415.1 The Fetal Circulation

Daniel Bernstein

The human fetal circulation and its adjustments after birth are similar to those of other large mammals, although rates of maturation differ. In the fetal circulation, the right and left ventricles exist in a parallel circuit, as opposed to the series circuit of a newborn or adult (see Fig. 415-1A on the Nelson Textbook of Pediatrics website at www.expertconsult.com). In the fetus, the placenta provides for gas and metabolite exchange. Since the lungs do not provide gas exchange, the pulmonary vessels are vasoconstricted, diverting blood away from the pulmonary circulation. Three cardiovascular structures unique to the fetus are important for maintaining this parallel circulation: the ductus venosus, foramen ovale, and ductus arteriosus.

Figure 415-1 A, The human circulation before birth (partly after Dawes). Red indicates more highly oxygenated blood, and arrows indicate the direction of flow. More highly oxygenated blood from the placenta passes through the foramen ovale from the right to the left atrium, thus bypassing the lungs. B, Percentages of combined ventricular output that return to the fetal heart, that are ejected by each ventricle, and that flow through the main vascular channels. Figures are those obtained from studies of late-gestation fetal lambs. Ao, aorta; DA, ductus arteriosus; IVC, inferior vena cava; LA, left atrium; LV, left ventricle; PA, pulmonary artery; PV, pulmonary veins; RA, right atrium; RV, right ventricle; SVC, superior vena cava.

(From Rudolph AM: Congenital diseases of the heart, Chicago, 1974, Year Book.)

The placenta is not as efficient an oxygen exchange organ as the lungs, so that umbilical venous PO₂ (the highest level of oxygen provided to the fetus) is only about 30-35 mm Hg. Approximately 50% of the umbilical venous blood enters the hepatic circulation, whereas the rest bypasses the liver and joins the inferior vena cava via the ductus venosus, where it partially mixes with poorly oxygenated inferior vena cava blood derived from the lower part of the fetal body. This combined lower body plus umbilical venous blood flow (PO₂ of ≈26-28 mm Hg) enters the right atrium and is preferentially directed by a flap of tissue at the right atrial–inferior vena caval junction, the eustachian valve, across the foramen ovale to the left atrium (see Fig. 415-1B). This is the major source of left ventricular blood flow, since pulmonary venous return is minimal. Left ventricular blood is then ejected into the ascending aorta where it supplies predominantly the fetal upper body and brain.

Fetal superior vena cava blood, which is considerably less oxygenated (PO₂ of 12-14 mm Hg), enters the right atrium and preferentially flows across the tricuspid valve, rather than the foramen ovale, into the right ventricle. From the right ventricle, the blood is ejected into the pulmonary artery. Because the pulmonary arterial circulation is vasoconstricted, only about 5% of right ventricular outflow enters the lungs. The major portion of this blood bypasses the lungs and flows right-to-left through the ductus arteriosus into the descending aorta to perfuse the lower part of the fetal body, including providing flow to the placenta via the 2 umbilical arteries. Thus, the upper part of the fetal body (including the coronary and cerebral arteries and those to the upper extremities) is perfused exclusively from the left ventricle with blood that has a slightly higher PO₂ than the blood perfusing the lower part of the fetal body, which is derived mostly from the right ventricle. Only a small volume of blood from the ascending aorta (10% of fetal cardiac output) flows all the way around the aortic arch (aortic isthmus) to the descending aorta.

The total fetal cardiac output—the combined output of both the left and right ventricles—is ≈450 mL/kg/min. Approximately 65% of descending aortic blood flow returns to the placenta; the remaining 35% perfuses the fetal organs and tissues. In the sheep fetus, where most of these circulatory pathways were studied, right ventricular output is approximately 2 times that of the left ventricle. In the human fetus, which has a larger percentage of blood flow going to the brain, right ventricular output is probably closer to 1.3 times left ventricular flow. Thus, during fetal life the right ventricle is not only pumping against systemic blood pressure but is also performing a greater volume of work than the left ventricle.