1.3.8 Explain how the structures of blood vessels (capillaries, arteries and veins) relate to their functions.

• blood vessel, function: adaptation
• capillaries, site of metabolic exchange: very thin wall (just one cell thick) allows rapid exchange between blood and tissues (short diffusion distance) which speeds up diffusion, the capillary beds (networks of capillaries in tissue) increase surface area for gas exchange. 
• arteries, carry oxygenated blood from heart to rest of body: relatively thick wall to helps it withstand high blood pressure, smooth muscle regulates flow by varying the diameter of lumen, elastic fibres allows walls to stretch when blood is pumped into the artery and recoil (squeeze) to smoothen blood flow and to maintain blood pressure during diastole, lined with smooth layer of endothelial cells to lower friction and ease blood flow, narrow lumen maintains the high pressure inside the artery. 
• veins, carry deoxygenated blood back to the heart: relative thin wall as blood is under low pressure, very little smooth muscle or elastic fibres as no pulse of blood so no stretching and recoiling, a wide lumen to carry large volumes of blood (acts as blood reservoir), valves to stop backflow of blood.