Fetal circulation differs from the circulation in postnatal infants. Of most importance is that the fetal lungs are functionally inactive, and the fetus depends completely on the placenta for O2 and nutrients. Oxygenated fetal blood from the placenta passes through the umbilical vein to the fetal liver. Approximately half the flow from the placenta passes through the liver, and the remainder bypasses the fetal liver and reaches the inferior vena cava through the ductus venosus (Fig. 17.44 ). Blood from the ductus venosus joins the blood returning from the lower part of the fetal trunk and the extremities in the inferior vena cava. This blood merges with blood from the fetal liver through the hepatic veins.
The streams of blood tend to maintain their characteristics in the inferior vena cava and are divided into two streams of unequal size by the edge of the interatrial septum (crista dividens). The larger stream, which contains mainly blood from the umbilical vein, is shunted from the inferior vena cava to the left atrium through the foramen ovale (see
Fig. 17.44 ). The other stream passes into the right atrium, where it merges with blood returning from the upper parts of the fetal body through the superior vena cava and with blood from the myocardium.
Unlike the ventricles in adults, those in a fetus operate essentially in parallel. Only a tenth of right ventricular output passes through the lungs because the pulmonary vascular resistance in the fetus is high. The remainder passes from the fetal pulmonary artery through the ductus arteriosus to the aorta at a point distal to the origins of the arteries to the fetal head and upper extremities. Blood flows from the pulmonary artery to the aorta because pulmonary vascular resistance is high, and the diameter of the ductus arteriosus is as large as that of the descending aorta.
The large volume of blood that passes through the foramen ovale into the fetal left atrium is joined by blood returning from the lungs, and it is pumped out by the left ventricle into the aorta. Most of the blood in the ascending aorta goes to the fetal head, upper thorax, and arms; the remainder joins blood from the ductus arteriosus and supplies the rest of the body. The amount of blood pumped by the left ventricle is approximately half that pumped by the right ventricle. The major fraction of the blood that passes down the descending aorta comes from the ductus arteriosus and right ventricle and flows by way of the two umbilical arteries to the placenta.
Oxygen saturation of fetal blood occurs at various loci (see
Fig. 17.44 ). Thus the fetal tissues that receive the most highly saturated blood are the liver, heart, and upper parts of the body, including the head.
At the placenta, the chorionic villi dip into the maternal sinuses, and O2, CO2, nutrients, and metabolic waste products are exchanged across the membranes. The barrier to exchange prevents equilibration of O2 between the two circulations at normal rates of blood flow. Therefore, the PO2 of the fetal blood that leaves the placenta is very low. Were it not for the fact that fetal hemoglobin has a greater affinity for O2 than adult hemoglobin does, the fetus would not receive an adequate O2 supply. The fetal oxyhemoglobin dissociation curve is shifted to the left. Therefore, at equal pressures of O2, fetal blood carries significantly more O2 than maternal blood does.
In early gestation, the high glycogen levels that prevail in cardiac myocytes may protect the heart from acute periods of hypoxia. Glycogen levels decrease in late gestation, and they reach adult levels by term.