The alveoli are normally kept dry by the:

Normally, Starling's forces across the alveolar wall, mainly the negative interstitial hydrostatic pressure, or fluid movement from the alveolar space into the interstitium. This will keep the alveoli dry. Surfactant reduces the surface tension within the alveolus, and decrease the tendency of fluid to be drawn in, keeping the alveoli dry. In the absence of surfactant, the surface tension within the alveoli would rise, increasing the collapsing tendency of the alveoli. Inward-acting forces that tend to collapse the alveolus would draw fluid into the alveolus from the interstitium or pulmonary capillaries