Change pattern of pleural deformation pressure on varying lung height and volume

Simultaneous measurements of pleural surface (Ppl) and liquid (Pliq) pressures were made at various lung heights in lateral and supine dogs on increasing lung volume from FRC to 80% TLC by decreasing abdominal pressure. The difference between Pliq and Ppl, /Pdef liq/, i.e. the pressure elicited by the deformation forces of the contacting pleurae over the areas of intervening liquid, was greater in the superior parts at all lung volumes. Previous data on pleural liquid thickness (9) and the present measurements show that an increase of /Pdef liq/ may occur either with a decrease or an increase of 9. delta Pdef liq/delta Ppl over a given range of Ppl, increases progressively from bottom up. For a given height the greater the initial /Pdef liq/, the greater delta Pdef liq/delta Ppl. When /Pdef liq/ was decreased by injecting liquid into the pleural space at FRC, delta Pdef liq/delta Ppl decreased. Hence, delta Pdef liq/delta Ppl depends on the initial Pdef liq, i.e. upon the initial tightness of fitting of the pleural membranes. A model interpretation of this relationship is provided.