2. Capillaries are capillaries microscopic vessels that carry blood from arterioles to small veins or venules and are found abundant in those tissues or organs where the rate of metabolism is very fast. Capillaries are made of single layered endothelial tissue with very narrow lumen.
Capillaries can dilate (vasodilation) or contract (vasoconstriction) which increases or decreases the supply of blood to various parts of the body.
Their wall is very thin, lacking both tunica media and tunica externa. They have a tunica intima layer only. They have a large lumen with valves.
Their walls are thin to facilitate the exchange of food material, gases and waste between the blood and the tissues.
3. Veins are the blood vessels that carry blood from the body parts to the heart. All veins (except the pulmonary vein) carry deoxygenated blood. The wall of a vein also has three layers as in an artery but is thinner as compared to the arterial wall.
Functions of Capillaries:
i. To allow inward and outward diffusion of glucose, amino acids, urea, etc.
ii. To allow diffusion of oxygen into intercellular fluids and inward diffusion of carbon dioxide from intercellular fluids.
iii. To allow movement of leucocytes through capillary walls.
Veins have semilunar valves, which prevent the backward flow of blood. These valves are formed from folds of the inner walls of the veins. Forward pressure of the blood forces the valve to open and the blood flows towards the heart whereas, backflow of blood causes the valve to close.