Capillaries have no wall but are constructed of a single layer of flattened endothelial cells which is continuous with the endothelial linings of both arteries and veins. Capillaries are the one type of vessel through which nutrients and waste products can be exchanged with the tissue cells. Blood vessels have the unusual property of being able to repair themselves and of growing into newly repaired or replaced tissues. Most of that mileage would be accounted for by the capillaries. A. Structure and Function of Blood Vessels.
All blood vessels have three layers called tunics .
Sympathetic nerves of the vasomotor type are found in this layer, especially in arterioles and arteries. The tunica media is the thickest portion with increased amounts of smooth muscle and less elastin. Metarterioles connect between thorough channels and capillary beds. They may be fenestrated, have fewer tight junctions between adjacent cells and are so much more permeable than other types that they are referred to as being leaky. And, in fact, even some proteins can pass through these membranes into the surrounding tissues - usually liver and spleen.