The heart is an organ about the size of your fist that pumps blood through your body. Your heart is in your chest, and with each heartbeat the heart sends blood throughout your body. Your heart is made of muscle, and you can feel your pulse on the inside of your wrist. Heart-muscle cells form a branched network joined by intercalated discs, which pass signals from cell to cell so the cells contract together. The heartbeat begins in the sinoatrial (SA) node, a cluster of pacemaker cells in the right atrium, and the SA node is the heart's pacemaker and sets the rate of the heartbeat. Each heartbeat starts with a signal from the SA node, and the 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. The period of heart contraction is called systole and the period of relaxation is called diastole, and 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. Between beats, the heart relaxes and its ventricles fill with blood. At rest, a heart rate of 60 to 100 beats per minute is normal for adults. To find your heart rate, count the beats you feel in 30 seconds and double the number.

The Greek physician Galen showed that arteries carry blood, not air, correcting what had been taught for 400 years, and 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. French physician René Laënnec invented the stethoscope in 1816, and his original one was a wooden tube. On December 3, 1967, South African surgeon Christiaan Barnard led a team of surgeons in the first human heart transplant. An electrocardiogram (EKG or ECG) is a quick test that records the heart’s electrical activity. An echocardiogram is a heart ultrasound, and a pacemaker is a small device that sends electrical pulses to help the heart beat at a normal rate and rhythm.

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 heart has its own blood vessels, called coronary arteries, that supply the heart muscle with blood. 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. When you exercise, your heart beats faster to get more oxygen to your muscles, and 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, and running, biking and swimming are exercises that are good for your heart. Eating a healthy, balanced diet helps keep your heart healthy.

Words to know

• pacemaker: beat setter
• diastole: relaxing phase
• coronary: heart's own
• echocardiogram: heart ultrasound
• cardiac: of the heart