How the Heart Works: Four Chambers, Four Valves and a Built-In Pacemaker

General information only, not medical advice. Talk to a doctor or pharmacist about your own health.

The heart is a muscular organ that pumps blood through the body, as part of a network of arteries, veins and capillaries that distribute oxygen and nutrients. The National Heart, Lung, and Blood Institute (NHLBI) says a healthy heart supplies the body with the amount of blood needed at the rate needed.^[1] This article is general science background, not individual health guidance.

Chambers, layers and valves

The heart has four hollow chambers surrounded by muscle. The two upper chambers, the atria, collect incoming blood, and the two lower chambers, the ventricles, pump it out. A wall called the septum divides right from left. Its wall has three tissue types: the endocardium lining the chambers and valves, the thick muscular myocardium that contracts, and the pericardium, a protective sac with fluid that reduces friction.^[2]

Four valves keep blood flowing one way. The tricuspid valve separates the right atrium and ventricle, the mitral valve does the same on the left, the pulmonary valve sits between the right ventricle and pulmonary artery, and the aortic valve sits between the left ventricle and aorta. Each valve has flaps that open to let blood out of a chamber and close so the chamber can refill.^[3]

The route of the blood

• Oxygen-poor blood enters the right atrium through the superior and inferior vena cava.^[3]
• The right ventricle pumps it through the pulmonary artery to the lungs.^[3]
• Oxygen-rich blood returns by the pulmonary veins to the left atrium, then the left ventricle.^[3]
• The left ventricle generates enough pressure to send blood through the aorta to the body.^[3]

The heart muscle itself is fed by coronary arteries that branch from the aorta, and oxygen-poor blood from the heart wall returns through coronary veins to the right atrium.^[3] A physiology review adds that cardiac output equals heart rate multiplied by stroke volume, the amount of blood ejected per minute, and that each cycle has a filling and relaxing phase, diastole, and a contracting and ejecting phase, systole.^[5]

What sets the rhythm

The beat is controlled by the cardiac conduction system. The signal starts in pacemaker cells in the sinoatrial (SA) node in the right atrium and spreads across the atria, making them contract and push blood into the ventricles. At the atrioventricular (AV) node it slows slightly so the ventricles can fill, and then a new signal makes the ventricles contract.^[4] The familiar lub-DUB sound comes from valves closing: first those between atria and ventricles, then the aortic and pulmonary valves.^[4]

A resting rate of 60 to 100 beats per minute is described as normal, and the rate rises during exercise to deliver more oxygen to muscles.^[4] The physiology review lists the same 60 to 100 range for the SA node, while giving 40 to 60 beats per minute as the intrinsic rate of the AV node, which can act as a backup pacemaker.^[5]

NHLBI notes that disease or injury can weaken the heart, and that problems in the electrical system or in the nervous and endocrine systems can impair pumping.^[1]