Limits to steady-state lymph flow rates derived from plasma-to-tissue uptake measurements

Upper and lower limits to basal lymph flow rates per unit tissue mass calculated from measurements of initial tissue uptake rates of 131-I albumin and steady-state lymph/plasma or interstitial fluid/plasma concentration ratios of endogenous albumin are compared with published measurements of lymph flow. The upper limit corresponds to purely convective transport of albumin, the lower to purely diffusive transport. Of a total of nine feasible comparisons, five show good agreement defined by the measured value falling at or between the calculated limits. These include two direct comparisons in the same or closely similar animal species and experimental conditions, and three cross-specific comparisons. Disagreement, with measured lymph flows much higher than the estimated upper limit, was observed in three comparisons, one of which was direct and conspecific and two that were cross-specific. In one instance, also cross-specific, reported lymph flows were below our minimal estimate. We suggest that estimates of lymph flow derived from tissue uptake measurements may be useful in setting limits to lymph turnover of fluid and plasma proteins in tissues and organs from which it is not feasible to collect lymph. They may also provide a means of evaluating experimentally induced disturbances of lymph drainage and over- or underestimation of the tissue mass from which lymph is collected, these being serious experimental problems associated with lymphatic cannulation and lymph collection. If both direct and indirect measurements can be made in the same tissue or organ under the same conditions, the relative contributions of convective and dissipative (diffusion and vesicular) transport processes to protein extravasation can be evaluated from the relation of the directly measured flow to the two calculated limits. These requirements were met for only two tissues among those surveyed: rat tail skin and rabbit leg muscles. For rat tail skin, our results suggest that approximately half the albumin transport is convective and half dissipative, and for rabbit muscle, about two-thirds is convective and one-third dissipative.