An Experimental Model for the Ultrafiltration of Sodium Ion from Blood or Plasma

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Abstract
The ultrafiltration of sodium ion, Na+, was studied in model aqueous quaternary solutions containing 7-14% albumin as a membrane-impermeable polyelectrolyte, 140 mEq/l NaCl as a membrane-permeable electrolyte and 60 mg/l00 ml urea as a membrane-permeable nonelectrolyte. Both the urea and Na+ concentrations were found higher in the ultrafiltrate than in the bulk retentate, a result attributed to the excluded volume of the relatively high concentration of albumin. To correct for this effect, Na+ and urea retentate concentrations were recalculated with reference to the aqueous (protein free) volume fraction. On this basis, urea concentration in the filtrate equalled that in the retentate whilst Na+ filtrate concentration was less than Na+ retentate concentration. Na+ holdback increased with increasing protein concentration, an effect which is compatible with either nonspecific ion binding or with Donnan-like effects. These studies provide a further understanding of Na+ transport during clinical hemofiltration.

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