Fetal hemoglobin (two alpha and two gamma chains) has a higher affinity for oxygen than adult hemoglobin, facilitating oxygen transfer across the placenta. The fetal hemoglobin-oxygen dissociation curve is shifted to the left of the adult curve (Fig. 58-1); at the same Pao2 level, fetal hemoglobin is more saturated than adult hemoglobin. Because fetal hemoglobin functions on the steep, lower end of the oxygen saturation curve (Pao2, 20 to 30 mm Hg), however, oxygen unloading to the tissue is not deficient. In contrast, at the higher oxygen concentrations present in the placenta, oxygen loading is enhanced. In the last trimester, fetal hemoglobin production begins to decrease as adult hemoglobin production begins to increase, becoming the only hemoglobin available to the infant by 3 to 6 months of life. At this time, the fetal hemoglobin dissociation curve has shifted to the adult position.
A portion of well-oxygenated umbilical venous blood returning to the heart from the placenta perfuses the liver. The remainder bypasses the liver through a shunt (the ductus venosus) andenters the inferior vena cava. This oxygenated blood in the venacava constitutes 65% to 70% of venous return to the right atrium.The crista dividens in the right atrium directs one third of thisblood across the patent foramen ovale to the left atrium, whereit subsequently is pumped to the coronary, cerebral, and upperextremity circulations by the left ventricle. Venous return from the upper body combines with the remaining two thirds of thevena caval blood in the right atrium and is directed to the rightventricle. This mixture of venous low-oxygenated blood fromthe upper and lower body enters the pulmonary artery. Only 8%to 10% of it is pumped to the pulmonary circuit; the remaining80% to 92% of the right ventricular output bypasses the lungsthrough a patent ductus arteriosus and enters the descendingaorta. The amount of blood flowing to the pulmonary systemis low because vasoconstriction produced by medial muscle
Figure 58-1 Hemoglobin-oxygen dissociation curves. The position of the adult curve depends on the binding of adult hemoglobin to 2,3-diphosphoglycerate (DPG), temperature, PCO2, and hydrogen ion concentration (pH).
hypertrophy of the pulmonary arterioles and fluid in the fetallung increases resistance to blood flow. Pulmonary artery tonealso responds to hypoxia, hypercapnia, and acidosis with vasoconstriction, a response that may increase pulmonary vascularresistance further. The ductus arteriosus remains patent in thefetus because of low Pao2 levels and dilating prostaglandins. Inutero, the right ventricle is the dominant ventricle, pumping65% of the combined ventricular output, which is a high volume (450 mL/kg/min) compared with that pumped by an olderinfant’s right ventricle (200 mL/kg/min).
The transition of the circulation, occurring between the fetal and neonatal periods, involves the removal of the low-resistance circulation of the placenta, the onset of breathing, reduction of pulmonary arterial resistance, and closure of in utero shunts. Clamping the umbilical cord eliminates the low-pressure system of the placenta and increases systemic blood pressure. Decreased venous return from the placenta decreases right atrial pressure. As breathing begins, air replaces lung fluid, maintaining the functional residual capacity. Fluid leaves the lung, in part, through the trachea; it is either swallowed or squeezed out during vaginal delivery. The pulmonary lymphatic and venous systems reabsorb the remaining fluid.