The normal BPG in the blood keeps the O2-hemoglobin dissociation curve shifted slightly to the right all the time. In hypoxic conditions that last longer than a few hours, the quantity of BPG in the blood increases considerably, thus shifting the O2-hemoglobin dissociation curve even faher to the right. This shift causes O2 to be released to the tissues at as much as 10 mm Hg higher tissue O2 pressure than would be the case without this increased BPG. Therefore, under some conditions, the BPG mechanism can be impoant for adaptation to hypoxia, especially to hypoxia caused by poor tissue blood flow.Ref: Guyton and Hall textbook of medical physiology 13th edition. Page:532