On the existence of stretchable pores in the exchange vessels of the isolated rabbit lung preparation.

In the present work our aim has been to seek evidence for or against the existence of stretchable pores in the exchange vessels of the lungs. In isolated rabbit lungs ventilated by positive pressure and perfused with homologous blood we performed repeated tests with fluid filtration from the exchange vessels. In these tests the outflow pressure was elevated to specific values for periods of 6 min. The rate of weight gain of the preparation during the last 2 min of each test period was taken as the rate of fluid filtration from the exchange vessels. We found a linear relationship between rate of filtration and outflow pressure in the range from 5 to 20 mm Hg. This indicates that the hydraulic conductivity of the exchange vessels did not change with outflow pressure and thus that no pore stretching occurred within this pressure range. An abrupt increase in filtration rate took place when the outflow pressure was set at 25 or 30 mm Hg. The hydraulic conductivity of the exchange vessels was therefore probably increased at these high pressures. Since in 3 lungs this increase in filtration rate was fully reversible we suggest that a stretching of pores in the exchange vessels of the lungs contributed to the increase in hydraulic conductivity. This stretching of pores occurred only when vascular pressures were at or above the upper level of the physiological pressure range for the lungs.