AIRWAY SURFACE LIQUID THICKNESS AS A FUNCTION OF LUNG-VOLUME IN SMALL AIRWAYS OF THE GUINEA-PIG

The average thickness and distribution of airway surface liquid (ASL) on the luminal surface of peripheral airways were measured in normal guinea pig lungs frozen at functional residual capacity (FRC) and total lung capacity (TLC). Tissue blocks containing cross sections of airways of internal perimeter 0.188-3.342 mm were cut from frozen lungs and imaged by low-temperature scanning electron microscopy (LTSEM). Measurements made from LTSEM images were found to be independent of freezing rate by comparision of measurements at rapid and slow freezing rates. At both lung volumes, the ASL was not uniformly distributed in either the circumferential or longitudinal direction; there were regions of ASL where its thickness was < 0.1 mu m, whereas in other regions ASL collected in pools. Discernible liquid on the surfaces of airways frozen at FRC followed the contours of epithelial cells and collected in pockets formed by neighboring cells, a geometry consistent with a low value of surface tension at the air-liquid interface. At TLC airway liquid collected to cover epithelial cells and to form a liquid meniscus, a geometry consistent with a higher value of surface tension. The average ASL thickness (h) was approximately proportional to the square root of airway internal perimeter, regardless of lung volume. For airways of internal perimeter 250 and 1,800 mu m, h was 0.9 and 1.8 mu m at FRC and 1.7 and 3.7 mu m at TLC, respectively. For a given airway internal perimeter, h was 1.99 times thicker at TLC than at FRC; the difference was statistically significant (P < 0.01; 95% confidence interval 1.29-3.08). These measurements suggest that the ASL layer is highly dynamic and that movement of liquid across the airway wall and along the airway tree are both significant factors in establishing ASL thickness under normal conditions. Transepithelial pressure gradients, ion transport, and intraluminal surface tension gradients are potential mechanisms driving ASL flow, and accumulation or depletion of the ASL via these mechanisms can occur on time scales of < 1 min.