Blood is made of plasma, red blood cells, white blood cells and platelets. Plasma is the liquid part. It carries dissolved substances around the body, including carbon dioxide, glucose, amino acids, mineral ions, hormones and urea from the liver to the kidneys. It also carries heat around the body.
Red blood cells (erythrocytes) carry oxygen. They contain the protein haemoglobin, which combines with oxygen in the lungs to form oxyhaemoglobin and releases it in the tissues. Red blood cells have no nucleus, which leaves more room for haemoglobin. Their biconcave disc shape gives them a large surface area for oxygen to diffuse in and out quickly.
White blood cells defend the body against pathogens. Phagocytes engulf pathogens and digest them, a process called phagocytosis. Lymphocytes produce antibodies, which destroy specific pathogens. Some lymphocytes also produce antitoxins, which neutralise the toxins that pathogens release.
Platelets are small fragments of cells with no nucleus. When a blood vessel is cut, platelets help the blood to clot and form a scab. This stops blood loss and stops microorganisms getting in.
