the lungs via the pulmonary circulation. Blood will be oxygenated in the lungs and return to the left heart, which will pump oxygen-rich blood out through the aorta to supply the rest of the body via the systemic circulation. In certain cases, the transition from fetal to postnatal circulation may not occur as described above due to complications leading to persistently high pulmonary vascular resistance. Preterm infants are born without fully mature lungs lacking a compound called surfactant that allows alveoli to remain open by overcoming the surface tension of water. The resulting difficulty in lung expansion prevents the necessary reduction in pulmonary vascular resistance for the infant to make the normal cardiopulmonary transition, resulting in infant respiratory distress syndrome. In addition, occasionally during birth, infants may inhale remnants of their fecal matter known as meconium, preventing adequate breathing. The presence of meconium within the lungs, known as meconium aspiration syndrome, can obstruct the airways and also deactivate the newborn’s surfactant. The inflammation that also results from the inhalation of meconium also causes airway constriction, resulting in poor ventilation of the alveoli and inadequate oxygenation of the pulmonary capillary beds. With a lack of oxygen entering the lungs, pulmonary vascular resistance will remain high and the newborn's blood will no longer be oxygenated, preventing fetal shunt closure. In both cases of infant respiratory distress syndrome and meconium aspiration syndrome, fetal shunts will remain open due to the high pulmonary vascular resistance until appropriate measures, such as administration of surfactant or mechanical ventilation, are taken to help the infant breathe on its own. If the problem is not corrected, the infant will undergo hypoxia, acidosis, and other serious complications, such as seizures. Adult remnants Remnants of the fetal circulation can be found in the adult. Physiology The core concept behind fetal circulation is that fetal hemoglobin (HbF) has a higher affinity for oxygen than does adult hemoglobin, which - together with the difference in partial pressure of oxygen - allows a diffusion of oxygen from the mother's circulatory system to the fetus. Blood pressure It is the fetal heart and not the mother's heart that builds up the fetal blood pressure to drive its blood through the fetal circulation. Intracardiac pressure remains identical between the right and left ventricles of the human fetus. The blood pressure in the fetal aorta is approximately 30 mmHg at 20 weeks of gestation, and increases