Due to this need for hemoglobin in the tissues, oxygen is delivered where it is needed. At the level of resting tissues at a Po2 of 40 mm Hg. Hb is 75% saturated, only 23% of the O2 transported by HB is released. However, in active tissues, Po2 is lower, with Po2 of 20 mm Hg being 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 erythrocytes does not struggle with the transport of oxyhemoglobin, because Co2 binds to the amino acids of globin while l Oxygen binds to heme. In the lungs, the Pco2 of the alveolar air is lower than in the blood, causing rapid dissociation of carbon dioxide from hemoglobin. While in tissues, Co2 easily binds to hemoglobin because Pco2 is higher than in blood. To conclude Co2 + H2O ----- H2CO3 ------ H + HCO3 When Co2 is present, Hb-o2 discharges more O2 more quickly, therefore O2 is more available to the tissues, This is called the Bohr effect. "The amount of oxygen carried by hemoglobin depends on the locally available Po2. This relationship ensures optimal oxygen collection and distribution." (Elaine, N., Katja H.,