As a result of this need in the tissues, hemoglobin ensures that oxygen is supplied where it is needed. At resting tissue level at a Po2 of 40 mm Hg. Hb is 75% saturated, only 23% of the O2 carried by HB is released. However, in an active tissue the Po2 is lower; a Po2 of 20 mmHg is only 40% saturated. So 35% of 02 must be discharged for tissue exercise. (Elaine,N.,Katja H, 2014). The binding of carbon dioxide to hemoglobin is determined by the fact that the transport of carbon dioxide in the erythrocytes does not struggle with the transport of oxyhemoglobin, consequently CO2 binds to the globin animacides while oxygen binds to the eme. In the lungs, the Pco2 of the alveolar air is lower than that of the blood, causing a rapid dissociation of carbon dioxide from hemoglobin. While in tissues Co2 binds easily with hemoglobin because Pco2 is higher than in blood. To conclude Co2 + H2O----- H2CO3 ------ H +HCO3When Co2 is present, Hb-o2 discharges more O2 faster, so O2 is more available to tissues, this is known as the Bohr effect. “The amount of oxygen carried by hemoglobin depends on the locally available Po2, this relationship ensures optimal oxygen uptake and release.” (Elaine,N.,Katja H,