated atrial contraction creates blood stasis within the LAA, promoting thrombus formation — the leading source of embolic stroke in AF patients.

Left Ventricle

The engine of the systemic circulation. Its walls are 8–12 mm thick — approximately three times the RV — because it must generate sufficient pressure to overcome systemic vascular resistance (aortic pressure: ~120/80 mmHg). It receives blood from the left atrium via the mitral valve (bicuspid valve) and ejects into the aorta via the aortic valve. The interventricular septum bulges into the RV cavity — visible on cross-sectional imaging — because of the higher LV pressure.

The Four Heart Valves

The heart contains four valves that ensure unidirectional blood flow. Two are atrioventricular (AV) valves and two are semilunar valves:

Valve	Type	Location / Auscultation Site	Cusps
Tricuspid	AV (right)	4th ICS, left sternal border	3 (anterior, posterior, septal)
Mitral (Bicuspid)	AV (left)	5th ICS, midclavicular line (apex)	2 (anterior, posterior)
Pulmonary	Semilunar	2nd ICS, left sternal border	3 semilunar cusps
Aortic	Semilunar	2nd ICS, right sternal border	3 semilunar cusps (L, R, posterior)

Mnemonic — Valve Auscultation Sites: "All Patients Take Medicine" — Aortic (2R), Pulmonary (2L), Tricuspid (4L sternal border), Mitral (5th ICS midclavicular).

Blood Flow Through the Heart

The complete circuit of blood through the heart follows this sequence:

1. Deoxygenated blood enters the right atrium from SVC, IVC, and coronary sinus
2. Passes through the tricuspid valve into the right ventricle
3. Ejected through the pulmonary valve into the pulmonary trunk → left and right pulmonary arteries
4. Oxygenated in the lungs; returns via four pulmonary veins to the left atrium
5. Passes through the mitral valve into the left ventricle
6. Ejected through the aortic valve into the ascending aorta → systemic circulation

The Coronary Circulation

The heart is supplied by the right coronary artery (RCA) and left coronary artery (LCA), both arising from aortic sinuses just above the aortic valve:

• Left Coronary Artery (LCA): Divides almost immediately into the Left Anterior Descending (LAD) — supplying the anterior wall of the LV, anterior two-thirds of the interventricular septum, and the apex — and the Left Circumflex (LCx) — supplying the lateral wall of the LV and the left atrium.
• Right Coronary Artery (RCA): Supplies the right ventricle, the inferior wall of the LV, the posterior one-third of the IVS, and (in most people — "right dominant") the SA node and AV node.

Clinical pearl: The LAD is commonly called the "widow maker" because occlusion of the proximal LAD produces massive anterior MI with the highest mortality of any STEMI territory. Prompt percutaneous coronary intervention (PCI) within 90 minutes is the standard of care.

The Cardiac Conduction System

The heart's electrical system generates and conducts action potentials that trigger each coordinated contraction. The pathway is:

1. SA Node (Sinoatrial Node): The pacemaker of the heart. Located in the upper right atrium near the SVC opening. Spontaneously depolarises at 60–100 bpm. Supplied by the SA nodal artery (branch of RCA in 60% of people).
2. AV Node (Atrioventricular Node): Located in the floor of the right atrium (Koch's triangle). Introduces a critical physiological delay (~120 ms) allowing atrial contraction to complete before ventricular systole begins. Intrinsic rate: 40–60 bpm.
3. Bundle of His: Penetrates the fibrous skeleton of the heart — the only electrical connection between atria and ventricles.
4. Left & Right Bundle Branches: Distribute impulses to their respective ventricles along the interventricular septum.
5. Purkinje Fibres: Specialised conduction cells that rapidly spread the impulse throughout the ventricular myocardium subendocardially, producing nearly simultaneous ventricular contraction. Intrinsic rate: 20–40 bpm.

The Cardiac Cycle — Systole & Diastole

Each heartbeat comprises two phases:

• Ventricular Systole (~0.3 seconds at normal heart rate): The ventricles contract. Pressure rises, the AV valves close (producing heart sound S1 — "lub"), and blood is ejected through the semilunar valves once ventricular pressure exceeds arterial pressure. At end-systole, the semilunar valves close (producing S2 — "dub").
• Ventricular Diastole (~0.5 seconds at normal heart rate): The ventricles relax and fill. AV valves open once ventricular pressure falls below atrial pressure. The Frank-Starling Law governs this relationship: the greater the ventricular filling (preload), the more forceful the subsequent contraction — up to a physiological limit.

Common Cardiac Pathology — A Rapid Review

• Myocardial Infarction (MI): Occlusion of a coronary artery → ischaemia and necrosis of the supplied myocardium. STEMI: complete occlusion → ST elevation on ECG. NSTEMI: partial occlusion → no ST elevation.
• Heart Failure: The heart cannot pump sufficient blood to meet metabolic demands. Left heart failure: pulmonary oedema (dyspnoea, orthopnoea). Right heart failure: peripheral oedema, raised JVP, hepatomegaly.
• Aortic Stenosis: Calcification and narrowing of the aortic valve orifice — the most common valvular disease in the elderly. Classic triad: exertional dyspnoea, angina, syncope. Ejection systolic murmur loudest at 2nd right ICS, radiating to carotids.
• Mitral Regurgitation: Incompetent mitral valve allows backflow into left atrium during systole. Pansystolic murmur loudest at apex, radiating to axilla.
• Atrial Fibrillation (AF): Chaotic atrial electrical activity — the most common sustained cardiac arrhythmia. Irregularly irregular pulse, absent P waves on ECG. Major risk: thrombus formation in LAA → embolic stroke.

Archive File #042 is for educational purposes only. Medical disclaimer →