Blood flow matter for individual tissues due to the fact the blood delivers nutrietns and removes waste. When a person is exercising, blood flow increases. Therefore, the delivery of nutrients and removal of waste also increases. The structure of blood vessels consist of three layers: tunica intima, tunica media, and tunica externa. The functional tissue of the tunica intima is endothelim with some elastic tissue in larger vessels. This layer is the internal layer of the vessel. The tunica media is the middle layer and consists of smooth tissue. The tunica externa is the outer layer which is made of mostly collagen. In the systemic system, the order of the vessels from the left ventricle to the right atrium are the aorta, elastic arteries, muscular arteries, arterioles, capillaries, venule, small vien, large viens, and then the vena cava. The capillaries are the smallest of the vessels, where exchange between extracellular space and blood occurs. The exchange occurs through a diffusion down the concentration gradient.
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