How does the heart impel the blood?

The heart is a muscular organ that having chambers (right atrium and right ventricle and left atrium and right ventricle) by which blood passes. The blood enters the heart in the atria, goes to the ventricles and then leaves the organ.

The blood is pumped out of the heart by the contraction of the muscle fibers that type the ventricular walls. The contraction reduces the ventricle volume therefore increasing the internal pressure and the blood then flows to the exit vessels (pulmonary artery for the right ventricle and aorta for the left ventricle). When ventricular muscle fibers distend the ventricles regain their original size and receive new blood flow coming from the atria.

The Circulatory System - Image Diversity: heart structure