Relationship between the measured and calculated colloid osmotic pressure of plasma and lung lymph in the sheep.

The colloid osmotic pressure of plasma, pi p, and lung lymph pi i, in the adult sheep was measured with a membrane osmometer and calculated from the protein content using the Landis and Pappenheimer equation. Measured (M) plasma oncotic pressure was 6 mm Hg lower than calculated (C), for normal sheep plasma. The difference between (M) and (C) decreased as protein content decreased. The (M) lymph value, considered to be equal to interstitial fluid, was only 1 mm Hg lower than (C) for normal lymph. This resulted in a difference between the calculated and measured oncotic gradient (pi p-pi i) of nearly 5 mm Hg. This difference decreased as protein content decreased in plasma and lymph. The difference between measured and calculated values may in part be explained by the differences in A/G ratio between human and sheep plasma and between sheep plasma and lymph. Measured oncotic pressure in plasma and lymph after severe hemorrhagic and endotoxic shock did not differ significantly from that in the normal animal.