Chloride-driven secretion in utero switches to sodium-driven absorption at birth
Expansion
In utero, the fetal lung actively secretes fluid through chloride channels, which is essential for normal lung growth. The volume is about 25 to 30 ml per kilogram, similar to functional residual capacity.
At birth, secretion switches to absorption:
- The surge of catecholamines and corticosteroids during labour activates epithelial sodium channels (ENaC)
- Sodium absorption draws water out of the airspaces into the interstitium and then into blood and lymphatics
- Mechanical compression during vaginal delivery contributes only a small amount
Transient tachypnoea of the newborn is delayed clearance of this fluid. It is commoner after elective caesarean section without labour, in prematurity, and in infants of diabetic mothers, precisely because the hormonal switch has not been triggered.
It presents with tachypnoea from soon after birth, usually resolves within 24 to 72 hours, and shows fluid in the fissures and prominent perihilar markings on radiography.
Conversely, oligohydramnios or prolonged rupture of membranes reduces the fluid needed for growth and causes pulmonary hypoplasia.