Molecular oxygen is carried in blood in two ways: bound to haemoglobin within the red blood cells and dissolved in plasma. Oxygen is poorly soluble in water, so only 3% of the oxygen is transported in the dissolved form. 97% of oxygen binds with haemoglobin in a reversible manner to form oxyhaemoglobin (HbO2). The rate at which haemoglobin binds with O2 is regulated by the partial pressure of O2 .
Each haemoglobin carries maximum of four molecules of oxygen. In the alveoli high pO2 , low pCO2 , low temperature and less H+ concentration, favours the formation of oxyhaemoglobin, whereas in the tissues low pO2, high pCO2 , high H+ and high temperature favours the dissociation of oxygen from oxyhaemoglobin.
A sigmoid curve (S-shaped) is obtained when percentage saturation of haemoglobin with oxygen is plotted against pO2 . This curve is called oxygen haemoglobin dissociation curve. This S-shaped curve has a steep slope for pO2 values between 10 and 50 mm Hg and then flattens between 70 and 100 mm Hg. Under normal physiological conditions, every 100 mL of oxygenated blood can deliver about 5 mL of O2 to the tissues.
