The heart is an organ about the size of your fist that pumps blood through your body, and with each heartbeat, the heart sends blood throughout your body. 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. 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 SA node is the heart's pacemaker and sets the rate of the heartbeat. At the AV node the electrical 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.

Your pulse is the rate your heart beats, also called your heart rate. To find it, count the beats you feel in 30 seconds and double the number. 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. 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. Your heart pumps more or less blood depending on what your body needs. When you exercise, your heart beats faster to get more oxygen to your muscles. 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, it can rise to as much as five times its resting level.

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 it is the highest pressure in the cycle, while diastolic blood pressure is the lower pressure measured between beats, while the heart is filling with blood. The period of heart contraction is called systole and the period of relaxation is called diastole, and diastole is the longer of the two phases, which lets the heart rest between contractions. 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.

A heartbeat makes a “lub-DUB” sound, and the “lub” of a heartbeat is the sound of the valves between the upper and lower chambers closing. 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, and 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.

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. The pulmonary circulation is the low-pressure circuit from the right side of the heart through the lungs and back to the left atrium, and 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. After the blood delivers oxygen, it returns to the heart, and the heart sends it to the lungs to pick up more oxygen.

Words to know

• sympathetic: speed-up nerves
• systolic: top number
• stethoscope: wooden-tube invention
• electrocardiogram: electrical record
• circulation: blood loop