The heart is an organ about the size of your fist that pumps blood through your body, and it sits in your chest. With each heartbeat, the heart sends blood throughout your body, and you can feel the beat as your pulse on the inside of your wrist. Your pulse is the rate your heart beats, which is 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.

Your heart is made of muscle, and the myocardium is the thick middle layer of the heart wall, made of muscle. Your heart sits between your lungs, a little to the left of the middle of your chest. Every heartbeat makes a “lub-DUB” sound. The “lub” of a heartbeat is the sound of the valves between the upper and lower chambers closing. The heart has four chambers and four valves, and valves inside the heart keep blood moving in the right direction. The ventricles, the two lower chambers of the heart, pump blood out of the heart.

Something sets the schedule for each beat. Each heartbeat starts with an electrical signal from the sinoatrial (SA) node, which is the heart's pacemaker and sets the rate of the heartbeat. The electrical signal first makes the upper chambers contract and push blood into the ventricles. Then the atrioventricular (AV) node carries the heart's electrical signal from the upper chambers to the lower chambers. The electrical signal travels down the Purkinje fibers and makes the ventricles contract. Between beats, the heart relaxes and its ventricles fill with blood. Your nervous system helps control your heart rate.

Each beat sends blood to every cell in your body, and blood carries oxygen and nutrients to the cells of your body. Human blood is made of red blood cells, white blood cells, platelets and a liquid called plasma. Arteries carry blood away from the heart, and veins carry blood back to the heart. Blood carries carbon dioxide to your lungs. After the blood delivers oxygen, it returns to the heart, and the heart sends it to the lungs to pick up more oxygen. Your body contains about 60,000 miles of blood vessels.

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. Running, biking and swimming are exercises that are good for your heart. Aerobic exercises such as walking, running or swimming raise your heart rate and keep your heart, lungs and circulatory system healthy. As of November 2026, adults are advised to get at least 150 minutes of moderate-intensity aerobic activity a week. Then you can press two fingers to your wrist and feel your heart keeping time.

Words to know

• pulse: heart rate
• valves: one-way doors
• pacemaker: beat setter
• ventricles: lower chambers