Fig. 5. Oxygen-hemoglobin dissociation curve. Percentage of hemoglobin saturation changes with a change in PO2 ( a ). Because of its greater affinity for oxygen, a curve with fetal hemoglobin is shifted to the left ( b ). Several factors shift curves: increases in (1) hydrogen ions, (2) CO2, (3) temperature, or (4) 2,3-diphosphoglycerate (DPG) shift the curve to the right ( c ). (Adapted from Hellegers AE, Schruefer JJ: Nomograms and empiric equations relating oxygen tension, percentage saturation, and pH in maternal and fetal blood. Am J Obstet Gynecol 81:377, 1961)