Interstitial fluid: exchange of macromolecules between plasma and skin interstitium.

Fluid was collected from blisters developed by suction on the abdominal skin in ten normal volunteers. During the same study the transcapillary escape rate of intravenously injected 131I-albumin and 125I-IgG together with the accumulation of these tracers in the blister fluid was measured. The ratios between the concentrations of endogenous albumin, transferrin, IgG and alpha 2-macroglobulin in the blister fluid (Cb) and serum (Cs) correlated directly with the free diffusion coefficients indicating that the sieving properties of the vascular endothelium remained intact during the suction. The ratios of intravascular to total masses of the four endogenous proteins calculated from plasma volume, Cs, Cb and extracellular fluid volume determined with inulin and Cb were very similar to those obtained in long-term metabolic turnover studies. The ratio between the accumulated IgG and albumin tracers in the blister fluid (0.80) was identical with the Cb/Cs ratio between endogenous IgG and albumin (0.78) but higher than the ratio between the IgG and albumin escape rates from the total microvasculature (0.58). From the protein tracer determinations it cannot be definitely excluded that suction increases the microvascular escape rates. If the distribution volume of inulin equates that of albumin in steady-state it can, however, be concluded from the data given on the endogenous plasma protein, that blister fluid from the abdominal skin corresponds to average interstitial tissue fluid.