Water movement during standard hemodialysis

Schematic representation of water movement during standard hemodialysis. Plasma at an osmolality of 320 mosmol/kg flows into the dialyzer (step 1), which is perfused with a dialysate having an osmolality of 280 mosmol/kg. Diffusive loss of urea (and other small solutes) lowers the osmolality of the fluid returning to the patient (step 2). The ensuing fall in extracellular osmolality creates an osmotic gradient favoring water movement into the cells. This effect, plus water loss by ultrafiltration across the dialyzer, leads to extracellular volume depletion and may cause hypotension. The rapid fall in plasma osmolality also may play a contributory role, perhaps by interfering with sympathetic responsiveness to volume depletion.