Haemoglobin has maximum affinity with carbon monoxide. CO combines with haemoglobin more rapidly than oxygen t form carboxy haemoglobin. It has 200−250 times more affinity for haemoglobin as compared to oxygen. Formation of carboxyhaemoglobin decreases the overall O2 carrying capacity of blood to cell resulting into oxygen deficiency- hypoxia. Most of oxygen is transported in the form of oxyhaemoglobin. One molecule of haemoglobin readily combines with 4 molecules of oxygen. Hb+4O2⇌HbO2 (oxyhaemoglobin) Formation of oxyhaemoglobin depends on pH, PCO2 and temperature. Haemoglobin also has affinity for CO2. About 23% of total blood CO2 is transported by loosely binding with haemoglobin forming a unstable compound called carbamino haemoglobin.