The heme entity is made up of four porphyrin groups with an iron atom in the centre. The iron atom (the reduced form, Fe²⁺) can combine with one molecule of oxygen in an reversible manner to form oxyhaemoglobin. When the iron releases the oxygen, it is still in the reduced form and the haemoglobin is called deoxyhaemoglobin. It is important to note that oxyhaemoglobin is not equivalent to oxidised haemoglobin. Oxidised haemoglobin is called methaemoglobin and in this state the iron atom is in the oxidised state (Fe³⁺). Methaemoglobin cannot participate in oxygen carriage as the iron does not form a bond with the oxygen. Normally only a small percentage of haemoglobin is oxidised though certain drugs and chemicals (hydorgen sulphide is one of them) can increase this percentage.

Another type of haemoglobin, is carboxyhaemoglobin. In this form, the haemoglobin is bound to carbon monoxide (CO) instead of oxygen and as this bond is around 250 times as strong as the haemoglobin/oxygen bond, CO displaces the oxygen and thus reduces the capacity of the haemoglobin to carry oxygen.