How the Heart Works: Chambers, Valves and the Beat

The heart has four chambers. The two upper chambers are the atria (one is an atrium), and the two lower chambers are the ventricles. A wall of tissue called the septum separates the left side from the right, according to the American Heart Association (AHA).

In general terms, the atria receive blood and the ventricles push it out. The right side handles blood that is low in oxygen, and the left side handles blood that is rich in oxygen. Because the left ventricle has to drive blood around the whole body, the US National Heart, Lung, and Blood Institute (NHLBI) describes it as generating "the high pressure needed to pump the blood to your whole body."

Blood only works if it keeps moving forward. Four valves act as one-way doors, opening and closing to let blood flow in a single direction. Each sits between two spaces:

• The tricuspid valve, between the right atrium and right ventricle.
• The pulmonary valve, between the right ventricle and the pulmonary artery.
• The mitral valve, between the left atrium and left ventricle.
• The aortic valve, between the left ventricle and the aorta.

The AHA adds that each valve has flaps, also called leaflets or cusps. The mitral valve normally has two, and the others have three. When a valve does not open fully or does not close tightly, the problem has its own page: see heart valve problems.

It helps to picture blood travelling a figure of eight, with the heart at the crossing point. The AHA describes the sequence as body, heart, lungs, heart, body.

In the first loop, blood low in oxygen returns from the body through veins and enters the right atrium. It passes into the right ventricle, which pumps it through the pulmonary artery to the lungs. There the blood picks up oxygen. In the second loop, oxygen-rich blood comes back from the lungs through the pulmonary veins into the left atrium, moves into the left ventricle and leaves through the aorta, the body's main artery, to reach the rest of the body (NHLBI).

The AHA notes that the pressure generated by the left ventricle shows up as the arterial blood pressure measured at the arm. That link between the heart and blood pressure is explored in high blood pressure explained.

The heart does not wait for instructions from the brain to beat. It has its own electrical system, which NHLBI says controls the rate and rhythm of the heartbeat. Each beat starts in a small cluster of pacemaker cells called the sinoatrial (SA) node, located in the right atrium.

From there, NHLBI describes four steps. The signal spreads through the atria, which pump blood into the ventricles. It then reaches the atrioventricular (AV) node, where it slows briefly so the ventricles can finish filling. The signal travels along the ventricle walls, and the ventricles contract to send blood out. Then they relax, and the cycle begins again at the SA node.

NHLBI states that "at rest, a heart rate of 60 to 100 beats per minute is normal." When the rhythm itself goes wrong, one well-known example is covered in atrial fibrillation explained.

The blood inside the chambers does not nourish the heart muscle. The muscle has its own supply line: the coronary arteries. NHLBI says these arteries supply blood to the heart and branch from the aorta. The left coronary artery and its branches serve the left atrium and much of the left ventricle, while the right coronary artery and its branches serve the right atrium and parts of both ventricles. Coronary veins then carry low-oxygen blood from the heart muscle back to the right atrium.

This supply line matters because a heart that pumps blood for everyone else still depends on these narrow vessels for its own oxygen. A heart attack involves this blood supply, and the warning signs and what to do are set out on the page about heart attack signs.

NHLBI notes that a healthy heart supplies the body with blood at the rate the body needs, and that disease or injury can leave organs short of blood. NHLBI adds that problems with the electrical, nervous or endocrine systems can make pumping harder. What happens when the pump cannot keep up is covered in heart failure basics.

Speak to a doctor or nurse about palpitations that keep returning, breathlessness that is new for you, fainting, or swelling in the legs. Sudden chest pain or pressure that does not go away, especially with sweating, nausea or breathlessness, needs emergency care. Emergency numbers differ by country, so use your local emergency number. This page is general education and cannot assess anyone's own symptoms.

The heart is a four-chambered muscular pump with four one-way valves. The right side sends blood to the lungs, the left side sends it to the body, and an internal electrical system sets the rhythm. The coronary arteries feed the heart muscle itself, which is why blockages there are so serious.