Heart Chambers and Circulation through the Heart
The human heart consists of four chambers: The left side and the right side each have one atrium and one ventricle. Each of the upper chambers, the right atrium (plural = atria) and the left atrium, acts as a receiving chamber and contracts to push blood into the lower chambers, the right ventricle and the left ventricle. The ventricles serve as the primary pumping chambers of the heart propelling blood to the lungs or to the rest of the body.
There are two distinct but linked circuits in the human circulation called the pulmonary and systemic circuits. The pulmonary circuit transports blood to and from the lungs, where it picks up oxygen and delivers carbon dioxide for exhalation. The systemic circuit transports oxygenated blood to virtually all of the tissues of the body and returns to the heart.
The right ventricle pumps deoxygenated blood into the pulmonary trunk which leads toward the lungs and splits into the left and right pulmonary arteries. These vessels in turn branch many times before reaching the pulmonary capillaries where gas exchange occurs: Carbon dioxide exits the blood and oxygen enters. The pulmonary trunk arteries and their branches are the only arteries in the body that carry relatively deoxygenated blood (blue blood). Highly oxygenated blood returning from the pulmonary capillaries in the lungs passes through a series of vessels that join together to form the pulmonary veins—the only veins (red) in the body that carry highly oxygenated blood. The pulmonary veins (red) bring blood into the left atrium which delivers the blood into the left ventricle which in turn pumps oxygenated blood into the aorta and on to the many branches of the systemic circuit. Eventually, these vessels will lead to the systemic capillaries where exchange with the tissue fluid and cells of the body occurs. In this case, oxygen and nutrients exit the systemic capillaries to be used by the cells in their metabolic processes, and carbon dioxide and waste products will enter the blood.
The blood that has traveled throughout the body is lower in oxygen concentration than when it entered. The superior vena cava and the inferior vena cava return blood to the right atrium. The blood in the superior and inferior venae cavae flows into the right atrium, which delivers blood into the right ventricle. This process of blood circulation continues as long as the individual remains alive. (Figure).