what is FETAL CIRCULATION

Oxygenated blood from the placenta the umbulical venous blood (P02, 35 mmHg; Saturation 80%) either bypass the liver through the ductus venosus and enter directly the right atrium (mostly rich O2 blood) or it transverse the liver and enter the inferior vena cava. (Diagram depicting the Fetal Circulation) Inferior vena cava (IVC) blood contains umbulical, lower body and portal venous blood. It represents more or less 70% of the total systemic venous return to the heart. 25% of the IVC blood passes directly into the left atrium through the foramen ovale. Here the blood mixes with the pulmonary venous blood. Left atrial blood (pO2 - 27 mmHg; O2 Saturation 65%) passes into the left ventricle and supplies the coronary arteries, the head (the brain), and the upper body. Only 10% of this blood ejected from the ventricle crosses the aortic isthmus to contribute flow to the descending aorta. 

Superior vena cava (SVC) blood contributes only about 25% of the total venous return to the heart and has a significantly lower pO2 and oxygen saturation (14 mmHg and 40%). Practically all SVC blood flows into the right ventricle and the pulmonary trunk. 8% is diverted into the highresistance pulmonary vascular circuit. The rest is ejected through the ductus arteriosus into the descending thoracic aorta, with a pO2 of 21 mmHg and an O2 saturation of 60%. Because of the crossing streams of the IVC and SVC blood in the right atrium, organs distal to the ductus arteriosus are supplied with blood that has a lower pO2 and O2 saturation than that of the brain and upper extremities. Thus the placenta receives blood with low O2 saturation, maximizing the effeciency of placental gas exchange. 

In the late gestation, pressures in the foetal left and right ventricles are the same (approximately 70 mmHg systolic). The pressure is transmitted into the pulmonary arteries and ascending and descending thoracic aorta. The left and right sides of the foetal circulation function more or less parralell, that is a major distinction from the postnatal (in series) circulation. Combined ventricular output (CVO), that passes through the placenta is more or less equal to the volume of right ventricular blood that passes through the lungs after birth.

In the foetus the right ventricle ejects approximately 65% and the left ventricle only about 35% of the CVO. Because 8% flows through the lungs, about 60% of the total CVO crosses the ductus arteriosus into the descending thoracic aorta.