Why does the exchange of substances and gases take place in capillaries, and not in other vessels?

In capillaries, the key task of blood circulation is solved: there is an exchange of various substances and gases between blood and tissue fluid. These processes are possible because the capillary walls are represented by a single layer of cells that are not tightly adjacent to each other (endothelium). Such a structure, favorable for metabolic processes, made the capillaries incapable of actively changing their inner diameter.

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