Fetal circulation

Compared with the postnatal circulation (in which the right ventricle and left ventricle are in series), in the fetal circulation, the two ventricles are in parallel. The parallel circulation is created by several shunts and preferential flow patterns that deliver relatively well-oxygenated blood from the placenta to those fetal organs that have increased metabolic demand. The most important structures that shunt blood in the fetal circulation are the ductus venosus (DV), the foramen ovale (FO), and the ductus arteriosus (DA).

From the placenta, blood with a partial pressure of oxygen (PO₂) of 30 to 35 mm Hg flows to the fetus via the umbilical vein (UV) (Figure 191-1), which, in the liver of the fetus, separates into two branches, with one branch joining the portal vein and the other becoming the DV, which joins the inferior vena cava (IVC). Approximately 30% to 50% of the oxygenated blood flowing through the UV will bypass the liver and flow directly through the DV into the IVC, flowing along its posterior wall. As this oxygenated blood enters the right atrium, it is directed across the FO into the left atrium by the eustachian valve, flowing through the left ventricle (∼35% of fetal circulation) into the aorta to supply the head and upper torso.

The deoxygenated blood returning from the superior vena cava, from the myocardium via the coronary sinus, and from the IVC flows through the right ventricle into the pulmonary artery. Most of this deoxygenated blood returns to the descending aorta via the DA; however, approximately 5% to 10% passes through the high-resistance pulmonary circulation. Blood in the descending aorta either flows through the umbilical arteries to be reoxygenated in the placenta or continues to supply the lower limbs. The fetal circulation therefore runs in parallel, with the left ventricle providing 35% and the right 65% of cardiac output. Fetal cardiac output is therefore measured as a combined ventricular output (CVO).

The three major shunts are under autonomic, neural, and hormonal control. The DV, for example, is not a passive shunt; the vessel is trumpet-shaped, with a sphincter at its distal end that regulates flow by β-adrenergic dilation or α-adrenergic constriction. Hypoxemia, presumably via release of endothelial nitric oxide, results in significant vasodilation. Prostaglandins ostensibly have an important role, as they do in the DA, in maintaining patency and in closure following birth.