Renal — nephron physiology, AKI/CKD, electrolyte disorders
**Nephron:** • PCT — reabsorbs ~65% of Na/H₂O, glucose (SGLT2), amino acids; site of ATN damage. • Descending loop — water permeable only (concentrates). • Thick ascending — NKCC2 (loop diuretic target). • DCT — NCC (thiazide target); PTH ↑ Ca reabsorption. • Collecting duct — principal cells (ENaC, ADH-V2); α/β-intercalated cells (H/HCO₃).
**GFR:** creatinine-based CKD-EPI 2021 (race-free). Normal 90–120 mL/min/1.73m². Cystatin C for confirmation.
**AKI (KDIGO):** • **Pre-renal** — FENa <1%, BUN/Cr >20, Uosm >500. Volume depletion, HF, cirrhosis, NSAIDs. • **Intrinsic** — ATN (FENa >2%, muddy-brown casts), AIN (eosinophils, drugs — PPI, sulfa, β-lactams), glomerular (RBC casts). • **Post-renal** — hydronephrosis on imaging.
**CKD staging (eGFR mL/min):** G1 ≥90 (with damage), G2 60-89, G3a 45-59, G3b 30-44, G4 15-29, G5 <15. ACEi/ARB + SGLT2i slow progression.
**Electrolytes:** • **Hyponatremia (<135):** check serum osm. Hypotonic — assess volume. SIADH: euvolemic, low Uosm inappropriately concentrated, low UA. Treat central pontine myelinolysis risk — correct <8 mEq/L per 24 hr. • **Hyperkalemia (>5.5):** EKG peaked T → wide QRS → sine wave. Calcium gluconate (stabilize), insulin + glucose + β-agonist (shift), Kayexalate/patiromer/furosemide (remove). • **Calcium:** corrected Ca = measured + 0.8 × (4 − albumin). Hypercalcemia: stones, bones, groans, psychiatric overtones.
**Acid-base (Winter's formula):** PaCO₂ = 1.5 × HCO₃ + 8 ± 2.