Renal Anatomy, Physiology, and Pathophysiology
The nephron
The functional unit is the nephron. The glomerulus filters; the proximal tubule reabsorbs the bulk of sodium, water, glucose, and amino acids, reclaims bicarbonate, and activates vitamin D; the loop of Henle builds the medullary gradient; the distal tubule handles sodium, chloride, and calcium; the collecting duct responds to ADH and aldosterone. Normal GFR is about 125 mL/min - roughly 180 L filtered daily against 1–2 L excreted.
Endocrine and metabolic roles
The kidney secretes renin (RAAS, blood pressure), produces erythropoietin from peritubular interstitial fibroblasts, converts 25-OH vitamin D to calcitriol, excretes potassium, hydrogen ion, and organic acids, and clears small peptides such as insulin.
Pathophysiology to know
CKD is staged by eGFR: G1 ≥90, G2 60–89, G3a 45–59, G3b 30–44, G4 15–29, G5 <15. Diabetes and hypertension account for most ESKD. AKI is classified as pre-renal (volume loss, poor perfusion), intrinsic (acute tubular necrosis, nephrotoxins, contrast), or post-renal (obstruction). Uraemia produces pericarditis, encephalopathy, pruritus, platelet dysfunction, and nausea.
Traps
- Creatinine and GFR are inverse but not linear; a creatinine that has "only doubled" can mean the GFR has halved.
- BUN reflects protein intake, steroids, GI bleeding, and volume status, not kidney function alone.
- Anaemia of CKD is mainly EPO deficiency, but iron status must be corrected first.
- Potassium rises with acidosis and with RAAS inhibitors.
Rule of thumb: the classic dialysis indications are AEIOU - Acidosis, Electrolytes (hyperkalaemia), Intoxication, Overload, Uraemia.