Heart Anatomy

The heart is located within the pericardium, inside the middle mediastinum, which is defined by the pericardial sac. It sits 2/3 in the left side of the chest, rotated to the left so that the right heart is more anterior and the left heart more posterior. The apex points anteroinferiorly and is located at the left 5th intercostal space, midclavicular line (L5icsmcl) — the point of maximal intensity (PMI) for auscultation. The right margin of the heart is at approximately the 4th intercostal space, right sternal edge (4icsRSE).

The mediastinum is divided into superior (above the sternal angle), anterior (between sternum and pericardium), middle (pericardium and heart), and posterior (descending aorta, oesophagus). The pericardium consists of an outer fibrous pericardium and an inner serous pericardium. The serous pericardium has two layers: visceral (adherent to heart) and parietal (adherent to fibrous pericardium). Reflections of serous pericardium around vessels form two sinuses: the transverse pericardial sinus (between arteries and veins) and the oblique pericardial sinus (behind the pulmonary veins).

The heart has four chambers. The atria have two anatomical parts: the sinus venosus derivative (smooth wall) and the atrium proper (rough, due to musculi pectinati — comb-like muscle fibres). Pectinate muscles are clinically significant because during atrial fibrillation, blood can stagnate in these recesses, forming thrombi that may embolise. The crista terminalis demarcates these two regions. The fossa ovalis is the remnant of the foramen ovale (foetal shunt); patent foramen ovale is a common heart defect.

The four valves control unidirectional flow, opening and closing passively based on pressure differences. AV valves (tricuspid on the right, mitral on the left) are secured by chordae tendineae attached to papillary muscles, which prevent valve prolapse during systole. Semilunar valves (pulmonary and aortic) have three cusps each. The free edge of semilunar cusps contains a nodule that ensures the cusp returns to the vessel centre to form a complete seal.

Valve auscultation sites are downstream to the direction of blood flow: aortic valve at 2icsRSE, pulmonary at 2icsLSE, mitral at L5icsmcl (apex), tricuspid at 5icsLSE. Heart sounds arise from valve closure: S1 (AV valve closure, mitral before tricuspid), S2 (semilunar valve closure, aortic before pulmonary).

The fibrous skeleton electrically isolates the atria from the ventricles, forcing conduction through the AV node. Cardiac innervation arrives via the superficial cardiac plexus from sympathetic cardiac nerves and the vagus (parasympathetic). Referred cardiac pain (chest and medial arm) occurs because visceral sensory fibres from the heart converge with somatic input from T1-T4 dermatomes at the dorsal horn.