We separated the solute and water flow across the alveolar epithelium from flow across airway epithelia of the adult rat. Small volumes (0.5-1.0 ml) of Krebs-Ringer
bicarbonate (KRB) were trapped in the distal air space of the isolated vascular-perfused left lung lobes while the airways were blocked by immiscible O2-carrying
fluorocarbon. Lobe weight was lost or gained in response to
colloid gradients and was raised by metabolic inhibitors but did not change with only
fluorocarbon in the air space or in response to modifiers of epithelial ion transport. When serum was added to the KRB-
colloid perfusion,
weight loss occurred in the absence of a
colloid gradient (3.4 ml/min) and was Na+ dependent (inhibited by
luminal Na(+)-free KRB). The change in the concentration of
blue dextran in liquid sampled by
micropuncture from subpleural alveoli was smaller than expected from lobe weight under basal conditions or with a
colloid gradient, even though the volume marker accurately detected
edema formation (
weight gain) induced by metabolic inhibitors. We conclude that 1) weight changes represent volume absorption from the air spaces, 2) serum stimulates a Na+ absorptive process, and 3) by exclusion, small airways and/or other subpopulations of alveoli are the site of this absorption.