Determination of fluid extraction and osmotic conductance sigma K in the lung with hypertonic NaCl infusion. II. Experiments.

The change in venous blood density in an in vitro rabbit lung preparation was measured after the osmolarity of the blood was elevated 17 to 50 mosmol/liter by a constant arterial infusion of hypertonic saline. We observed a transient density decrease of the blood flowing from the lung and then a return to the preinfusion density within 10 sec, an indication of the rapid completion of fluid extraction from the interstitia by the elevation in osmotic pressure. By fitting the time course of the density change with the prediction of an extraction model, we obtained the osmotic conductance sigma K (the product of the reflection and filtration coefficient) of the lung due to the increase in NaCl osmotic pressure as 1.33 +/- 0.18 ml/[hr.(mosmol/liter).g wet lung tissue] (mean +/- SEM), the interstitial volume participating in the extraction process as 0.27 +/- 0.04 ml/g of lung tissue, and the volume of fluid extracted as 1.12 +/- 0.16 microliter/g tissue for every mosmol/liter elevation. Since we also found no significant difference between the osmotic extraction parameters determined in blood-perfused lungs and those determined in plasma-perfused lungs, we concluded that the rapid density change is the result of the fluid extraction and not the flow impediment of red blood cells caused by the increase in osmolarity.