The oxygen-hemoglobin dissociation curve is sigmoid because
The characteristic sigmoid shape is due to 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 oxygen is first bound, the bond holding the globin units are released, producing a relaxed(R) configuration, which exposes more oxygen binding sites.Ref: Ganong&;s review of medical physiology Kim E Barrett, Susan M Barma, Scott Boitano, Heddwen L Brooks 23rd edition 610
Binding of one oxygen molecule increases the affinity of binding other Oz molecules
Binding of one oxygen molecule decrease the affinity of binding other Oz molecules
Oxygen affinity of Hemoglobin decreases when the pH of blood falls
Binding of oxygen to Hemoglobin reduces the affinity of Hb for C02
Respiratory system