[[Image:Semipermeable membrane (svg).svg|thumb|250px|Màng bán thấm]]
Nguyên tắc thẩm phân máu
is the same as other methods of [[ dialysis]]; it involves [ [diffusion]] of solutes across a semipermeable membrane. Hemodialysis utilizes [[Counter-current exchange|counter current flow]], where the dialysate is flowing in the opposite direction to [[blood flow]] in the [[extracorporeal]] circuit. Counter-current flow maintains the concentration gradient across the membrane at a maximum and increases the efficiency of the dialysis.
Fluid removal ([[ultrafiltration (renal)|ultrafiltration]]) is achieved by altering the [[hydrostatic pressure]] of the dialysate compartment, causing free water and some dissolved solutes to move across the membrane along a created pressure gradient.
The dialysis solution that is used may be a sterilized solution of mineral ions or comply with [[British Pharmacopoeia]]. [[Urea]] and other waste products, [[ potassium]], and [[phosphate]] diffuse into the dialysis solution. However, concentrations of [[ sodium]] and chloride are similar to those of normal [[ Blood plasma|plasma]] to prevent loss. [[Sodium bicarbonate]] is added in a higher concentration than plasma to correct blood acidity. A small amount of glucose is also commonly used.
Note that this is a different process to the related technique of [[hemofiltration]].