(i) Blood carries oxygen from the lungs to the heart and from the heart to various body parts.
(ii) About 3 % of oxygen in the blood is dissolved in the plasma which carries oxygen to the body cells.
(iii) About 97% of oxygen is carried in combination with haemoglobin of the erythrocytes.
(iv) The haem portion of haemoglobin contains four atoms of iron, each capable of combining with a molecule of oxygen.
(v) Oxygen and haemoglobin combine in an easily reversible reaction to form oxyhaemoglobin (HbO2).
(vi) Under the high partial pressure, oxygen easily binds with haemoglobin in the pulmonary (lung) blood capillaries.
(vii) When this oxygenated blood reaches the different tissues, the partial pressure of oxygen declines and the bonds holding oxygen to haemoglobin become unstable.
(viii) As a result, oxygen is released from the blood capillaries.