Blood Vessel Anatomy
There are five types of blood vessels in the human body. These are the arteries, arterioles, capillaries, veins and venules. Arteries carry blood away from the heart. As the arteries get smaller in diameter they are called arterioles. Arterioles lead into very small capillaries. The capillaries enable the exchange of water, oxygen, carbon dioxide, and many other nutrient and waste chemical substances between the blood and surrounding tissues. From the capillaries, the blood flows into venules which then flow into larger veins returning blood to the heart.
Veins and arteries have the same internal structure. The inner most layer consists of endothelial cells surrounded by an elastic-like tissue. The endothelial cells are in direct contact with the blood. The next outer layers are elastic fiber and connective tissue containing vascular smooth muscle. These muscles control the diameter of the blood vessel. This layer allows the blood vessel to expand and contract, controlling the amount of blood flow and blood pressure. By controlling the blood vessel diameter, the body controls distribution blood to areas where it is needed most, such as muscles during exercise. The outer layers are connective tissue that contain nerves control the muscles, and small capillaries providing nutrients to the cells.
Veins have one feature that is not seen in arteries or capillaries. When the heart contracts, it creates high blood pressure forcing blood through the arteries. By the time the blood is returning to the heart in veins, the blood is now at a much lower pressure. This potentially makes it difficult for the blood to flow upward from the feet back to the heart. Gravity could cause the blood to flow back toward the feet. To prevent blood from flowing backward, veins have one way valves allowing the blood to flow in one direction, toward the heart. This is somewhat like climbing a ladder. Each heart beat, and corresponding pulse of blood, pushes the blood up another valve or rung of the ladder, until it eventually reaches the heart.