Hemodialysis Fundamentals and Renal Physiology
What the exam expects
Understand the twin jobs of dialysis - solute clearance by diffusion and fluid removal by ultrafiltration - and the physiology that makes them safe. Know the components of the extracorporeal circuit (access, blood pump, dialyzer, dialysate delivery, air detector, venous alarm), the difference between diffusion (concentration gradient), convection (solvent drag with transmembrane pressure), and osmosis. Be able to explain clearance of small solutes such as urea versus middle molecules, and why dialyzer surface area and blood/dialysate flow rates matter.
Physiology candidates must hold
Recall normal glomerular filtration rate, the kidney's endocrine roles (erythropoietin, vitamin D activation, renin), and acid-base and potassium handling. Know that dialysis patients commonly have anemia, secondary hyperparathyroidism, and volume-dependent hypertension. Understand intradialytic hypotension mechanisms: rapid ultrafiltration, reduced plasma refilling, impaired baroreflex, and autonomic neuropathy.
Traps
Questions often swap diffusion and convection, or ask which parameter most directly changes clearance. Another classic trap is assuming a low blood pressure reading always means hypovolemia - it may reflect vasodilation, arrhythmia, or a machine artifact. Do not confuse dialysate sodium with plasma sodium.
Worked rule of thumb
If ultrafiltration rate exceeds the plasma refill rate, blood pressure falls. Estimate: a 70 kg patient removing 3.5 L over 4 hours is about 12.5 mL/min/kg - above the commonly cited 10 mL/min/kg threshold, flagging hypotension risk. Slow the rate or extend time before reaching for pressors.
Biofeedback link
Because hypotensive episodes are driven by autonomic and volume dynamics, biofeedback targets arousal and breathing to stabilize sympathetic tone during treatment.