Evolution of thinking in fetal respiratory physiology.

An outstanding characteristic of intrauterine life is the low oxygen pressure (Po2) of fetal blood. It seemed at one time that in order to tolerate the Po2 of the uterine environment the fetus must have a relatively slow rate of oxidative metabolism or depend upon anaerobic pathways of substrate utilization. Subsequent studies have shown that the fetus has a high rate of oxygen consumption and that lactic acid is a substrate rather than a net product of normal fetal metabolism. The circulatory response of the fetus to acute hypoxia is centered on the requirement of maintaining the arterial oxygen flow to the heart and central nervous system without increasing cardiac output. The arterial oxygen flow concept helps in understanding the physiologic meaning of the high oxygen affinity of fetal blood and the boundary conditions of the fetal defense against hypoxia.