FETAL CIRCULATION
In utero circulation is maintained via placenta, with oxygenated blood from mother entering the fetal circulation via single umbilical vein and unoxygenated blood leaving via two umbilical arteries.
Normal sequence of fetal circulation (Fig. 17.2) is as follows:• Oxygenated blood (PaO2 30-35 mm Hg) enters the fetus from placenta via single umbilical vein.
• Nearly half of this flow passes via portal vein to hepatic circulation and then to inferior vena cava (IVC), while the rest enters the IVC directly via Ductus venosus. Ultimately all placental flow mixes with less oxygenated blood from lower extremities.
• IVC returns this mixed blood (PaO2 26-28 mm Hg) to RA, where another less-oxygenated flow (PaO2 18-22 mm Hg) also streams in via superior vena cava
(%) indicate percentage oxygen saturation of blood
Fig. 17.2: Hemodynamics: Fetal circulation.
UV: Umbilical vein; DV: Ductus venosus; IVC: Inferior vena cava; SVC: Superior vena cava; PA: Pulmonary artery; DA: Ductus arteriosus; PV: Pulmonary vein; UA: Umbilical artery.
(SVC). However, mixing of these two flows is minimal at atrial level, as IVC flow is preferentially directed towards Foramen ovale to enter LA, while SVC flow mainly goes to RV via tricuspid valve.
• LA blood (PaO2 26-28 mm Hg) passes to LV and then to aorta to supply head and neck vessels, before its mixing with RV outflow via Ductus arteriosus.
• RV flow (PaO2 18-22 mm Hg) enters pulmonary artery. However, due to high pulmonary resistance of unexpanded lungs, only ~10% of this flow enters pulmonary circulation. Remaining 90% of RV output is shunted through ductus arteriosus (Rt gt; Lt shunt) into aorta, bypassing the lungs.
• Mixing of less oxygenated RV flow with LV output drops the PaO2 in distal aorta beyond the ductus arteriosus.
Thus, cerebral and coronary circulation in fetus is supplied by more-oxygenated pre-ductal flow, while other organs receive less-oxygenated post-ductal blood supply.• Nearly 60% of flow in descending aorta goes back to placenta via two umbilical arteries, for recycling.
In brief, fetal circulation is markedly different from postnatal circulation, on following points:
• It is primarily dependent on umbilical flow with all gaseous/metabolic exchange through placenta rather than the unaerated fetal lungs.
• Systemic circulation runs parallel to pulmonary flow, rather than serially as in postnatal life, with mixing at three levels — ductus venosus (umbilical artery gt; IVC), foramen ovale (RA gt; LA), ductus arteriosus (pulmonary artery gt; aorta). Actual pulmonary flow is negligible.
• These mixing sites are vital to maintain parallel flow in fetal life though none of them serves any physiological purpose after birth and get obliterated in early neonatal period.
• Due to high pulmonary resistance, RV is the dominant ventricle in fetus, unlike LV in adults.
17.1.3