The circulatory system of the human body is responsible for providing oxygen and nutrients to cells as well as removing waste products. This crucial system is composed of the heart, blood and blood vessels, which work in unison to maintain the normal function of our bodies. The three types of blood vessels--the arteries, capillaries and veins--carry blood to cells, tissues and organs throughout the body.
Arteries
Arteries carry oxygenated blood from the heart to various tissues and organs in the body, such as the brain or kidneys. The Franklin Institute in Philadelphia, Pennsylvania, explains that arteries are composed of three layers: an outer tissue layer, a muscular middle layer and a smooth inner layer. The flexible muscular layer of arteries functions to pump blood along the arteries from the heart. The smooth inner layer of arteries ensures that blood can flow through these small passageways unobstructed. The oxygen within the blood flowing along the arteries causes the arteries to appear red in color. As the heart contracts (or beats), the arteries expand and fill with blood. The arteries contract when the heart relaxes, which pumps the oxygen-rich blood along the arteries to the capillaries.
Capillaries
Capillaries are the smallest blood vessels in the body and act to carry blood to and from each cell in the body. These blood vessels are incredibly thin--the width of a capillary is equivalent to one red blood cell. As each individual blood cell passes through the capillary in single file, the oxygen within the red blood cell is released. This oxygen passes through the thin wall of the capillary and is absorbed by the surrounding tissue. In exchange, the surrounding tissue releases waste products back through the wall of the capillary into the red blood cell. The red blood cells then pass along the waste-laden blood to the veins.
Veins
The blood received by the veins from the capillaries contains very low levels of oxygen, causing veins to appear blue in color. Veins are similar to arteries and consist of three layers: an outer tissue layer, a muscular middle layer and a smooth inner. The blood is passed through veins at a lower pressure than the arteries, which means that the veins are not as muscular or thick as arteries, according to Boston Scientific in Natick, Massachusetts. Veins contain several valves, which prevent the oxygen-poor blood from flowing backwards through the circulatory system. These valves within the veins also help keep the blood flowing against the force of gravity. Vein valves ensure that blood is able to be pumped from the feet back up to the heart, where the blood will be refueled with oxygen to begin another journey along the arteries.
