Mechanism of Solute Removal in Hemodialysis

Introduction to Solute Removal

The American Society of Nephrology recommends that hemodialysis primarily removes solutes from the plasma through diffusion across a concentration gradient, with the solutes moving from areas of high concentration to areas of low concentration until equilibrium is reached 1, 2

The National Kidney Foundation states that hemodialysis functions mainly through the diffusion of solutes from the blood into a physiological saline solution, separated by a thin semipermeable membrane, following Fick's law 2

The European Renal Association notes that diffusion is the dominant mechanism for the removal of small solutes, including urea, creatinine, and electrolytes, which are the primary markers of uremic toxins 1, 2
The rate of diffusion across the dialyzer is driven by blood concentration and is proportional to it, following first-order kinetics 2
The clearance of urea, determined by the concentration ratio, is a sensitive marker of small solute diffusion across the dialyzer 3, 4

The American College of Physicians states that convection occurs when the solvent (water) drags solutes across the membrane through solvent drag, which is particularly important for the removal of larger molecular weight molecules that diffuse less efficiently 1
During ultrafiltration, fluid is removed along with solutes by convection, contributing significantly to the total clearance of urea and should be included in dialysis dose calculations 1, 5

The International Society of Nephrology notes that hemodialysis removes solutes primarily through diffusion, but both mechanisms (diffusion and convection) proceed simultaneously during treatment 1
Dialytic clearance achieves only 10-20% of normal renal clearance for index substances like urea or creatinine, and even less for larger molecular weight solutes 1, 7