The oxygen hemoglobin dissociation curve is sigmoid because
Co-operativity- Hb is a tetramer. If one molecule of O2 binds to the tetramer more easily if another O2 is already bound. Since it is most difficult for the first molecule of O2 to bind to Hb, the shape becomes sigmoid.
Binding of one oxygen molecule increases the affinity of binding other O2 molecules
Binding of one oxygen molecule decrease the affinity of binding other O2 molecules
Oxygen affinity of Hemoglobin decreases when the pH of blood falls
Binding of oxygen to hemoglobin reduces the affinity of Hb for CO2