Pleural liquid pressure in the zone of apposition and in the lung zone

Pleural liquid pressure in zone of apposition (Pliq,ap) and in lung zone (Pliq,L) was measured simultaneously through liquid filled cannulas in anesthetized dogs in lateral posture. At top Pliq,ap and Pliq,L at iso-height were -9.7 +/- 0.4 and -11.9 +/- 0.3 cm H2O, respectively, at end expiration (eE; P of delta less than 0.01; 11 dogs); -9.6 +/- 0.7 and -19.8 +/- 0.7 cm H2O at end inspiration (eI). At bottom they were -0.6 +/- 0.2 and -2.4 +/- 0.4 cm H2O, respectively, at eE (P of delta less than 0.05; 4 dogs); -1.5 +/- 0.3 and -8.5 +/- 1.0 cm H2O at eI. Vertical gradient of Pliq,ap was -0.97 +/- 0.02 cm H2O/cm. A 132% increase in ventilation (after 10 min dead space breathing) did not change eE and eI Pliq,ap, and eE Pliq,L, but decreased eI Pliq,L. These results imply: (1) no transmission to zone of apposition of tidal changes in Pliq,L; (2) a liquid flow from zone of apposition to lung zone; (3) a net filtration into zone of apposition. They suggest that absorption pressure of capillaries of visceral pleura is greater than that of lymphatics of zone of apposition. Lymphatic role in setting Pliq,L is discussed.