The transition at birth requires replacement of fetal alveolar fluid with air and an increase in pulmonary blood flow.
Fetal lung fluid and its clearance
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During intrauterine life, the placenta provides gas exchange, and fetal alveoli contain fluid secreted by type II alveolar cells.
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Fluid removal starts with labor. Fluid is reabsorbed into the perivascular space, blood, and lymphatic channels.
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Labor facilitates clearance; removal is slower when labor is absent, as in elective cesarean section.
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The first breaths expand the alveoli and replace fluid with air. Apnea or weak initial respiratory efforts associated with prematurity or sedation can impair clearance.
Pulmonary blood flow and asphyxia
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Fetal pulmonary vessels are constricted, allowing very little pulmonary blood flow. After birth, vasodilation reduces pulmonary vascular resistance and increases blood flow.
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Oxygenation requires both air reaching the alveoli and adequate pulmonary circulation.
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Hypoxemia and acidosis in an asphyxiated infant impair pulmonary vasodilation. Poor pulmonary blood flow may prevent adequate tissue oxygenation even when ventilation is adequate.
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Rapid restoration of ventilation may improve pulmonary blood flow in mild asphyxia. Severe asphyxia may require oxygenation together with correction of metabolic acidosis.