The heart is an organ about the size of your fist that pumps blood through your body. With each heartbeat, the heart sends blood throughout your body. Arteries carry blood away from the heart, and veins carry blood back to the heart. Capillaries are tiny blood vessels that connect small arteries to small veins, and your body contains about 60,000 miles of blood vessels. Your lungs add oxygen to your blood.

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. Today the loop can be followed in detail. After the blood delivers oxygen, it returns to the heart, and the heart sends it to the lungs to pick up more oxygen. Oxygen-rich blood returns from the lungs to the heart through the pulmonary veins, and the left ventricle pumps oxygen-rich blood to the whole body.

The aorta is the body's largest artery, and it carries oxygen-rich blood out of the heart. The pulmonary artery carries oxygen-poor blood from the heart to the lungs. A wall of tissue called the septum divides the right and left sides of the heart. Valves inside the heart keep blood moving in the right direction, and the heart has four valves. Blood travels through three main kinds of blood vessels: arteries, veins and capillaries.

A heartbeat makes a “lub-DUB” sound. The “lub” of a heartbeat 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. 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.

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. A pacemaker is also a small device that sends electrical pulses to help the heart beat at a normal rate and rhythm. At rest, a heart rate of 60 to 100 beats per minute is normal for most adults, and athletes typically have lower resting heart rates than other people, sometimes even in the 40s.

Blood pressure is written as two numbers, such as 120/80, with the systolic number first and the diastolic number second. As of November 2026, a healthy blood pressure for adults is a systolic number under 120 and a diastolic number under 80 mm Hg. 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. With each heartbeat, the heart sends blood throughout your body.

Words to know

• circulation: blood loop
• physician: doctor
• electrocardiogram: electrical record
• pacemaker: beat setter
• capillaries: tiny vessels