Muscular, or distributing, arteries deliver blood to specific body organs, and have the greatest proportion of tunica media of all vessels, making them more active in vasoconstriction. Arterioles are the smallest arteries and regulate blood flow into capillary beds through vasoconstriction and vasodilation.Capillaries are the smallest vessels and allow for exchange of substances between the blood and interstitial fluid.Continuous capillaries are most common and allow passage of fluids and small solutes. Fenestrated capillaries are more permeable to fluids and solutes than continuous capillaries.inusoidal capillaries are leaky capillaries that allow large molecules to pass between the blood and surrounding tissues. Capillary beds are microcirculatory networks consisting of a vascular shunt and true capillaries, which function as the exchange vessels.