Blood vessels from a closed circuit of tubes that carries blood from the heart throughout the rest of the body. These include arteries, arterioles, capillaries, venules, and veins There are about 62,000 miles of blood vessels in the average human.
Arteries
Arteries are strong vessels that carry blood away from the heart under high pressure. They divide into smaller vessels throughout the body, called arterioles. The wall of an artery has three layers. The inner layer, the tunica interna, is made of simple squamous epithelium that lines a connective tissue layer. This layer prevents blood clots from forming by providing a smooth surface for blood cells to pass by. The middle layer is called the tunica media, which consists of smooth muscle fibers and elastic connective tissue. The outer layer, the tunica externa, attaches the artery to the surrounding tissues, and is made up of connective tissue with abnormal elastic and collagenous g |
Capillaries
Capillaries are the smallest blood vessels in the human body, and are responsible for connecting the smallest arterioles and the smallest venules. Their walls are made of endothelium, which is semipermeable, allowing some substances in the blood to be released or absorbed by the surrounding tissues. The density of a capillary depends on the type of tissue. This also reflects the rate of metabolism of the surrounding tissues. For example, muscle and nerve fibers contain capillaries due to their need for oxygen and nutrients, while cartilage and the cornea lack capillaries altogether. Blood flow within capillaries is regulated by the smooth muscles that encircle these blood vessels. They form precapillary sphincters, which contract to control the amount of blood flow depending on the needs of a particular cell. |
Veins
Veins are formed by the merging of many venules, microscopic blood vessels that are extensions of capillaries. Veins carry blood towards the atria of the heart. The walls of veins are composed of three layers similar to arteries. However, they are much thinner due to the middle layer not being as thick. The walls of veins have less smooth muscle and less elastic connective tissue than arteries. Many veins have flap-like valves, which are responsible for returning blood to the heart from the rest of the body. They also serve as blood reservoirs, by constricting to help maintain blood pressure. Even if a lot of blood is lost, veins have the ability to maintain a normal rate of blood flow throughout the body. |