Blood circulates through the body in a circle that travels from the heart, through the tissues, organs and lungs, and then back to the heart. Three types of blood vessels make up the circulatory system: arteries, veins and capillaries. Smaller blood vessels known as arterioles are types of arteries; venules are a type of vein. If placed end to end, blood vessels from one person would stretch over 60,000 miles, the Texas Heart Institute says. Each has a specific function; all are necessary for life.
Arteries
Arteries are tough, three-walled blood vessels that carry blood pumped from the heart. The arteries from the left ventricle carry oxygenated blood to the rest of the body; arteries from the right ventricle return de-oxygenated blood to the lungs. The contraction and relaxation of the ventricles of the heart as the heart beats force blood into the arteries and throughout the body. Large arteries that carry blood from the heart are also known as elastic or conducting arteries. Medium-sized arteries--called distributing or muscular arteries--branch off from the large arteries, and even smaller arteries known as arterioles branch off and supply oxygenated blood to the capillaries. There are 20 major arteries, according to the Texas Heart Institute. Arterial blood is bright red, because it has a high oxygen content; it also "spurts" if you cut an artery. Because arterial blood is under high pressure, you can lose a lot of blood very quickly from an arterial cut.
Veins
Veins, like arteries, are three-walled blood vessels, but their task is to carry de-oxygenated blood back to the lungs for oxygenation--with one exception. The pulmonary vein carries oxygenated blood from the lungs back to the left ventricle. Veins have valves that keep blood flowing toward the heart, because they don't have as much force pushing through them as the arteries do. Veins are the blood vessels used to draw blood and to insert intravenous catheters. Venous blood is a very dark red, because it's not oxygenated. It only appears blue under the skin because the skin refracts light, the Franklin Institute says. Only higher-energy wavelengths are reflected back to the eye and appear blue.
Capillaries
Capillaries are microscopic blood vessels that connect veins and arteries. They transport nutrients and waste between tissues and blood via the interstitial fluid. There are more capillaries in areas that require more oxygen and nutrients, such as the brain, and fewer in areas that need less. Capillaries carry waste products back to veins through venules that combine to form veins. Capillary blood flows intermittently 5 to 10 times a minute rather than continuously, the University of North Carolina Wilmington says, and can speed up or slow down in response to body requirements. Capillaries are also part of the body's cooling system. Flushing occurs when capillaries dilate to release heat, for example, the Franklin Institute says.
