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