Blood vessels

Arteries: transport oxygenated blood away from the heart at a high pressure so it can reach the rest of the body (apart from the pulmonary artery which carries deoxygenated blood). They have a thick outer protective wall, and a thick muscle and elastic layer that can stretch to withstand high-pressure blood flow. There is a small lumen to maintain high pressure and smooth lining to minimise friction.

Veins: transport deoxygenated blood back to the heart at a low pressure (apart from the pulmonary vein that carries oxygenated blood from the lungs). Veins have a thinner walls and muscle layers than arteries as blood pressure is lower. They have a large lumen and smooth lining to minimise friction, and valves to ensure blood only flows one way.

Capillaries: surround respiring cells and transport blood at a very low pressure so it moves slowly to allow time for gas exchange with neighbouring cells. Blood plasma passes through capillary walls into tissues where it is called tissue fluid. Capillaries have a very small lumen so blood is in contact with capillary wall to create a short distance of diffusion for gases. Their walls are one cell thick so there is a very short distance of diffusion.