Heart

The heart is a muscular organ that functions as the core of the circulatory system. Located in the thoracic cavity between the lungs, it is enclosed by the pericardium and weighs about 300 g in adults. The heart consists of four chambers: two atria and two ventricles. Blood flows through valves—tricuspid and mitral control input to the ventricles, while the pulmonary and aortic valves regulate flow to the lungs and the rest of the body. The heart wall comprises three layers: the endocardium (inner), myocardium (muscle), and epicardium (outer). The conduction system, including the sinoatrial node, atrioventricular node, bundle of His, and Purkinje fibers, coordinates rhythmic contractions.

Blood returns to the heart through the right atrium via the superior and inferior vena cavae; from there it moves to the right ventricle through the tricuspid valve and is pumped to the lungs via the pulmonary artery for oxygenation. Oxygenated blood returns from the lungs to the left atrium via the pulmonary veins, passes through the mitral valve into the left ventricle, and is ejected into the aorta for systemic distribution. The valves ensure unidirectional flow, and the cardiac cycle alternates between systole (contraction) and diastole (relaxation).

Cardiac output, the product of heart rate and stroke volume, reflects the amount of blood the heart pumps per minute and is regulated by autonomic nerves, circulating hormones, and intrinsic mechanisms such as the Frank-Starling law, which links stretch of the ventricular wall to contraction strength. Blood supply to the heart muscle itself is provided by the coronary arteries.

Common clinical considerations include coronary artery disease, myocardial infarction, heart failure, arrhythmias, and valve disorders. Diagnosis relies on history, physical examination, electrocardiography, echocardiography, and other imaging and laboratory tests. Treatments range from lifestyle modification and medications to interventional procedures and surgery, depending on the condition.