Info

Pulmonary Artery

To Brain / Ductus

Arteriosus

Foramen Ovale \ )V

Superior Vena Cava

Inferior Vena Cava

Pulmonary Artery

To Brain / Ductus

Arteriosus

Superior Vena Cava

Inferior Vena Cava

Descending Aorta

Placenta

FIGURE ll Schematic of the normal fetal circulation.

Descending Aorta

Placenta

FIGURE ll Schematic of the normal fetal circulation.

from the placenta is delivered directly to the liver via the umbilical vein (Fig. 11). A shunt called the ductus venosus allows some of the umbilical vein blood to bypass the liver and empty into the inferior vena cava. There is little mixing of the poorly oxygenated blood from the lower inferior vena cava with the well-oxygenated blood from the ductus venosus. Hence, the two streams remain distinct from one another in the inferior vena cava, and as they enter the right ventricle the larger, well-oxygenated bloodstream from the ductus venosus is shunted to the left atrium via an opening, the foramen ovale, in the septum. (The foramen is covered by a flap that allows for right-to-left atrial flow, but it opposes flow in the opposite direction.) This diversion of well-oxygenated blood directly into the left heart, and into the ascending aorta, ensures that the heart and brain receive an adequate supply of oxygen.

Another shunt that diverts blood away from the fetal lungs is the ductus arteriosus. This large opening between the pulmonary artery and aortic arch diverts most of the right ventricular output into the descending aorta, rather than into the lungs. The mixture of poorly oxygenated inferior vena caval blood and superior vena caval blood that is pumped from the right ventricle traverses the ductus arteriosus and mixes with blood pumped out of the left ventricle. This arrangement, therefore, ensures that blood with the lowest O2 content perfuses the viscera and lower extremities.

There are several important physiological consequences of the cardiovascular arrangement in the fetus. Pulmonary blood flow is low and pulmonary vascular resistance is high. Pulmonary artery pressure is higher than aortic pressure because of the high pulmonary vascular resistance as well as the low peripheral vascular resistance resulting from the substantial distribution of cardiac output through the low-resistance placental circulation. Another consequence of the low pulmonary blood flow is a left atrial pressure that is lower than right atrial pressure.

At birth, the fetal circulatory and respiratory systems undergo profound changes to ensure survival in the extrauterine environment. Clamping the umbilical cord and the taking of the first breath produce dramatic effects on the circulation. Interruption of the umbilical circulation leads to closure of the ductus venosus, and systemic vascular resistance almost doubles by removing the low-resistance placental circulation to produce an increase in arterial pressure. Inflation of the lungs with the first breath leads to a profound fivefold reduction in pulmonary vascular resistance and increased lung blood flow. The resultant increase in left atrial filling elevates left atrial pressure by a few mmHg, which is sufficient to reverse the pressure gradient between left and right atria and abruptly close the valve over the foramen ovale. The reduction in pulmonary artery pressure, coupled to the elevated aortic pressure, reverses flow in the ductus arteriosus, which then closes gradually over a period of 1-2 days. The net result of these changes after birth is the establishment of an adult pattern in the circulation.

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