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Arteries, veins, and capillaries are in fact all forms of blood vessels, just with different shapes and roles in the body. Blood vessels are an integral part of the circulatory system, which transfers oxygen and important components of life around the body and removes waste. Each of the three major types of blood vessels play their own role in this complex system, helping to keep a human body functioning at full strength and health.
The arteries are those blood vessels that carry blood away from the heart. This means that, with only two exceptions, arteries are carrying highly oxygenated blood to transport oxygen to the tissue of the body. Arteries are the higher-pressure part of the circulatory system, as they are getting blood from the heart. The pressure in the arteries differs between when the heart contracts and when it expands, the systolic and diastolic pressure, respectively. It is this pressure shift that can be felt as a pulse.
The largest artery in the body is the aorta, in the heart. The aorta receives blood from the heart’s left ventricle, then branches off into smaller and smaller arteries, eventually turning into arterioles, which supply the capillaries with blood. Pulmonary arteries are another special type of artery, which carry deoxygenated blood from the heart to the lungs, when they can be replenished, disposing of their carbon monoxide and gathering oxygen.
Veins are those blood vessels that carry blood back to the heart, with a few minor exceptions. For the most part, veins are carrying deoxygenated blood back to the heart, although this is not the case in either pulmonary or umbilical veins, where they carry oxygenated blood. Veins are basically tubes that just collapse when not filled with blood. Within veins are flaps that keep the blood flowing towards the heart, rather than being pulled down and pooling by the effects of gravity.
The blood carried by veins, in addition to having little oxygen, is also filled with carbon dioxide and various forms of cellular waste. Blood moves through the veins back to the heart, where it enters in the right ventricle, where it is then pumped into the lungs by the pulmonary artery, and then back through the heart via the left atrium.
Both veins and arteries are most easily defined not by the oxygen content of the blood, which is generally high for arteries and low for veins, but not always, but rather by the direction of blood flow. Arteries are always moving blood away from the heart, while veins are always moving blood towards the heart. Capillaries, on the other hand, act as intermediaries, connecting arterioles and venules.
Capillaries serve the function in the circulatory system of helping to facilitate the exchange of various things between blood and tissue. When the arteries bring blood to an area of tissue, they pump the blood into the capillaries, which can then essentially drop off the oxygen, water, and nutrients. The tissue can then dispose of its cellular waste and carbon dioxide, which the capillaries then pump back into the veins to be returned to the heart and lungs.