Oxygen dissociation curves in vitro. (A) A gel consists of an equilibrium between Hb molecules free in solution and polymerized Hb. Only the T conformation of hemoglobin can polymerize, whereas the R conformation is excluded at all oxygen pressures.^5,6 (B) True equilibrium ODCs are what would be observed if the oxygen pressure were changed infinitely slowly. The equilibrium gel ODC is shown at 3 different total Hb concentrations in a red cell with a Hb composition of 92% HbS, 4%HbF, and 4% HbA₂, which is the average composition for 23 sickle cell patients with Hb(F+A₂) levels less than 12%. Although the oxygen affinity of HbA and nonpolymerized HbS are identical,^31,54 the p50 for sickle red cells is ∼4 torr higher than normal red cells because of the elevated 2,3-DPG in sickle cell disease. The right shift of the gel ODCs relative to the solution curve (green) is related to the presence of low-affinity polymerized Hb (red curve). The T conformation of free Hb in solution has a higher affinity for oxygen (p50 = 130 torr) than polymerized Hb.⁵⁵