Why is the oxygen-hemoglobin dissociation curve sigmoid is shape?

Oxygen hemoglobin dissociation curve, is the curve relating percentage saturation of the oxygen carrying power of hemoglobin to the Po2. It has a characteristic sigmoid shape due to the T-R interconversion. In deoxyhemoglobin, globin units are tightly bound in a tense (T) configuration which reduces the affinity of the molecule for oxygen. When the first oxygen is bound, the bonds holding the globin units are released, producing a relaxed (R) configuration which exposes more oxygen binding sites. Thus, binding of one oxygen molecule increases the affinity of hemoglobin to bind other O2 molecules. Ref: Review of Medical Physiology, 22nd Edition By William F Ganong, Pages 667, 669; Textbook of Medical Physiology By Khurana, Page 440; Textbook of Physiology By Guyton and Hall, 10th Edition, Page 475