The blood and the heart

The components of blood

Blood plasma carries red blood cells, white blood cells, and platelets suspended in it. It carries many dissolved substances around the body. Waste carbon dioxide is carried to the lungs, urea is carried to the kidneys, and the products of digestion pass into plasma from the small intestine and are taken to individual cells.

Red blood cells pick up oxygen from the air in the lungs. They have a biconcave shape, which increases the surface area to volume ratio for diffusion. They have haemoglobin (a red pigment) which binds to oxygen to form oxyhaemoglobin. They have no nucleus so there is more space for the haemoglobin.

White blood cells are part of the body's immune system. Lymphocytes produce antibodies to pathogens and antitoxins to the pathogen's toxins; phagocytes engulf and digest pathogens.

Platelets are small cell fragments which help blood clot, which converts fibrinogen into fibrin. This produces a network of protein fibres which captures red blood cells and platelets to form a clot. This dries and hardens to form a scab which protects the skin as it heals and prevents the entry of pathogens.

The blood vessels

Arteries carry blood away from the heart to the organs. They stretch as blood is forced through them and return to shape afterwards. This can be felt as a pulse. They have thick walls with muscle and elastic fibres.

Veins carry blood to the heart away from the organs. They have valves which prevent blood backflow. They open if the blood that flows through them goes to the heart but close if blood flowing through…