O 2 binding by hemoglobin exhibits a _________ shape, which can be viewed as a hybrid S-shaped binding curve reflecting a transition from a low-affinity to a high-affinity state. Because of its cooperative binding, hemoglobin is more sensitive to the small differences in O 2 concentration between the tissues and the lungs, allowing it to bind oxygen in the lungs where p O 2 is high and release it in the tissues where p O 2 is low.