Blood Has Four Main Components

1. Plasma is the liquid that suspends the blood cells and carries dissolved materials and heat.
2. Red blood cells, also called erythrocytes, transport oxygen.
3. White blood cells defend the body against pathogens.
4. Platelets are cell fragments involved in blood clotting.

Red Blood Cells Carry Oxygen

1. A red blood cell is a biconcave disc, so its curved surfaces give a large surface area relative to its volume for rapid oxygen diffusion.
2. The thin centre gives oxygen a short diffusion distance into and out of the cell.
3. A mature red blood cell has no nucleus, leaving more internal space for haemoglobin.
4. Haemoglobin binds oxygen reversibly in the lungs to form oxyhaemoglobin and releases oxygen where cells are respiring.
5. The cell is small and flexible, allowing it to bend through capillaries that are narrower than its resting diameter.

Phagocytes Engulf Pathogens

1. White blood cells have a nucleus and can change shape as they move through tissues.
2. A phagocyte detects a pathogen, surrounds it with its cell membrane and encloses it inside the cell.
3. Digestive enzymes then break down the engulfed pathogen in a process called phagocytosis.
4. The defence is non-specific because a phagocyte can engulf different types of pathogen.

Lymphocytes Make Specific Antibodies

1. A lymphocyte recognises an antigen, a molecule on the surface of a pathogen.
2. The lymphocyte produces an antibody with a binding site complementary to that antigen.
3. The antibody binds only to its matching antigen, which helps the immune system identify and destroy the pathogen.
4. This response is specific because a different antigen requires a different antibody.

Platelets Form Blood Clots

1. When a blood vessel is damaged, platelets gather at the wound and trigger a series of clotting reactions.
2. A mesh of insoluble fibres forms and traps blood cells, producing a clot.
3. The clot reduces blood loss and blocks pathogens from entering through the break in the skin.
4. The clot later dries into a scab while the tissue underneath repairs.

Plasma Carries Dissolved Materials

1. Plasma carries carbon dioxide from respiring tissues towards the lungs.
2. It transports soluble products of digestion, including glucose and amino acids, from the small intestine to cells.
3. It carries urea from the liver to the kidneys for excretion.
4. It transports hormones from endocrine glands to their target organs.
5. It distributes heat from active organs to other parts of the body.
6. Blood cells and platelets are suspended in plasma and carried through the circulation.