Fetal Physiology and the Transition to Extrauterine Life

Abstract

The physiology of the fetus is fundamentally different from the neonate, with both structural and functional distinctions. The fetus is well-adapted to the relatively hypoxemic intrauterine environment. The transition from intrauterine to extrauterine life requires rapid, complex, and well-orchestrated steps to ensure neonatal survival. This article explains the intrauterine physiology that allows the fetus to survive and then reviews the physiologic changes that occur during the transition to extrauterine life. Asphyxia fundamentally alters the physiology of transition and necessitates a thoughtful approach in the management of affected neonates.

Keywords: Fetal physiology; Intrauterine circulation; Transition from intrauterine to extrauterine life; Transition physiology.

Figure 1. Fetal circulation

This schematic summarizes the fetal circulation. The placenta provides oxygen and nutrients to the fetus via the umbilical vein (UV). The UV splits at the level of the liver with some blood perfusing the hepatic circulation and the remainder entering the ductus venosus. While most of the blood from the ductus venosus is directed across the foramen ovale to the left atrium, the inferior and superior vena caval blood preferentially enters the right atrium. Right ventricular output is directed across the patent ductus arteriosus into the descending aorta while left ventricular output provides blood flow to the preductal vessels supplying the brain, coronary arteries, and upper body. Intrauterine pulmonary blood flow is initially limited because of high pulmonary vascular resistance and the right-to-left shunting across the patent foramen ovale and patent ductus arteriosus.

Figure 2. Estimated Intrauterine Oxygen Saturations^,

Blood within the umbilical vein has the highest oxygen saturation (70% to 80%, estimated pO2=32–35 torr) compared with the rest of the fetal circulation. Because of the preferential shunting of ductus venosus blood into the left atrium, and the poorly oxygenated inferior and superior vena caval blood (40% to 45%, estimated pO2=12–14) preferentially entering the right atrium, the left side of the heart has a slightly higher oxygen saturation (65%, estimated pO2=26–28 torr) compared with the right side of the heart (55%, estimated pO2=20–22 torr). As a result, the left ventricular output to the brain, coronary arteries, and the upper body, has a slightly higher oxygen saturation/oxygen content compared with the lower body, which is mostly provided by the right ventricular output.