Systemic circulation is the part of the cardiovascular system which carries oxygenated blood away from the heart to the body, and returns deoxygenated blood back to the heart. This physiologic theory of circulation was first described by William Harvey. This term is opposed and contrasted to the term pulmonary circulation first proposed by Ibn al-Nafis.
In the systemic circulation, arteries bring oxygenated blood to the tissues of the body. The pulmonary circulation (for arterial blood sent to the lungs) is excluded from this definition. As blood circulates through the body, oxygen diffuses from the blood into cells surrounding the capillaries, and carbon dioxide diffuses into the blood from the capillary cells. Veins bring deoxygenated blood back to the heart.
Oxygenated blood enters the systemic circulation when leaving the left ventricle, through the aortic semilunar valve. The first part of the systemic circulation is the aorta, a massive and thick-walled artery. The aorta arches and branches into major arteries to the upper body before passing through the diaphragm, where it branches further into arteries which supply the lower parts of the body.
Major arteries branch successively into smaller vessels, the arterioles.
Arterioles divide finally into capillaries, which are the thinnest and most numerous of the blood vessels. These capillaries help to carry arterial blood to the close vicinity of body cells, facilitating the diffusion of oxygen and nutrients from the blood into the cells. At the same time, carbon dioxide and waste products diffuse from the cells into the bloodstream.
After their passage through body tissues, capillaries merge once again into venules, which continue to merge into veins. The venous system finally coalesces into two major veins: the superior vena cava (roughly speaking draining the areas above the heart) and the inferior vena cava (roughly speaking from areas below the heart). These two great vessels empty into the right atrium of the heart.
The heart itself is supplied with oxygen and nutrients through a small "loop" of the systemic circulation.
The general rule is that arteries from the heart branch out into capillaries, which collect into veins leading back to the heart. Portal veins are a slight exception to this. In humans the only significant example is the hepatic portal vein which combines from capillaries around the gut where the blood absorbs the various products of digestion; rather than leading directly back to the heart, the hepatic portal vein branches into a second capillary system in the liver.
Because the systemic circulation is powered by the left ventricle (which is very muscular), one advantage of this form of circulation - as opposed to open circulation, or the gill system that fish use to breathe - is that there is simultaneous high-pressure oxygenated blood delivered to all parts of the body.
- Lua error in package.lua at line 80: module 'Module:Citation/CS1/Suggestions' not found.