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...