In a normal and healthy person, the haemoglobin is approximately 15gms per 100ml of blood. The capacity of 1gm of Hb is to combine with 1.34ml of oxygen. Thus, on an average, 100ml of blood carries about 19.4ml of oxygen. It is called oxygen capacity of Hb. When blood reaches the tissues, its oxygen concentration is reduced to 14.4ml, which is then collected by the venules and veins. Thus under normal conditions, approximately 5ml of oxygen is transported to tissues by 100ml of blood.
While under strenuous conditions or during exercise, the muscle cells needs more oxygen, this demand of the muscle cells is fulfilled by the oxygen which is left unused in the human blood. During exercises the partial pressure of O2 in the tissue falls, as a result of which, the oxygen concentration of blood at the tissue level is reduced to 4.4ml of oxygen/100ml of blood. Thus, approximately 15ml of oxygen is transported to tissues by 100ml of blood during muscular exercise.