Clinical meaning
Renal replacement therapy (RRT) replaces the filtration, solute clearance, and fluid removal functions of the failed kidneys. Hemodialysis (HD) uses an extracorporeal circuit where blood passes through a semipermeable membrane dialyzer in contact with dialysate flowing in the opposite direction (countercurrent flow maximizing diffusion gradients). Solutes move by diffusion (concentration gradient — urea, creatinine, potassium move from blood to dialysate) and convection (solvent drag with ultrafiltration). Typical schedule: 3-4 hours, 3 times per week via arteriovenous fistula (AVF — preferred, lowest infection/thrombosis risk, requires 2-3 months maturation), arteriovenous graft (AVG — usable in 2-3 weeks, higher thrombosis rate), or central venous catheter (CVC — immediate use but highest infection risk). Peritoneal dialysis (PD) uses the peritoneal membrane as the dialysis membrane, with dialysate instilled into the peritoneal cavity via a Tenckhoff catheter. Solutes diffuse across peritoneal capillaries into the dialysate, and water moves by osmotic gradient (dextrose-based dialysate creates osmotic pressure). PD modalities include CAPD (continuous ambulatory — 4-5 manual exchanges daily) and APD (automated overnight using a cycler). Continuous renal replacement therapy (CRRT) is used in hemodynamically unstable ICU patients, providing gentle, continuous solute and fluid removal over 24 hours using either diffusion (CVVHD), convection (CVVH), or both (CVVHDF).