How The Heart Works

3. How The Heart Works

Each side of the heart performs a different pumping job. The right side takes blood from the body and pumps it to the lungs. The left side collects blood from the lungs and pumps it to the body.

Blood entering the right side of the heart contains carbon dioxide, a waste product of the body. All blood entering the right side of the heart goes to the lungs before it reaches the left side of the heart. In the lungs, the carbon dioxide is removed, and oxygen is added to the blood, blood that flows to the body from the left side of the heart contains fresh oxygen. The oxygen is used in the body cells to produce energy.

Blood from the body flows into the right atrium through two large veins. One of these veins, the superior vena cava, carries blood from the head and arms. The other vein, the inferior vena cava, carries blood from the trunk and legs.

Blood from the body fills the right atrium. The atrium then contracts, squeezing blood through the tricuspid valve into the ventricle. The tricuspid valve is made of three little triangular flaps of thin, strong fibrous tissue. These flaps permit the blood to flow into the ventricle, but they prevent it from flowing back into the atrium. They are like doors that open only in one direction.

At first, the ventricle is relaxed, but it contracts when it is filled with blood. The resulting pressure closes the tricuspid valve and opens the semilunar valve between the ventricle and the pulmonary artery. Blood gushes through the valve into the artery, which leads to the lungs. The valve is called semilunar because it has three flaps that are shaped like half-moons. Blood squeezed from the ventricle pushes the flaps against the walls of the pulmonary artery.

From the lungs, the blood flows back to the heart through the four pulmonary veins. It flows out of the pulmonary veins into the left atrium. The left atrium, like its neighbor on the right, then contracts, squeezing blood through the mitral valve into the left ventricle. The mitral valve is similar to the tricuspid valve, except that it has only two flaps. The left ventricle contracts, forcing blood through another semilunar valve into the aorta. The aorta, with its numerous branches, carries blood throughout the body.