The neonatal period is the period of the most dramatic physiologic changes that occur during human life. While the respiratory and cardiovascular systems change immediately at birth, other organ systems evolve slowly with time until the transition from intrauterine to adult physiology is complete. The transitional period of the newborn is a critical time for humans to adapt to life outside the womb. There are distinct physiologic changes during this period, especially regarding the respiratory and cardiovascular systems. The loss of the low-pressure placenta and its ability to facilitate gas exchange, circulation, and waste management for the fetus creates a need for physiologic adaptation.
Premature birth can significantly thwart these physiologic changes from occurring as they should. The endocrine system, specifically the release of cortisol via the hypothalamus, is responsible for lung maturation of the fetus and the neonate. There is a “cortisol surge” that begins with cortisol levels of 5 to 10 mcg/ml at 30 weeks gestational age, 20 mcg/ml at 36 weeks, 46 mcg/ml at 40 weeks, and 200mcg/ml during labor. Cortisol is responsible for lung maturation, thyroid hormone secretion, hepatic gluconeogenesis, catecholamine secretion, and the production of digestive enzymes. Mature thyroid function appears to help prepare the neonatal cardiovascular system and aid in the regulation of temperature.
Following clamping of the umbilical cord and the first breath of life, arterial oxygen tension increases, and pulmonary vascular resistance decreases, facilitating gas exchange in the lungs. Subsequent pulmonary blood flow will cause an increase in left atrial pressure and a reduction in right atrial pressure. Changes in the PO2, PCO2, and pH are contributing factors to these physiologic changes in the newborn. Lung surfactant plays a critical role in these changes allowing the lungs to mature upon delivery. Remnants of fetal circulation (ductus arteriosus, foramen ovale, ductus venosus) will also gradually recede during this neonatal period, defined as up to 44 weeks postconceptual age.
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