Affinity for O2 in the blood depends on?

Ans. a (2-3 diphosphoglycerate). (Ref. Ganong, Physiology, 21 st/pg.537)2-3 DPG/ 2-3 BPG is essential for normal transport function of Hb, eg., storing blood in acid citrate dextrose (formerly used medium) leads to decreased 2, 3 DPG in RBCs and such blood displays an abnormally high oxygen affinity, acting virtually as an oxygen "trap" rather than oxygen transport system, failing to unload properly its bound oxygen in tissues.However, decrease in 2-3 DPG can be prevented by adding inosine to storage medium as it can enter RBC where it is converted to 2, 3 DPG.