Fetal circulation

"Fetal circulation differs physiologically and anatomically from postnatal circulation because of the presence of fetal shunts and altered metabolic needs of the various organs.

In addition, the fetal liver is only partially functional; therefore the majority of blood is diverted away from these areas through fetal shunts. Because the fetal brain requires maximum concentrations of oxygen and nutrients for growth, fetal circulation is streamlined to ensure optimal perfusion to the brain.

In utero the fetus receives blood carrying oxygen and nutrients from the placenta through the

. . .The blood travels to the liver, where a portion enters the portal and hepatic circulation; approximately half the flow is diverted away from the liver through the ductus venosis and into the inferior vena cava. Because the blood received from the inferior vena cava yields a higher pressure, blood entering the right atrium from the inferior vena cava is shunted through the foramen ovale and into the left atrium and is then pumped through the left ventricle and into the aorta. Approximately two thirds of the blood flows to the head and upper extremities.

. The remaining blood flows into the descending aorta.

Less-saturated blood. . .returns from the upper body, head, neck and arms and travels from the superior vena cava into the right atrium. A small portion of this blood flows into the right ventricle and out the pulmonary artery and enters the nonfunctioning lungs. Most of the blood, however, bypasses the lungs by flowing through the ductus arteriosus and into the descending aorta.

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. . .At birth a series of circulatory changes occur that affect blood flow, vascular resistance, and oxygen tension. The most important change that takes place in the circulation is the shift of gas exchange from the placenta to the lungs. . ."