PULMONARY FLUID EXTRACTION AND OSMOTIC CONDUCTANCE, SIGMA-K, MEASUREDIN-VIVO

The change in aortic blood density in an in vivo rabbit preparation wa s measured to assess fluid movement at the pulmonary capillaries cause d by infusion of hypertonic solution (NaCl, urea, glucose, sucrose, or raffinose in isotonic saline) into the vena cava over 20 s. The hyper tonic disturbance increased the plasma osmotic pressure by less than o r equal to 30 mosmol/l. The density change indicates that the fluid ex traction from the lung tissue was completed within 10 s. It was follow ed by a fluid filtration into the lung tissue and then an extraction a nd filtration from peripheral organs. An exchange model with flow disp ersion yields two equations to estimate the osmotic conductance (sigma K; where sigma is the reflection coefficient of the test solute and K is the filtration coefficient including the total capillary surface a rea), and the tissue fluid volume from the area and first moment of th e measured density change over the extraction phase. The values of sig ma K are 1.40 +/- 0.11, 1.00 +/- 0.10, 1.71 +/- 0.10, 2.60 +/- 0.23, a nd 3.73 +/- 0.34 (SE) ml.h(-1).mosmol(-1).l.g(-1) for NaCl, urea, gluc ose, sucrose, and raffinose, respectively. Consistent with the model p rediction, the tissue fluid volume (0.28 +/- 0.04 ml/g wet lung tissue ) was independent of the solute used. This value suggests that all flu id spaces in the alveolar septa participate in the process of fluid ex traction due to an increase in plasma osmotic pressure.