Acid-base balance of pleural liquid in dogs

Acid-base balance and electrolyte concentrations were measured in dogs on small artificial hydrothoraces and in vitro on bicarbonate buffered Ringer solution on serosal and interstitial side of specimens of parietal pleura. Under steady conditions, pleural liquid PCO 2 was similar to and pH higher (delta = 0.022 +/- 0.006 SE) than that in mixed venous blood. Computed pleural liquid [HCO-3] was similar to that in venous plasma and hence less than that set by the Donnan effect, with which Na+ and Cl- approximately complied. In vitro, pH, [Na+], [Cl-], and computed [HCO-3] were significantly lower (delta = 0.030 +/- 0.004; -2.6 +/- 0.5; -1.2 +/- 0.5 and -1.7 +/- 0.2 meq/L, respectively) on the serosal than on interstitial side of pleural specimens, PCO2 being 42 mm Hg on both sides . HCO-3 and Na+ were not distributed according to transpleural potential (-0.4 +/- 0.1 mV on serosal side), suggesting an active transport of Na+ and HCO-3 from pleural liquid to blood. This, however, does not seem to add to the absorption pressure of plasma proteins in setting pleural liquid pressure.