Perinatal circulatory physiology.

Fetal circulation is designed to utilize placenta for gas exchange whereas postnatal circulation uses lungs for gas exchange. Fetal circulatory pathways, namely, umbilical vessels, ductus venosus, foramen ovale and ductus arteriosus facilitate placental gas exchange and promote distribution of oxygenated blood to the vital organs of the fetus. Mechanical factors, prostaglandins and low PO in the lung keep the fetal circulatory pathways open. Postnatal circulatory changes include elimination of the placenta, development of pulmonary circulation, and closure of fetal circulatory pathways. Postnatal circulatory changes markedly influence the clinical presentation and clinical course of the neonate with congenital heart defects.