Globular protein, found in red blood cells, it combines with oxygen to form oxyhaemoglobin, which is a bright red colour, it can combine with four oxygen molecules, and will release them at low partial pressures of oxygen (such as in respiring tissues) and take it up at high partial pressures of oxygen (such as the lungs). It is made up of four polypeptide chains and four prosthetic haem (heme) groups (these contain iron, and are the primary use of iron in the body). There are several different types of h(a)emoglobin, such as foetal haemoglobin and haemoglobin from differnet animals, which differ mainly in their disociation curves (name for the graph of the partial pressure of oxygen against the percentage saturation of the haemoglobin molecules ie. how much oxygen it takes up at different partial pressures). Haemoglobin also combines with carbon dioxide to form carbaminohaemoglobin, and with carbon monoxide irreversably to form carboxyhaemoglobin, which can seriously reduce the oygen-carrying capacity of the blood.
The degree to which heamoglobin is adapted to its environment is incredible. High altitude animals, such as llamas have heamoglobin that binds releases heamoglobin at lower partial pressures than human haemoglobin, so it is better able to cope with the low levels of oxygen in the air. Crocodile heamoglobin, which is one of the primary reasons they can stay underwater for up to 30 minutes at a time has a version of haemoglobin that is sensitive to bicarbonate ions, which build up in the blood as the crocodile goes underwater. This causes the haemoglobin to bind tighter to the oxygens and only release them at lower partial pressures, effectively streching out the crocodile's oxygen supply.