The heart is an organ about the size of your fist that pumps blood through your body. The heart has four chambers and four valves. Valves inside the heart keep blood moving in the right direction. 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. The four heart valves are the tricuspid, pulmonary, mitral and aortic valves.
A heartbeat makes a “lub-DUB” sound, and both parts come from valves. The “lub” of a heartbeat is the sound of the valves between the upper and lower chambers closing. The “dub” of a heartbeat 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.
A wall of tissue called the septum divides the right and left sides of the heart. 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. Oxygen-rich blood returns from the lungs to the heart through the pulmonary veins.
The period of heart contraction is called systole, and the period of relaxation is called diastole. 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. Diastolic blood pressure is the lower pressure measured between beats, while the heart is filling with blood.
Your pulse is the rate your heart beats, also called your heart rate. To find your heart rate, 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. When you exercise, your heart beats faster to get more oxygen to your muscles. Your heart pumps more or less blood depending on what your body needs.
A typical adult heart weighs about 10 ounces, or about 300 grams, and it sits between your lungs, a little to the left of the middle of your chest, while your body contains about 60,000 miles of blood vessels. Heart rate is a number that changes from one kind of animal to the next. 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.
Words to know
- cusps: valve flaps
- septum: dividing wall
- aorta: largest artery
- systole: squeezing period
- diastole: relaxing period
