Ans. is 'b' i.e., Low affinity for 2,3 DPG o Make it very clear in mind that higher affinity' of fetal hemoglobin for oxygen is due to low affinity of HbF for 2,3- DPG (not due to decreased 2, 3-DPG concentration). HbF does not combines to 2, 3-DPG as avidly as adult hemoglobin."Fetal hemoglobin is left - shifted because it has a low affinity' for DPG", --Principles of medical physiology'"It is worth noting that DPG does not combine with fetal hemoglobin. That is perhaps one reason why HbF has greater affin ity for oxygen than adult hemoglobin (HbF) ".-- Understanding of medical physiology"Fetal hemoglobin binds less 2-3 DPG it has consequently a greater affinity for O2" -- Pediatric physiologyo All other textbooks have mentioned the same thing.o So, high affinity of HbF to O2 is due to low affinity of HbF for 2,3 DPG (and not due to decreased 2,3-DPG concentration).Effect of 2-3 diphosphoglvcerete (2.3 PPG)o Another important factor which influences oxygen-hemoglobin dissociation curve is 2,3 DPG in RBC. DPG is an optional by-product of the glycolytic pathway. DPG binds with deoxygenated hemoglobin but not with oxygenated hemoglobin. Therefore raised DPG concentration releases oxygen from oxyhemoglobin by shifting the following reversible reaction to the right: -HbO2 + DPG - Hb - DPG + O2o One molecule of DPG binds with one mole of deoxyhemoglobin.o That is why in the presence of raised DPG concentration, less oxygen can combine with hemoglobin at a given PO2. Hence an increase in DPG concentration shifts the oxygen-hemoglobin dissociation curve to the right. Thus 2,3 DPG causes delivery (unloading) of O2 to the tissues.o Factors which increases 2,3 DPG level and therefore lower the oxygen affinity of hemoglobin (right shift of O2 dissociation curve and increased delivery of oxygen to tissues) are : -# Hypoxia# High altitude# Alkalosis# Exercise# Pregnancy# Anemia# Hormones (thyroid, GH, androgen)# Cyanotic congenital heart disease# Chemical agents (Inosine, Pyruvate, PEP, phosphate, dihydroxyacetone)o Factors which decrease 2,3 DPG levels and therefore increase oxygen a ffinity of hemoglobin (left shift of O2 dissociation curve and decreased delivery' of O2 to the tissues) are : - Stored blood, and fetal hemoglobin (HbF).o Fetal hemoglobin has considerably less afinity for 2,3 - DPG than does adult hemoglobin therefore fetal hemoglobin has greater affinity for oxygen than adult hemoglobin. In human blood, the affinity of fetal hemoglobin for 2,3-DPG is only about 40% that of adult hemoglobin. This makes fetal hemoglobin behave as if 2,3-DPG levels are low. Therefore, the oxygen dissociation curve is shifted to the left. Although fetal red cells do contain 2,3 DPG, the poor binding of 2,3-DPG by fetal hemoglobin is an important factor in facilitating transport of oxygen across the placenta from mother to fetus. On the maternal side of the placenta, the reduction of hemoglobin effectively shifts the curve to the right, increasing oxygen release. On the fetal side, 2,3-DPG does not interfere with the affinity' of fetal hemoglobin for oxygen and thus the uptake of oxygen by fetal erythrocytes.