The Blood Vessels
The blood vessels are the tubes through which blood passes as it moves from one part of the body to another. They represent a closed system that allows substances to travel to or from any body cell, with the heart as the central pumping organ that allows circulation to occur. Blood vessels are of three major types.
- Arteries: Arteries are major vessels that carry blood away from the heart. Arterioles are smaller branches of arteries that ultimately lead to capillaries or capillary beds. Since the blood in arteries and arterioles has recently been pumped into them by the heart, it is under considerable pressure, and the walls of arteries are thickened to withstand this blood pressure. They consist of a layer of specialized epithelium called endothelium, and are reinforced by smooth muscle and connective tissue rich in elastin.
- Capillaries: Capillaries are the smallest and most permeable of the blood vessels, and it is here that the major function of the cardiovascular system is carried out. Composed of a layer of endothelium only one cell thick, the walls of capillaries allow the diffusion of materials between the blood and the tissue fluid surrounding body cells. This includes the diffusion of nutrients and oxygen into cells, and the diffusion of carbon dioxide and other waste molecules out of cells. In addition, plasma fluid is forced through capillary walls due to the pressure of the blood. Most of this fluid is reabsorbed by the venules which are connected to the other end of the capillaries, but some fluid remains in the intercellular spaces. This fluid is normally returned to the cardiovascular system via the lymphatic system.
- Veins: After blood passes through an artery, arteriole, and capillary, the exchange of substances between blood and tissue is complete. The blood must now be returned to the heart, and this is accomplished by its passage through venules (small veins connected to capillaries) and veins. Veins are major vessels that carry blood towards the heart. By this point, most of the initial blood pressure created by the heart has dissipated, so blood flow through veins occurs by contraction of muscles surrounding veins. Valves ensure a unidirectional flow of blood through the veins towards the heart.