In conventional dialysis blood is circulated through a dialyzer (flow rate, 250-350 ml/min) in the opposite direction to the dialysate (flow rate, 500-800 ml/min). The two flows are separated by a semipermeable membrane and most solute transport is achieved by diffusion based on a concentration gradient. Fluid removal is achieved by ultrafiltration driven by a hydrostatic pressure gradient ( Fig 1)
Diffusive solute clearance is determined by several variables: time on dialysis (duration and frequency), membrane surface area, membrane diffusive permeability (thickness and porosity), solute molecular weight, blood flow, and dialysate flow. Diffusion is very efficient for the elimination of small solutes, but the removal of larger solutes depends largely on the convective component (ultrafiltration).
Fluid removal is controlled by calculating the transmembrane pressure required to achieve the desired fluid balance, given the ultrafiltration coefficient of the membrane and the time on dialysis. Inaccuracies can be overcome by ultrafiltration control systems which automatically adjust transmembrane pressure and allow more uniform weight reduction over time.
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