Role of the diaphragm in setting liquid pressure in serous cavities

Supraphrenic and subphrenic liquid pressures in supine dogs and rabbits were found to be about 2 cm H2O more negative than costal and mediastinal liquid pressures at the same lung height. Peritoneal liquid pressure was negative at all lung heights in dogs and slightly positive below 25% of lung height in rabbits. The vertical gradients of liquid pressure did not differ significantly in either species between the pleural and peritoneal cavities and were lower than 1 cm H2O/cm. From the peritoneal protein concentration (2.3 and 3.5 g/100 ml in dogs and rabbits, respectively) we calculated a filtration pressure for the peritoneal mesothelium and a lymphatic role was hypothesized to explain negative liquid pressures in the peritoneal cavity. The greater negativity of the subphrenic as compared with the peritoneal data could be related to a strong respiratory-dependent lymphatic action. A similar mechanism and/or a low filtration conductance of the diaphragmatic pleura may explain the costo-supraphrenic liquid pressure difference. The present results, extending previously published data on the mediastinal side of the lung, further support the existence of local differences in fluid dynamics in the pleural cavity.