COMPOSITION OF INTERSTITIAL FLUID

In several previous experiments to determine the composition of inters titial fluid, the results varied depending on the collecting technique , and the electrolyte concentrations differed from those of a hypothet ical ultrafiltrate of plasma. In our approach, since a change of posit ion from standing to supine is accompanied by hemodilution with inters titial fluid, we used the changes in hematocrit and composition of pla sma in 20 subjects before and after lying down to calculate the compos ition of added interstitial fluid. The estimated protein concentration was 20.6 g/L, and the concentrations of total calcium and magnesium w ere low, in accord with a lower concentration of protein-bound calcium and magnesium. The activity of free cations was also lower, in agreem ent with a Donnan equilibrium potential of 1 mV across the endothelium . The concentration of leukocytes and platelets decreased according to the hemodilution, implying no escape or mobilization of these element s.