Blood Vessel Anatomy
We need to briefly discuss the anatomy of the vessels. There are three types of vessels - arteries, veins, and capillaries. Arteries, veins, and capillaries are not anatomically the same. They are not just tubes through which the blood flows. Both arteries and veins have layers of smooth muscle surrounding them. Arteries have a much thicker layer, and many more elastic fibers as well. The largest artery, the aorta leaving the heart, also has cardiac muscle fibers in its walls for the first few inches of its length immediately leaving the heart. Arteries have to expand to accept the blood being forced into them from the heart, and then squeeze this blood on to the veins when the heart relaxes. Arteries have the property of elasticity, meaning that they can expand to accept a volume of blood, then contract and squeeze back to their original size after the pressure is released. A good way to think of them is like a balloon. When you blow into the balloon, it inflates to hold the air. When you release the opening, the balloon squeezes the air back out. It is the elasticity of the arteries that maintains the pressure on the blood when the heart relaxes, and keeps it flowing forward. if the arteries did not have this property, your blood pressure would be more like 120/0, instead of the 120/80 that is more normal. Arteries branch into arterioles as they get smaller. Arterioles eventually become capillaries, which are very thin and branching.
Image courtesy of Carolina Biological Supply/Access Excellence
Capillaries are really more like a web than a branched tube. It is in the capillaries that the exchange between the blood and the cells of the body takes place. Here the blood gives up its carbon dioxide and takes on oxygen. In the special capillaries of the kidneys, the blood gives up many waste products in the formation of urine. Capillary beds are also the sites where white blood cells are able to leave the blood and defend the body against harmful invaders. Capillaries are so small that when you look at blood flowing through them under a microscope, the cells have to pass through in single file. As the capillaries begin to thicken and merge, they become venules. Venules eventually become veins and head back to the heart. Veins do not have as many elastic fibers as arteries. Veins do have valves, which keep the blood from pooling and flowing back to the legs under the influence of gravity. When these valves break down, as often happens in older or inactive people, the blood does flow back and pool in the legs. The result is varicose veins, which often appear as large purplish tubes in the lower legs.
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