<prompt> The oxygen-hemoglobin dissociation curve is sigmoid becauseBinding of one oxygen molecule increases the affinity of binding other Oz moleculesBinding of one oxygen molecule decrease the affinity of binding other Oz moleculesOxygen affinity of Hemoglobin decreases when the pH of blood fallsBinding of oxygen to Hemoglobin reduces the affinity of Hb for C02  <response>: The characteristic sigmoid shape is due to T-R interconversion. In deoxyhemoglobin, globin units are tightly bound in a tenseT 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 relaxedR 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 