Pulmonary alveolar air–liquid interface (ALI)

The pulmonary alveolar air-liquid interface is a specialized physiological boundary where the alveolar epithelium meets the thin layer of pulmonary surfactant and the air within the lung's alveoli. This interface is vital for respiratory mechanics, as the surfactant layer significantly reduces surface tension, preventing the collapse of small airways and alveoli during expiration (StatPearls, 2023). Beyond its mechanical role, it serves as the primary site for gas exchange between the atmosphere and the pulmonary capillaries, and acts as a critical component of the innate immune system through surfactant proteins like SP-A and SP-D (PubMed, PMID: 11588008). In diseases such as Neonatal Respiratory Distress Syndrome (NRDS), the absence or dysfunction of this interface leads to severe respiratory failure, which is treated clinically with exogenous surfactant replacements like Beractant (NIH, 2022). For drug development, this interface represents the primary barrier for inhaled therapeutics, determining the rate and extent of drug absorption into the systemic circulation.

Other names

Alveolar-capillary barrierAir-blood barrierAlveolar surfacePulmonary surfactant filmAlveolar lining layer

Exogenous surfactants lower surface tension at the air-liquid interface to prevent alveolar collapse (atelectasis) and improve gas exchange; the interface also serves as a semi-permeable barrier for the passive diffusion of inhaled pharmacological agents into the pulmonary vasculature.