Dialysis Principles
The Dialysis Principles domain tests the physical basis of solute and water removal: diffusion, convection, osmosis, ultrafiltration and clearance, plus how blood and dialysate flow interact in a dialyzer.
Core rules
- Diffusion moves solutes down a concentration gradient; countercurrent dialysate flow maintains that gradient.
- Convection drags solute with solvent during ultrafiltration; the sieving coefficient describes passage.
- Ultrafiltration removes plasma water; transmembrane pressure and membrane permeability determine the rate.
- Clearance is the virtual volume of blood cleared per minute; it depends on Qb, Qd, membrane area and KoA.
Worked example
Qb is 300 mL/min; arterial urea is 60 mg/dL and venous urea is 15 mg/dL. Clearance = Qb × (A − V)/A = 300 × (45/60) = 225 mL/min. This simplified blood-side estimate shows a large arteriovenous difference but does not correct for plasma water or recirculation.
Traps
Do not confuse UF (water removal) with clearance (solute removal); do not use whole-blood Qb for plasma clearance without conversion; recirculation shrinks the A−V difference and makes clearance look lower. Also, do not assume a high Qb always raises clearance if membrane or access limits it.