The microscopic blood vessels arranged in a network at the junction of arteries and veins are called capillaries. The capillary wall is composed of flattened endothelial cells. The wall is covered by a sheath made of collagen and reticular fibers. This is called the basal lamina. The capillary network carries blood from the aorta to the venules. The capillary network adjacent to the aorta is called the arteriole and the capillary network adjacent to the vena cava is called the venous network. Exchange of nutrients, respiratory air, excreta etc. takes place through capillaries.
Vasa vasorum are the fine blood vessels that supply nutrients to the walls of large arteries and veins.