The function of the blood in animals

   All animals need to get food and oxygen to their tissues and to have waste products carried away. In one-celled creatures this is quite simple. The substances they need dissolve in water and pass in through their thin covering membrane. Substances they need to get rid of seep out.

   The contents of the cell flow around in circles to distribute the chemicals within the cell. Larger, more complicated animals like earthworms, snails, crabs, insects and vertebrates, however, need a transport system to carry chemicals around their bodies.

   The blood in these animals usually has a liquid plasma and a few cells. Usually the cells are rather like amebas. They have no special shape, and move by flowing.

To eat, they send out arms to surround a particle and take it in. This method of eating by surrounding a particle is called 'phagocytosis'. Phagocytic cells remove unwanted particles from the blood and carry food around the animal.

   All vertebrate animals from fish and salamanders up to mammals and people have blood that contains three types of cells floating in the liquid plasma. These are white blood cells, red blood cells, and tiny cells called platelets. The platelets clot the blood so that it does not all run out when a blood vessel is cut. The red blood cells of vertebrate animals contain the coloured chemical (pigment) hemoglobin, which carries oxygen from the gills or lungs to the tissues. Hemoglobin contains iron and gives blood its red colour. Some invertebrate animals transport oxygen by the pigment hemocyanin. This contains copper instead of iron and is blue.