Klaesner, Joseph W., N. Adrienne Pou, Richard E. Parker, Charlene Finney, and Robert J. Roselli. Optical measurement of isolated canine lung filtration coefficients at normal hematocrits. J. Appl. Physiol. 83(6): 1976-1985, 1997.In this study, lung filtration coefficient (K_fc) values were measured in eight isolated canine lung preparations at normal hematocrit values using three methods: gravimetric, blood-corrected gravimetric, and optical. The lungs were kept in zone 3 conditions and subjected to an average venous pressure increase of 10.24 ± 0.27 (SE) cmH₂O. The resulting K_fc (ml · min¹ · cmH₂O¹ · 100 g dry lung wt¹) measured with the gravimetric technique was 0.420 ± 0.017, which was statistically different from the K_fc measured by the blood-corrected gravimetric method (0.273 ± 0.018) or the product of the reflection coefficient (_f) and K_fc measured optically (0.272 ± 0.018). The optical method involved the use of a Cellco filter cartridge to separate red blood cells from plasma, which allowed measurement of the concentration of the tracer in plasma at normal hematocrits (34 ± 1.5). The permeability-surface area product was measured using radioactive multiple indicator-dilution methods before, during, and after venous pressure elevations. Results showed that the surface area of the lung did not change significantly during the measurement of K_fc. These studies suggest that _fK_fc can be measured optically at normal hematocrits, that this measurement is not influenced by blood volume changes that occur during the measurement, and that the optical _fK_fc agrees with the K_fc obtained via the blood-corrected gravimetric method.