Abstract

Pleural liquid pressure was measured with saline-filled cannulas in rats, rabbits, cats, pups (4-5 days and 2 mo), small and large dogs, and pigs. On both the costal and mediastinal sides pleural liquid pressure at a level corresponding to the tricuspid valve decreased with increasing body mass; because this behavior was more marked on the mediastinal than on the costal side, a horizontal costomediastinal pleural liquid pressure gradient developed. No differences in liquid pressure were found in animals of different age but of similar size. The protein concentration and the volume-to-body weight ratio of the collected pleural liquid were also found to decrease with increasing body mass. For plasma colloid osmotic pressure, no size-related differences in the Starling balance seem to occur on costal side, whereas such balance shifts toward filtration at mediastinal level with increasing body mass. The lower liquid pressure found with increasing body mass is discussed in terms of lower conductance of the mesothelia and possibly greater lymphatic action. In terms of the Starling-lymphatic interaction we suggest that lymphatics can generate a pressure only slightly lower than that reflecting the equilibrium of the Starling balance of forces.