Figure 48.12 Binding of O₂ to Hemoglobin Depends on P_O₂ (A) The sigmoidal shape of the hemoglobin–oxygen binding/dissociation curve reflects positive cooperativity among hemoglobin’s subunits. (B) Hemoglobin in blood leaving the lungs is 100 percent saturated (four O₂ molecules are bound to each hemoglobin molecule). Most hemoglobin molecules drop only one of their four O₂ molecules as they circulate through the body and are still 75 percent saturated when the blood returns to the lungs. The steep portion of this O₂-binding/dissociation curve comes into play when tissue P_O₂ falls below 40 mm Hg, which is a normal mixed venous blood P_O₂.