The structure of arteries, arterioles and veins in relation to their function. The structure of capillaries and their importance in metabolic exchange. The formation of tissue fluid and its return to the circulatory system.

All the blood vessels have a lining or endothelial layer which prevents friction.

Arteries

• Carry blood away from the heart
• Blood inside is high pressure to push blood all the way round the body
• Thick muscle layer so the amount of blood going through can be controlled
• Thick elastic lining to maintain pressure
• Having thick walls also helps prevent it from bursting
• Small lumen keeps the pressure high

Arterioles

• Are in between arteries and veins to decrease the pressure before the blood reaches the capillaries so they don't burst
• Thick muscle layer so that it can control the blood flow into the capillaries
• Thinner elastic layer as pressure is not so high

Veins

• Carry blood back to the heart
• Doesn't need a thick elastic layer because the blood pressure is low (no danger of bursting)
• Thin elastic and muscle layers make it easy to compress so that blood can be pushed through
• Valves make sure blood doesn't flow in the wrong direction

Capillaries

• Thin lining layer and no muscle or elastic layer so there is a small diffusion distance
• It is also small so that it can get in between tissues so cells are close, decreasing the diffusion distance again
• Spaces in lining to let white blood cells through
• There are lots of them and they are small to increase the surface area

Large surface area and small diffusion distance are key to aiding fast diffusion, this is key because it is through the capillaries that crucial metabolic substances like oxygen are delivered to the cells of the body

stevegallik

Tissue fluid
At the arteriole end of capillaries there is a lot of blood pressure (hydrostatic pressure), this forces substances out by pressure filtration. Water and some other substances are pushed out of the capillaries into what is know as the tissue fluid (however large molecules like proteins cannot fit through the gaps); this surrounds cells and delivers substances to them like glucose, amino acids and oxygen.

Because a lot of water moved out of the capillary but the proteins remained, at the venous ends of the capillary, the water potential is very low: this causes water in the tissue fluid move back into the blood by osmosis. Some of the liquid enters the lymphatic system, but most then renters the blood at the neck.