The heart is a muscular organ located in the mediastinum, slightly left of the mid‑line. Its external features can be grouped into protective coverings, surface markings and major vessels.
Pericardium
Fibrous pericardium – tough outer layer that anchors the heart to the diaphragm and sternum.
Serous pericardium – consists of the parietal and visceral (epicardial) layers with a thin pericardial cavity containing lubricating fluid.
Epicardium – the visceral layer of the serous pericardium; also considered the outermost layer of the heart wall.
Coronary sulcus (atrioventricular groove) – encircles the heart, marking the division between atria and ventricles; contains the coronary arteries and the coronary sinus.
Anterior (sternocostal) surface – faces the sternum; shows the right and left ventricles and the left anterior descending artery.
Posterior (diaphragmatic) surface – rests on the diaphragm; displays the left and right atria and the opening of the inferior vena cava.
Major vessels attached to the heart
Superior and inferior vena cava – return de‑oxygenated blood to the right atrium.
Aorta – carries oxygenated blood from the left ventricle to the systemic circulation.
Pulmonary trunk and pulmonary veins – connect the right ventricle to the lungs and return oxygen‑rich blood to the left atrium.
Suggested diagram: External view of a mammalian heart showing pericardium, coronary sulcus, chambers and attached vessels.
Internal Structure of the Mammalian Heart
Chambers
The heart has four chambers that are separated by septa and valves to ensure unidirectional blood flow.
Atria – thin‑walled upper chambers that receive blood.
Right atrium (RA) receives systemic venous blood via the superior and inferior vena cava.
Left atrium (LA) receives pulmonary venous blood from the lungs.
Ventricles – thick‑walled lower chambers that pump blood.
Right ventricle (RV) pumps de‑oxygenated blood to the pulmonary artery.
Left ventricle (LV) pumps oxygenated blood into the aorta; its wall is the thickest.
Valves
Valves prevent backflow and are positioned at the atrioventricular (AV) and semilunar (SL) junctions.
Atrioventricular valves
Tricuspid valve – between RA and RV; composed of three leaflets.
Bicuspid (mitral) valve – between LA and LV; composed of two leaflets.
Semilunar valves
Pulmonary valve – between R \cdot and pulmonary trunk.
Aortic valve – between L \cdot and aorta.
Internal Walls and Septa
The internal architecture includes muscular walls, fibrous skeleton, and septa that separate chambers.
Interventricular septum – divides the right and left ventricles; contains a muscular portion and a small membranous portion.
Atrial septum – separates the atria; includes the fossa ovalis, a remnant of fetal circulation.
Fibrous skeleton (cardiac skeleton) – dense connective tissue that provides attachment points for valves and insulates electrical conduction between atria and ventricles.
Blood Supply – Coronary Circulation
The myocardium receives oxygenated blood via the coronary arteries, which arise from the aortic sinuses just above the aortic valve.
Right coronary artery (RCA) – supplies the right atrium, right ventricle, part of the interventricular septum and the SA node in most individuals.
Left coronary artery (LCA) – quickly divides into the left anterior descending (LAD) artery and the circumflex artery, supplying the left atrium, left ventricle and most of the interventricular septum.
Coronary veins – collect de‑oxygenated blood and drain into the coronary sinus, which empties into the right atrium.
Comparison of Wall Thickness
Chamber
Relative Wall Thickness
Function‑related Reason
Right Atrium
Thin
Low pressure reservoir for systemic venous return.
Left Atrium
Thin‑moderate
Receives blood from pulmonary veins; must stretch during ventricular systole.
Right \cdot entricle
Moderate
Generates pressure sufficient to overcome pulmonary vascular resistance.
Left \cdot entricle
Thickest
Produces high systemic arterial pressure.
Key Points for Revision
Identify the pericardial layers and their protective roles.
Know the four chambers, their positions and the direction of blood flow.
Distinguish the atrioventricular valves from the semilunar valves and recall their leaflet numbers.
Understand the composition and function of the interventricular and atrial septa.
Recall the origin and distribution of the coronary arteries and the drainage pathway of the coronary veins.
Remember that wall thickness correlates with the pressure each chamber must generate.
Suggested diagram: Cross‑sectional view of the heart showing chambers, valves, septa and major coronary vessels.