We can build up the structure of haemoglobin by considering a single unit or monomeric form caned myoglobin. This consists of a single polypeptide chain the globin in which the haem group is embedded. Myoglobin is found in striated muscles of vertebrates and combines with one molecule of oxygen. The vertebrate haemoglobins are tetramers formed by the aggregation of four polypeptide chains each containing a discrete haem group. Thus each haemoglobin can combine with four molecules of oxygen forming oxyhaeomoglobin in a reversible reaction and the unoxygenated compound is called deoxyhaemoglobin.
Two monomers in human haemoglobin are of a type called s and the other two are P type. Nearly all vertebrate haemoglobins are tetramers but invertebrate haemoglobins are more diverse. The most distinctive feature is that the subunits often form large aggregates of relative high molecular weights. In mammalian blood the amount of physically dissolved oxygen is about 0.2 ml of oxygen per 100 ml of blood. The amount found bound to haemoglobin is 20 ml oxygen per 100 ml of blood. The dissolved oxygen is therefore, almost insignificant in animals without respiratory pigments. The only exception is the antarctic fish which lacks a respiratory pigment altogether. The explanation could be that at low temperature, the metabolic rate is low and oxygen like other gases has higher solubility at low temperature.