When blood enters the heart it first enters the atria, which are above the ventricles. The right atrium receives blood that is returning from the body and is low in oxygen. The left atrium receives blood that is returning from the lungs where it has just gained oxygen. The atria contract first and push the blood down into the ventricles.
The cardiac muscle in the ventricles is thicker and stronger, and when the ventricles contract, the blood is forced out of the heart. The blood from the right ventricle goes to the lungs to pick up oxygen, and the blood from the left ventricle leaves the heart through the aortic valve and the aorta. The blood will travel through the arteries that get progressively smaller until it is in the capillaries where diffusion will occur to bring oxygen and nutrients to the cells.
This is a continually cycling process that depends on precise timing of muscle contractions to keep the blood circulating smoothly.
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