The outer covering of the heart is a strong, thin membrane called the pericardium. The superior vena cava, aorta, and other large blood vessels lead in and out of the heart. Smaller blood vessels nourish the heart itself.

The ventricles make up the largest part of the heart. They form the whole lower portion. Their thick, strong muscles pump blood throughout the body.

The atria form the upper, smaller part of the heart. They have thin walls, and are smaller than the ventricles. Each atrium serves as a collecting station for the blood that flows into the heart.

Source

The World Book Encyclopedia Chicago: World Book-Childcraft International, Inc., 1979

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This page was last updated on 02/08/2019.