Pleural liquid and surface pressures at various lung volumes

Simultaneous measurements of pleural surface (Ppl) and liquid (Pliq) pressures at similar lung height were performed in supine dogs from FRC to about 80% TLC. Lung volume was increased by lowering abdominal pressure so that pressure in the lung remained equal to atmospheric. The difference between Pliq and Ppl (i.e. the pressure elicited by the deformation forces of the contacting pleurae over the areas of intervening liquid) increased progressively with the increase of lung volume, being about -7 cm H2O at Ppl=-2.5 cm H2O and -30 at Ppl=-14. The relationship between Pliq and Ppl was similar in the 2nd and 5th intercostal spaces. From this relationship and previous data on pleural surface area, pleural liquid thickness and cells, and pleural microvilli an attempt was made to get indirect information on pleural contact area through cells and microvilli and pleural contact pressure at various lung volumes.