Pulmonary Physiology and Lung Development
Core concepts
Lung development follows embryonic, pseudoglandular, canalicular, saccular, and alveolar stages. A useful ordering is that conducting airways form before gas-exchange units. The pseudoglandular stage builds the bronchial tree; the canalicular stage brings vascularization and early surfactant; the saccular stage creates thin-walled sacs; the alveolar stage adds septation and continues into early childhood. Surfactant production begins around 24–26 weeks, and SP-B is essential for normal surfactant function. Fetal hemoglobin has a left-shifted oxygen dissociation curve, and pulmonary vascular resistance falls sharply after birth with lung inflation and oxygen exposure.
Pediatric physiology differs from adult physiology: the chest wall is compliant, the diaphragm is the main inspiratory muscle, and closing volume may exceed functional residual capacity in infants, promoting atelectasis. Smaller airways have much higher resistance because resistance is inversely proportional to the fourth power of radius. Oxygen consumption is also higher in young children.
Traps
Candidates often confuse the canalicular and saccular stages or assume alveoli are complete at birth. Another trap is applying adult pulmonary function thresholds to children. Surfactant is not present from the embryonic stage.
Worked example and rule of thumb
A 24-week infant with grunting and ground-glass lungs has surfactant deficiency at the canalicular-to-saccular transition, not transient tachypnea of the newborn. Rule of thumb: “airways, vessels, sacs, septa” corresponds to pseudoglandular, canalicular, saccular, and alveolar stages.