Fig. 4.

Rate of MetHb production in vitro at varying oxyHb saturations and pH. The initial rate of MetHb production in whole fetal (○) and adult (•) blood after introducing nitrite (1.44 mM initial concentration) is taken as a useful index of rates of nitrite reduction and NO formation. A: rates for fetal blood exceed those for adult blood by nearly 2-fold, and both rates are maximal in the 55–65% oxyHb saturation range. Lines and 95% confidence intervals represent a best-fit second-order polynomial equation with R² = 0.90 for fetus (dashed line) and 0.80 for adult (solid line). B: in both fetal and adult blood, the rate of MetHb production was inversely proportional to pH, with slopes of −5.2 ± 0.5 and −4.0 ± −0.4, respectively (P = 0.06).