Clinical meaning
The transition from intrauterine to extrauterine life represents the most dramatic physiological adaptation a human being will ever undergo. During fetal life, the placenta serves as the organ of gas exchange, nutrient delivery, and waste removal. At birth, the newborn must rapidly establish independent respiratory function, circulatory adaptation, thermoregulation, and metabolic homeostasis within minutes. The first breath is triggered by multiple stimuli including the sudden exposure to cooler ambient air, tactile stimulation, mild hypoxia, and hypercarbia. Lung fluid, which fills the alveoli in utero, must be cleared through a combination of thoracic compression during vaginal delivery and active lymphatic and capillary reabsorption. The first effective breath generates a negative intrathoracic pressure of approximately -40 to -100 cmH2O, which inflates the alveoli and establishes the functional residual capacity. With the initiation of breathing, pulmonary vascular resistance drops dramatically as oxygen causes pulmonary arteriolar vasodilation. Simultaneously, systemic vascular resistance rises as the low-resistance placental circuit is removed when the umbilical cord is clamped. These pressure changes cause functional closure of the three fetal shunts: the foramen ovale closes as left atrial pressure...