Transport of large molecules from plasma to interstitial fluid and lymph in dogs.

CARLICK, DAVID G., AND EUGENE M. RENKIN. Transport of large molecules from plasma to interstitial jluid and lymph in dogs. Am. J. Physiol. 219(6): 1595-1605. 1970-A study was made of the transport of endogenous serum albumin and a graded series of dextrans of 8,000-500,000 mol wt from blood to lymph in the dog’s paw. Sucrose was used as a representative small molecule. Plasma levels of sucrose and one or more test dextrans were maintained constant, and samples of lymph were collected until steady-state concentration ratios relative to plasma (R) were reached. R was unity for sucrose, progressively less for dextrans of increasing molecular weight. Analysis of the approach to equilibrium showed that both diffusion and bulk flow are important in saturation of the lymph space, even for dextrans of up to 500,000 mol wt and that the principal barrier to blood-lymph transport is at the blood capillary wall, and not at the lymph capillary, nor within the interstitial space. Steady-state R values for serum albumin were inversely related to lymph flow, corresponding to a constant permeability-surface area product (PS). PS for serum albumin was the same as that for dextran molecules of the same Stokes-Einstein radius (a,), but lower molecular weight. The relation of PS to a, is consonant with the hypothesis of a system of 40A radius pores, pluseither a few large pores (radius ‘v 8OOA) or pinocytotic vesicles (radius N 250 A) to carry the larger molecules.