Renal Anatomy, Physiology, and Pathophysiology
Core anatomy to hold in mind
Know the nephron segment by segment: glomerulus (filtration barrier of fenestrated endothelium, basement membrane, podocyte slits), proximal tubule (bulk reabsorption of sodium, water, glucose, amino acids, bicarbonate), loop of Henle (countercurrent multiplier; thick ascending limb is impermeable to water and drives the dilute/concentrated urine switch), distal convoluted tubule (thiazide-sensitive sodium chloride reabsorption, calcium handling), and collecting duct (aldosterone and ADH action, final potassium and acid handling). Understand the two capillary beds and why glomerular pressure differs from peritubular pressure.
Physiology that questions test
Glomerular filtration rate depends on filtration surface, permeability, and net filtration pressure; autoregulation via afferent arteriolar tone protects GFR across a range of pressures. Juxtaglomerular apparatus links tubular sodium delivery (macula densa) to renin release. Erythropoietin, vitamin D activation, and acid-base handling are endocrine roles that explain anemia, bone disease, and metabolic acidosis in kidney failure. Tubular transport explains drug toxicity: aminoglycosides injure proximal tubules, loop diuretics act on the thick ascending limb, potassium-sparing agents act distally.
Pathophysiology and traps
Distinguish prerenal, intrinsic, and postrenal injury by mechanism, not by creatinine alone. Trap: assuming a low fractional excretion of sodium always means volume depletion-it is unreliable with diuretics or contrast. Trap: confusing nephrotic (protein loss, edema, hypoalbuminemia) with nephritic (hematuria, hypertension, oliguria, red cell casts). Remember that CKD progression involves hyperfiltration and glomerulosclerosis, and that nephron loss reduces both concentrating and diluting ability, narrowing the urine osmolality range.
Worked example
A patient with vomiting has creatinine 2.0 mg/dL, urine sodium 8 mmol/L, and bland sediment. The low urine sodium with prerenal physiology suggests tubular sodium avidity, so restore volume and reassess rather than assuming intrinsic disease. Rule of thumb: match the mechanism to the segment-loop problem, distal problem, or filtration barrier problem-then predict the expected urine findings.