READ: Cardiovascular and Respiratory Systems

The Heart

What is the heart? How does it pump blood? The heart is divided into four chambers, or spaces: the left and right atria, and the left and right ventricles. An atrium (singular for atria) is one of the two small, thin-walled chambers on the top of the heart where the blood first enters. A ventricle is one of the two muscular V-shaped chambers that pump blood out of the heart. You can remember they are called ventricles because they are shaped like a "V." The four chambers of the heart are shown in Figure below.

The atria receive the blood, and the ventricles pump the blood out of the heart. Each of the four chambers of the heart has a specific job.

• The right atrium receives oxygen-poor blood from the body.
• The right ventricle pumps oxygen-poor blood toward the lungs.
• The left atrium receives oxygen-rich blood from the lungs.
• The left ventricle pumps oxygen-rich blood out of the heart to the rest of the body.

Where is the Heart?

The heart is closer to the center of the body than you may think. It is usually found in the left to middle of the chest, with the largest part of the heart slightly to the left. It always feels like the heart is on the left side of the body because the left ventricle is bigger and stronger than the right ventricle. The heart is also surrounded by the lungs.

Blood Flow Through the Heart

Blood flows through the heart in two separate loops. You can think of them as a “left side loop” and a “right side loop." The right side of the heart collects oxygen-poor blood from the body and pumps it into the lungs, where it releases carbon dioxide and picks up oxygen. The left side carries the oxygen-rich blood back from the lungs into the left side of the heart, which then pumps the oxygen-rich blood to the rest of the body.