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Unformatted text preview: portal systems in the body where blood flows from an artery --> capillary bed (where it picks up nutrients or hormones ) --> portal vein --> capillary bed (where it drops off nutrients or hormones) --> vein. (See Figure 2). The Vessel Wall As with most internal organs, the blood vessels have an outer connective tissue layer, the tunica externa, a middle smooth muscle layer, the tunica media, and an internal endothelial layer, the tunica intima . The tunica intima is very smooth to keep blood flowing without turbulence and coated with prostacyclins to repel platelets. The relative thickness of these layers in the arteries and veins varies dependent upon their distance from the heart; generally, the layers are thinner at greater distances from the heart. The thinnest blood vessels, containing only a single layer of endothelial tissue and a basement membrane , are the capillaries. It is at the level of the capillaries that all solute exchange takes place....
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- Fall '10