All You Ever Wanted To Know
A dialyzer is actually a semipermeable membrane designed into thousands of small 'capillary' like tubes and placed into a cartridge as shown in the adjoining picture. This effectively creates two compartments, one for the blood flow and another for the dialysate flow.
A dialysate is a solution that contains electrolytes and other substances at physiological concentration.
The blood flow and the dialysate flow are in opposite directions.
Hemodialysis machine is required to pump the blood at requisite flow. Apart from this, the machine mixes three different components of dialysis solution to make it into a dialysate with physiological concentrations of electrolytes.
The machine also monitors the flow parameters and the pressures in the circuit.
The amount of ultrafiltration ( removal of excess fluid from the body) can be programmed into the machine and current day machines will precisely remove the fluid to the last milliliter.
Having seen the hardware and the circuit, understanding the actual process of dialysis becomes easy.
When blood with toxins and high concentrations of substances is exposed to a solution with normal concentrations of substances, there is a natural gradient towards the dialysate and all the waste products and excess water are 'washed' away by the dialysate.
Since the blood and the dialysate are flowing in the opposite directions, this concentration gradient is constantly maintained across the length of the dialyzer.
Having understood this, one can make out that if efficiency of dialysis is to be enhanced, one can increase the blood flow (by adjusting the 'pump rate') and/or increasing the dialysate flow.
A similar effect can also be achieved by exposing the two solutions to each other for a longer time, i.e., increasing the duration of dialysis.
Next: Adequacy of Hemodialysis