Dialysis & Renal Replacement Therapy




What is the difference between dialysis and renal replacement therapy?

The term “dialysis” is a generic one which most people use in lieu of “renal replacement therapy” (RRT). In reality, dialysis refers to only one part of the RRT spectrum – the other is ultrafiltration, and the two of these are generally used in combination.

Can you explain the difference between dialysis and ultrafiltration?

Two fundamental processes underlie continuous renal replacement therapy – diffusion and convection

  1. Diffusion / dialysis– the movement of solutes from a compartment in which they are in high concentration to one in which they are in lower concentration – along an electrochemical gradient. An electrolyte solution runs countercurrent to blood flowing on the other side of a semipermeable (small pore) filter. Small molecules such as urea move along the concentration gradient into the dialysate fluid. Larger molecules are poorly removed by this process. Solute removal is directly proportional to the dialysate flow rate.


  1. Convection / ultrafiltration – solute is carried (in solution) a fluid across a semipermeable membrane in response to a transmembrane pressure gradient (a process known as solvent drag). This mimics what actually happens in the normal human kidney. The rate of ultrafiltration depends upon the porosity of the membrane and the hydrostatic pressure of the blood, which depends upon blood flow.  This is very effective in removal of fluid and middle-sized molecules, which are thought to cause uremia. Moreover, most of the cytokines involved in sepsis are “middle molecules”.


Conventional hemodialysis blood flow is 350-450 ml/min and dialysate flow is 500-800 ml/min. In continuous hemodialysis (CVVHD) blood flow is usually set at 100-200 ml/min, and dialysate flows at 1000-2000 ml/hr.




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