The circulatory system, made up of a pumping organ--the heart--and a network of pipes--the blood vessels--is responsible for carrying oxygen and nutrition toward and metabolic waste away from the body cells. While the circulatory system forms a single, closed loop, it’s actually made up of two subsystems: systemic and pulmonary circulation. Working together, these subsystems ensure that blood reaching the body cells is always high in oxygen.
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Destination of Blood
The primary distinction between the systems is that systemic circulation runs to and from the body cells. Pulmonary circulation, however, runs to and from the oxygen-exchange surfaces of the lungs. Each circulatory subsystem forms a loop, and the two are connected by the heart. A schematic of circulation would look a bit like a figure eight, notes Lauralee Sherwood, Ph.D., in her text “Human Physiology.” Systemic circulation, the larger loop of the figure eight, involves blood flow from the heart to the body cells and back again, while pulmonary circulation, the smaller loop of the figure, involves blood flow from the heart to the lungs and back again.
Oxygenation of Blood
Unlike systemic circulation, pulmonary circulation does not provide cells with nutrition and oxygen. Instead, it serves the purpose of reoxygenating blood. As oxygen-deficient blood returns to the heart from the body cells via systemic circulation, it is sent to the lungs. Here, the blood picks up oxygen, and then returns to the heart for systemic distribution. Blood leaving the heart, therefore, is in different states of oxygenation depending upon which system it’s entering. Blood leaving the heart via the arteries of systemic circulation is oxygenated, and this same blood returns in a deoxygenated state to the heart via the veins of systemic circulation. It then leaves the heart via the arteries of pulmonary circulation, travels to the lungs, and returns as oxygenated blood via the pulmonary veins.
Chambers of the Heart
To keep oxygenated blood returning from the lungs separate from deoxygenated blood returning from the body, the heart is composed of four chambers forming two distinct sides. The right side of the heart, notes Dr. Gary Thibodeau in his text, “Anatomy and Physiology,” receives deoxygenated blood from the body cells into a chamber called the right atrium, and sends that blood to the lungs via a chamber called the right ventricle, through which the blood enters pulmonary circulation. The left atrium of the heart receives oxygenated blood from the lungs, and sends that blood out to the body cells via the left ventricle, through which the blood enters systemic circulation. In this way, the two subsystems of circulation are kept distinct from one another, and all blood is forced through pulmonary circulation before it can return to systemic circulation, ensuring optimal oxygenation.