The atria are the small receiving chambers for blood returning from the body (right atrium) or from the lungs (left atrium). Although the atria contract, their contraction is relatively weak and serves mainly to push blood into the ventricles.

Most people's hearts could pump sufficient blood throughout the body even if the atria did not contract at all. However, the beating of the atria contributes to the overall efficiency of the heart and allows it to pump more blood with less effort. This added efficiency is particularly important if disease damages other parts of the heart that are responsible for pumping.

Each atrium is about the size of a golf ball, or slightly larger. The walls of the right atrium are less than 1\8 inch thick. The left atrial wall is thicker and more powerful than the right atrial wall, but its volume is about the same.

The Ventricles

The ventricles are the main pumping chambers that propel the blood to the lungs (from the right ventricle) and to the body (from the left ventricle). Each holds about 1/8 cup of blood after the heart contracts and about 1/2 cup after the heart is filled.

The walls of the right ventricle are 1\4 inch thick. The walls of the left ven¬tricle are three times thicker; the left ventricle is thus by far the most power¬ful chamber in the heart. The left ven¬tricle must generate enough pressure

to drive the blood to every part of the body.

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Cardio & Blood