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Four-chambered muscular pump in the chest that drives blood through the pulmonary and systemic circulations.
Medically reviewed & updated
The heart is a muscular organ about the size of a closed fist, located in the middle of the chest (mediastinum) behind the sternum and tilted slightly to the left. It sits inside a protective double-layered sac called the pericardium and rests on the diaphragm. Its wall has three layers: the inner endocardium, the thick muscular myocardium, and the outer epicardium.
The heart contains four chambers. The two upper chambers, the right and left atria, receive incoming blood. The two lower chambers, the right and left ventricles, are the main pumping chambers. A muscular wall called the septum separates the right and left sides, keeping oxygen-poor and oxygen-rich blood from mixing. The myocardium is far thicker in the left ventricle because it must pump blood to the entire body, whereas the thinner-walled right ventricle pumps only to the nearby lungs.
The heart works as two pumps in one organ. The right heart receives deoxygenated blood from the body through the superior and inferior vena cavae into the right atrium, then through the tricuspid valve to the right ventricle, which sends it through the pulmonary valve and pulmonary arteries to the lungs (pulmonary circulation). The oxygenated blood returns from the lungs to the left atrium, passes through the mitral valve into the left ventricle, and is ejected through the aortic valve into the aorta to supply the body (systemic circulation). One-way valves keep blood moving forward, and a built-in electrical conduction system coordinates each heartbeat so the chambers contract in the correct sequence. An average adult heart beats roughly 60 to 100 times per minute at rest.
Because the heart depends on continuous oxygen delivery through the coronary arteries, blockage of these vessels can cause a heart attack (myocardial infarction). Weakening of the pumping muscle leads to heart failure, and damaged or stiffened chambers can cause arrhythmias. This is educational information and not medical advice; anyone with chest pain, breathlessness, or palpitations should seek professional care.