Birth involves changes in numerous organ systems in the infant, but only a few of these are required to undergo relatively rapid postnatal adjustment. The most important of these birth-related changes involve transitional function in the respiratory, cardiovascular, gastrointestinal, renal, thermoregulatory, and metabolic systems. Common factors that influence transition at birth include delivery by cesarean section, prematurity, and delayed cord clamping.
Within minutes after birth, regular breathing efforts are sustained, lung compliance improves, airway resistance diminishes, and a functional residual capacity is established. With these changes, the tensions of both oxygen and carbon dioxide in the blood gradually approach those expected in the mature postnatal infant. The physiological triggers for establishing regular breathing efforts are probably the loss of the umbilical circulation and the increase in systemic oxygen content.
A critical element necessary for pulmonary adaptation after birth is surfactant (Fig. 45-1). Surfactant facilitates respiratory and gas exchange by lowering the surface tension of the alveolar lining layer, which is the shallow pool of liquid that overlies the cells of the distal airspaces. Without a very low surface tension at end-expiration, the airspaces become atelectatic with exhalation. Surfactant deficiency underlies the respiratory distress syndrome (RDS) observed after premature birth.
Respiratory transition at birth. During fetal life, the alveoli are filled with chloride (Cl–)-rich fluid secreted by the alveolar epithelium. In a term fetus, the type II cells have lamellar bodies and tubular myelin. The pulmonary vascular resistance is high secondary to pulmonary vasoconstriction. Following birth, air enters the alveoli and lung liquid is absorbed secondary to sodium (Na+) transport via the epithelial sodium channels (ENaC). Pulmonary vasodilation and surfactant release lead to the formation of a layer of surfactant at the air-fluid interphase, which leads to establishment of lungs as the site of gas exchange. (Used with permission from Satyan Lakshminrusimha.)
Surfactant is synthesized in the type II cells that line the distal airspaces. It is composed primarily of lipids, including the desaturated phospholipids, which are responsible for its biophysical properties. Associated with these lipids are a small but critical collection of proteins, the surfactant-associated proteins A, B, C, and D. Proteins B and C are water-insoluble, hydrophobic proteins that are closely associated with the lipid component of surfactant. The pivotal role of these latter 2 proteins is exemplified by RDSs that result from their genetic alteration. Specifically, the absence of surfactant protein B results in severe neonatal respiratory failure unresponsive to routine supportive care.
During the latter stages of intrauterine development, the enzymes important in surfactant production increase and this results in an increase in intracellular surfactant content. Surfactant release into the alveolar space is stimulated by lung inflation and ...