Red blood cells carry oxygen from the lungs to every cell of the body. They contain the red pigment haemoglobin, which binds to oxygen. In the lungs, where there is a lot of oxygen, haemoglobin combines with oxygen to form oxyhaemoglobin. In the tissues, where oxygen is used up, oxyhaemoglobin splits and releases the oxygen.
Red blood cells are adapted for this job. They are shaped like a biconcave disc, which gives a large surface area for oxygen to diffuse in and out. They have no nucleus, which leaves more room inside for haemoglobin. They are small and flexible, so they can squeeze through narrow capillaries.
Plasma is the pale yellow liquid part of the blood. Blood cells and platelets are suspended in it. Plasma carries many dissolved substances around the body, including glucose and amino acids from the gut, urea from the liver to the kidneys, mineral ions, hormones and carbon dioxide from the cells to the lungs. It also carries heat from the organs that release it, helping to spread it around the body.
