These are tiny pieces of cell which have no nucleus. Platelets Clump together when blood vessels are damaged and help to clog a ‘meshwork’ of fibres which create a clot, to help stop bleeding. Their main role is to: Function

These have no nucleus and are very flexible so they can pass through the extremely tiny capillaries of the body. Red Blood Cells Their main role is to: Collect and carry oxygen to all the cells of the body so they can create energy. Function

Red Blood Cells (x6000 Magnification).

Arteries Veins Capillaries Blood Vessels Lumen Muscle + elastic fibres Non-elastic fibres Muscle + elastic fibres Non-elastic fibres Muscle + elastic fibres Arteries carry blood away from the heart and have a thick, elastic, muscular wall. They stretch as blood is pumped in and the muscle wall contracts to force blood along. Veins have a relatively thinner and less muscular wall than arteries. The blood is under a lower pressure than in the arteries. Capillary walls are one cell thick. Exchange of nutrients and respiratory gases occurs across their surface.

Valve Closed Veins - Valves The blood pressure is lower in the veins than the arteries. Valve Open Muscle contraction and relaxation also squeezes the walls of the veins to help blood flow back towards the heart. To ensure that blood does not flow back towards the heart, between heartbeats, valves are present every few centimetres. The valves only allow blood to flow in one direction.

Arteries Veins The Major Blood Vessels