What the heart does

The heart contains four chambers. The heart pumps blood by contracting (squeezing blood out of its chambers) and relaxing (allowing blood to enter its chambers). The two upper chambers are called the atria. They receive blood returning from the body through veins. The two atria contract with only a small amount of force, sending the blood to the ventricles; the ventricles are the muscular, lower chambers of the heart. The ventricles contract with greater force, delivering blood to the body via your arteries. The right ventricle pumps blood to the lungs. Of all the chambers in the heart, the left ventricle does the greatest amount of work. Powerful contraction of the left ventricle sends oxygen-rich blood to all of the body’s organs.

The heart also has four valves. Their purpose is to ensure that blood flows only in one direction. The heart’s valves open at the appropriate times to allow forward flow of blood, then close to prevent back-flow of blood. The mitral and tricuspid valves control the flow of blood from the atria to the ventricles. The aortic and pulmonic valves control the flow of blood out of the ventricles. The image below shows the chambers and valves of the heart.

The purposes of blood are to deliver oxygen and nutrients to tissues and organs and to carry away waste products. There are actually two loops of circulation in the body.

The first is a circulation loop which goes from the heart to the lungs, where blood is mixed with oxygen and carbon dioxide in the blood is released. Carbon dioxide is a byproduct that the body makes when it uses oxygen. The other circulation loop goes to the body, where oxygen and other nutrients in the blood are exchanged for carbon dioxide and waste products produced by the body. Veins are the blood vessels which return blood to the heart. Arteries are the blood vessels which carry blood away from the heart to the lungs and the rest of the body.

The muscle of the heart (the myocardium) produces the forces necessary to eject (push) blood from the ventricles. When the ventricles contract, they force blood out of the heart through valves and into the aorta (from the left ventricle) or pulmonary artery (from the right ventricle). This phase of the heart cycle—when the ventricles are contracting and forcing blood from the heart— is called “systole."

After contracting, the ventricles must relax to receive the next volume of blood that needs to be pumped. When the ventricles relax, blood flows into them from the atria. This phase of the heart’s activity is called “diastole.’’

The combination of contraction (systole) and relaxation (diastole) is called the cardiac cycle. You may have encountered these terms when your blood pressure is measured from an artery in your arm. Your blood pressure measurement includes two numbers, which correspond to systole and diastole in your cardiac cycle. For example your blood pressure may be 120 over 80. 120 is your systolic blood pressure, representing the pressure generated by contraction of your left ventricle. 80 is called the diastolic blood pressure, and is the blood pressure in your body during the time when your left ventricle is relaxed and receiving blood.

Like every other organ or muscle in the body, the heart has arteries that carry blood nutrients and oxygen. The heart’s arteries are called the coronary arteries. The three coronary arteries are the right coronary artery, the left anterior descending coronary artery, and the circumflex coronary artery. When these arteries become narrowed by build-up of cholesterol-containing plaque, a person is said to have coronary artery disease. If a coronary artery becomes completely blocked, a portion of the heart muscle is deprived of its blood supply, and a heart attack occurs. A heart attack causes damage to the heart muscle as some muscle cells die.