The function of the human circulatory system is to transport oxygen and nutrients to the tissues, but also to transport wastes to the kidneys and lungs for excretion. The system, known collectively as circulation, is a series of blood vessels filled with plasma and blood cells. What is contained within the plasma and what is bound to red blood cells depends on the vessels and direction of flow at that point in the body.
The circulation consists of arteries and veins. In both the systemic and pulmonary circulation, arteries carry blood away from the heart, and veins transport blood to the heart. The function of the entire system rests on the pulmonary circulation. Any disruption in this relatively short course makes the systemic circulation obsolete. The pulmonary vessels shuttle blood between the heart and lungs. It is in the lungs where gas exchange takes place. Without this exchange, the systemic circulation cannot do its job.
Both systems are closed systems, in that blood that flows from the heart returns to the heart. The relay between arteries and veins in both systems are capillaries, thin walled vessels the width of a single red blood cell. The structure of the capillary allows the passage of the blood cells and the easy exchange of nutrients. In the lungs, the capillaries are adjacent to thin walled air sacs that allow for efficient gas exchange.
The major differences between the systemic and pulmonary circulations are:
-The systemic circulation consists of different types (muscular, elastic) of blood vessels, which branch into smaller sizes into all tissues. The pulmonary circulation consists of two main blood vessels, which branch off only into the lungs.
-In the systemic circulation, arteries carry oxygenated blood to the tissues from the left ventricle of the heart. In the pulmonary circulation, the pulmonary artery carries deoxygenated blood to the lungs via the right ventricle.
-In the systemic circulation, veins carry deoxygenated blood to the heart, emptying into the right atrium of the heart. In the pulmonary circulation, the pulmonary vein carries oxygenated blood to the heart, emptying into the left atrium.
-In the systemic tissues, the blood delivers oxygen and carries away carbon dioxide. In the lungs, the blood exchanges carbon dioxide for oxygen.
-The systemic circulation not only delivers oxygen to the tissues (exchanging it for carbon dioxide), but it picks up nutrients from other tissues, such as the intestines, and delivers cellular wastes to the kidneys. The pulmonary circulation is concerned primarily with gas exchange, making it more specialized.
-Though the circulation in the lungs consists of a lot of surface area to fulfill its functions (an estimated 500-1000 square feet of capillary surface area), the systemic circulation is larger. In total, all of the blood vessels in the body are estimated to cumulatively stretch to 60,000 miles in length (Reference: Henry S. Cabin, MD. “The Heart and Circulation” from the Yale Medical Library)