The heart is an organ about the size of your fist that pumps blood through your body. It sits in your chest. Your heart is made of muscle, and the myocardium is the thick middle layer of the heart wall. A sac called the pericardium surrounds the heart, holds it in place and protects it. The heart has four chambers, and a wall of tissue called the septum divides the right and left sides of the heart. With each heartbeat, the heart sends blood throughout your body.

Valves inside the heart keep blood moving in the right direction. The heart has four valves, and they are the tricuspid, pulmonary, mitral and aortic valves. Each heart valve is a set of flaps called cusps or leaflets that open to let blood out of a chamber and close so the chamber can refill. A heartbeat makes a “lub-DUB” sound. The “lub” is the sound of the valves between the upper and lower chambers closing, and the “dub” is the aortic and pulmonary valves closing after the ventricles squeeze blood out. The ventricles, the two lower chambers of the heart, pump blood out of the heart. Between beats, the heart relaxes and its ventricles fill with blood.

Arteries carry blood away from the heart, and veins carry blood back to the heart. Blood travels through three main kinds of blood vessels: arteries, veins and capillaries. Capillaries are tiny blood vessels that connect small arteries to small veins, and your body contains about 60,000 miles of blood vessels. Two large veins, the superior and inferior vena cava, bring oxygen-poor blood into the right atrium. The pulmonary artery carries oxygen-poor blood from the heart to the lungs, and oxygen-rich blood returns from the lungs to the heart through the pulmonary veins. The pulmonary circulation is the low-pressure circuit from the right side of the heart through the lungs and back to the left atrium. The systemic circulation carries blood from the left side of the heart through the aorta to the body’s tissues and back to the right atrium.

The heartbeat begins in the sinoatrial (SA) node, a cluster of pacemaker cells in the right atrium. The SA node is the heart's pacemaker and sets the rate of the heartbeat. The cells of the SA node, AV node, bundle branches and Purkinje fibers are specialized heart-muscle cells whose resting electrical level drifts upward on its own until it fires a contraction signal. The electrical signal first makes the atria contract and push blood into the ventricles. At the atrioventricular (AV) node the signal slows down slightly, giving the ventricles time to finish filling with blood. After the AV node, the signal passes along the bundle of His to the Purkinje fibers, which make the ventricles contract and pump blood out. Heart-muscle cells form a branched network joined by intercalated discs, which pass signals from cell to cell so the cells contract together.

The period of heart contraction is called systole and the period of relaxation is called diastole. Diastole, the relaxing phase, is the longer of the two phases, which lets the heart rest between contractions. The left ventricle makes the high pressure needed to send blood around the whole body, and blood pressure is greatest in the left ventricle and the aorta. Blood pressure is written as two numbers, such as 120/80, with the systolic number first and the diastolic number second. Systolic blood pressure is the pressure on the artery walls while the ventricles pump blood out, and diastolic blood pressure is the lower pressure measured between beats, while the heart is filling with blood. Blood pressure in the capillaries is usually about 20 to 30 mmHg, far below the pressure in the large arteries.

As of November 2026, a healthy blood pressure for adults is a systolic number under 120 and a diastolic number under 80 mm Hg. Blood pressure is lower in children than in adults, rises with age, drops during sleep and rises during exercise. Cardiac output is the volume of blood pumped by each ventricle per minute, found by multiplying stroke volume by heart rate. In a healthy adult at rest, cardiac output averages a little over five liters of blood per minute. During vigorous exercise, cardiac output can rise to as much as five times its resting level. Your heart pumps more or less blood depending on what your body needs.

Your nervous system helps control your heart rate. Sympathetic nerves speed up the heart and contract it harder, while parasympathetic activity slows the heart rate. Hormones such as epinephrine and norepinephrine, together with pressure and oxygen sensors in the body, adjust cardiac output to fit the body’s needs. At rest, a heart rate of 60 to 100 beats per minute is normal for adults, and athletes typically have lower resting heart rates than other people, sometimes even in the 40s. As of November 2026, heart rate varies inversely with animal size: an elephant’s heart averages about 25 beats per minute and a canary’s about 1,000. The heart makes a hormone too: the atria release atrial natriuretic peptide (ANP) when atrial pressure rises, and it helps the kidneys remove salt and water and lowers blood pressure.

The heart has its own blood vessels, called coronary arteries, that supply the heart muscle with blood. The coronary arteries branch off the aorta and bring oxygen-rich blood to the heart muscle itself. Human blood is made of red blood cells, white blood cells, platelets and a liquid called plasma. Before birth, the foramen ovale lets blood pass from the right atrium to the left; at birth, higher pressure in the left atrium presses the two flaps together and closes it.

The Greek physician Galen showed that arteries carry blood, not air, correcting what had been taught for 400 years. Before Harvey, blood was not thought to circulate; people believed the body used it up as fast as it was made. In 1628 English physician William Harvey published his book on the motion of the heart and blood, the first to show the full circulation of the blood. Harvey said the valves in veins led him to circulation, and showed that they all point toward the heart. Harvey could not see capillaries; their existence was not confirmed until later in the 17th century, after the microscope was invented.

French physician René Laënnec invented the stethoscope in 1816, and his original one was a wooden tube. An electrocardiogram (EKG or ECG) is a quick test that records the heart’s electrical activity. An echocardiogram is a heart ultrasound: sound waves bounce off heart tissue and a computer turns the echoes into moving pictures of the heart beating. A pacemaker is a small device that sends electrical pulses to help the heart beat at a normal rate and rhythm. In 1953 American surgeon John H. Gibbon, Jr. performed the first successful open-heart operation using a heart-lung machine, which keeps blood moving and oxygenated outside the body. On December 3, 1967, South African surgeon Christiaan Barnard led a team of surgeons in the first human heart transplant.

As of November 2026, heart disease is the leading cause of death in the United States. Aerobic exercises such as walking, running or swimming raise your heart rate and keep your heart, lungs and circulatory system healthy. Running, biking and swimming are exercises that are good for your heart, and eating a healthy, balanced diet helps keep your heart healthy. As of November 2026, adults are advised to get at least 150 minutes of moderate-intensity aerobic activity a week. Your pulse is the rate your heart beats, also called your heart rate. When you exercise, your heart beats faster to get more oxygen to your muscles.

Words to know

• pericardium: heart sac
• septum: dividing wall
• capillaries: tiny vessels
• systemic: whole-body
• cardiac: of the heart
• stethoscope: wooden-tube invention