ABSTRACT
In adults, blood from the left ventricle is ejected into the systemic circulation and then returns to the right side of the heart. From the right ventricle, blood circulates through the lungs, then after reoxygenation returns to the left heart. Because oxygenation in the fetus occurs in the placenta, and not in the lungs, the adult circulatory design is inappropriate for intrauterine life. Well-oxygenated blood returns from the placenta to the fetus through the umbilical vein. This blood is highly oxygenated, its oxygen saturation reaching up to 80 percent. In spite of the great advances in noninvasive imaging methods, most of the information on the physiology of blood distribution is still based on data obtained from chronically catheterized term sheep fetuses. Baroreceptors are stretch receptors found in the walls of the heart and in the blood vessels. The effect of variations in blood chemistry on ventilation and blood circulation is mediated by the chemoreceptors.
