Now, the atrium is the portion of the heart. It's the compartment of the heart that accepts the blood. But it's the ventricle that contains the strong cardiac muscle that is responsible for creating that contraction and that hydrostatic pressure that moves the blood through the blood vessels of our body. So let's begin at a certain portion of our cardiovascular system. Let's suppose we begin in the right atrium of the heart. So number one, so the deoxygenated blood returns to the right atrium of the heart.