The Circulatory System
The major artery that supplies blood to the body is the aorta.
The blood passing through the arterioles passes through a bed of minute vessels called capillaries, which are a single cell thick. These capillaries are so small that the red blood cells must line up single file to pass through. The exchange of nutrients and waste products takes place between the capillary blood and the tissue fluids. The arterialized blood that enters the capillaries thus becomes venous blood as it passes through them.
The capillaries empty the venous blood into collecting tubes called venules, and these in turn empty into small veins, which empty into larger veins, and so on until finally all the blood returns to the heart through two large veins, the superior and inferior vena cavae. These terminate in the right atrium, and the systemic circulation is complete.
A one-way flow of blood in this system is maintained by valves located, not only in the heart, but in the veins as well. Some veins also have semilunar valves and the pressure of contracting muscles against the veins works with the action of these valves to increase the venous return to the heart. This is the reason that exercise is so important for the circulation.
The tiniest of the blood vessels, and the place where the exchange of nutrients and waste products takes place between the blood and the tissue fluids, is the ________.
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