Plateau of oxygen-hemoglobin dissociation curve signifies ?

Ans. is 'b' i.e., No Hb molecule is available to bind with O2Each molecule of hemoglobin can combine with upto four molecules of oxygen.Combination with the first molecule alters the conformation of the hemoglobin molecule in such a way as to facilitate combination with the next oxygen molecule.In light of this, if we look at the curve, as the PO2 stas rising from 0 mm Hg upwards, initially all hemoglobin molecules in blood stas combining with their first oxygen molecule.This is the most difficult molecule to combine with.Hence saturation rises only slowly with initial rise in PO2. As PO2 rises fuher, hemoglobin molecules combine with their second, third and fouh molecules, which are progressively easier to combine with.Hence saturation rises steeply between PO2 of 15 mm Hg and 40 mm Hg.When PO2 rises still fuher, oxygen finds most of the hemoglobin molecules carrying four molecules of oxygen each.Since no molecules of hemoglobin can carry more than four molecules of oxygen, there is not much scope for more O2 combining with hemoglobin.Hence the curve becomes almost flat again beyond the PO2 of 60 mm Hg.