Fetal Physiology

FETAL PHYSIOLOGY

The placenta, which receives nearly half the fetal cardiac output, is responsible for respiratory gas exchange. As a result, the lungs receive little blood flow and the pulmonary and systemic circula-tions are parallel instead of in series, as in the adult (Figures 40–4 and 40–5). This arrangement is made possible by two cardiac shunts—the foramen ovale and the ductus arteriosus:

·        Well-oxygenated blood from the placenta (approximately 80% oxygen saturation) mixes with venous blood returning from the lower body (25% oxygen saturation) and flows via the inferior vena cava into the right atrium.

·        Right atrial anatomy preferentially directs blood flow from the inferior vena cava (67% oxygen saturation) through the foramen ovale into the left atrium.

·        Left atrial blood is then pumped by the left ventricle to the upper body (mainly the brain and the heart).

·        Poorly oxygenated blood from the upper body returns via the superior vena cava to the right atrium.

·        Right atrial anatomy preferentially directs flow from the superior vena cava into the right ventricle.

·        Right ventricular blood is pumped into the pulmonary artery.

·        Because of high pulmonary vascular resistance, 95% of the blood ejected from the right ventricle (60% oxygen saturation) is shunted across the ductus arteriosus, into the descending aorta, and back to the placenta and lower body.

The parallel circulation results in unequal ven-tricular flows; the right ventricle ejects two thirds of the combined ventricular outputs, whereas the left ventricle ejects only one third.

Up to 50% of the well-oxygenated blood in the umbilical vein can pass directly to the heart via the ductus venosus, bypassing the liver. The remain-der of the blood flow from the placenta mixes with blood from the portal vein (via the portal sinus) and passes through the liver before reaching the heart.

The latter may be important in allowing relatively rapid hepatic degradation of drugs (or toxins) that are absorbed from the maternal circulation.

In contrast to the fetal circulation, which is estab-lished very early during intrauterine life, maturation of the lungs lags behind. Extrauterine survival is not possible until after 24–25 weeks of gestation, when pulmonary capillaries are formed and come to lie in close approximation to an immature alveolar epithe-lium. At 30 weeks, the cuboidal alveolar epithelium

flattens out and begins to produce pulmonary sur-factant. This substance provides alveolar stability and is necessary to maintain normal lung expan-sion after birth . Sufficient pulmo-nary surfactant is usually present after 34 weeks of gestation. Administration of glucocorticoids to the mother may accelerate fetal surfactant production.