Effects of graded increases in pulmonary vascular pressures on lung fluid balance in unanesthetized sheep

Fourteen experiments were conducted on 12 chronically instrumented unanesthetized sheep in which we monitored pulmonary arterial, left atrial, and aortic pressures, lung lymph flow, and lymph-to-plasma ratios (L/P) of total proteins and four endogenous protein fractions during baseline and progressive elevations of left atrial pressure. We found that L/P for total proteins decreased as lung lymph flow increased until lymph flow exceeded four to five times baseline and thereafter remained nearly constant (filtration independent) at 0.26. The four protein fractions exhibited a filtration independent L/P that was related to the effective molecular radius of the protein fraction. The minimal L/P were 0.36, 0.28, 0.18, and 0.09 for the protein fractions that had effective molecular radii of 36, 38.5, 59, and 101 A, respectively. In addition, we found no evidence supporting a stretched pore phenomenon over the pressure range utilized in this investigation.