Fluid exchanges across the parietal peritoneal and pleural mesothelia

In 31 anesthetized rabbits, after removal of superficial tissues, glass micropipettes filled with 0.5 M NaCl solution and connected to an electrohydraulic servo-null system were used to measure extraperitoneal interstitial fluid pressure (Pi,per) and peritoneal liquid pressure (Pliq,per) at various heights. Linear regressions relating pressure to recording height (H) were Pi,per = 1.07 - 0.27H and Pliq,per = 0.9 - 0.64H, respectively. Protein concentration (C(p); g/dl) and colloid osmotic pressure (II; cmH₂O) of plasma and of peritoneal and pleural liquids were 5.48 ± 0.38 and 24.61 ± 3.23, 3.07 ± 0.5 and 13.29 ± 1.87, and 1.76 ± 0.42 and 8.45 ± 2, respectively. The equation relating II to C(p) was II = 4.64C(p) + 0.0027C(p)². Tissue fluid samples were collected with saline-soaked wicks implanted in vivo or dry wicks inserted postmortem in extraperitoneal and extrapleural interstitial spaces. After 60 and 15 min, respectively, wicks were withdrawn and centrifuged; wick fluid was analyzed in colloid osmometer for small samples. Average extraperitoneal and extrapleural II values were 14.2 ± 2.49 and 11.94 ± 1.52 cmH₂O, corresponding to C(p) of 3.07 and 2.57 g/dl, respectively. The average net pressure gradient, assuming reflection coefficient and hydraulic conductivity (Negrini et al. J. Appl. Physiol. 69: 625-630, 1990; 71: 2543-2547, 1991), was 1.18 and 0.98 cmH₂O for parietal peritoneal and pleural mesothelia, respectively, favoring filtration from the extraserosal interstitia into the serosal cavities. Total parietal peritoneal filtration was 1.49 ml · kg^-1 · h^-1, ~15-fold higher than that for pleural mesothelium.