Water movement during isolated ultrafiltration

Schematic representation of water movement during isolated ultrafiltration. Plasma at an osmolality of 320 mosmol/kg flows into the dialyzer (filter) (step 1), which is not perfused with dialysate. The favorable hydrostatic pressure gradient results in the isosmotic removal of fluid in the dialyzer. The fluid returning to the patient is reduced in volume, but not changed in osmolality (step 2). The ensuing rise in plasma oncotic pressure results in fluid movement from the interstitial fluid and the cells into the vascular space (step 3). This last effect will tend to minimize the degree of plasma volume depletion and protect against the development of hypotension.