countercurrent multiplier

**Mechanism** - Descending limb of the loop of Henle: water-permeable (AQP1), impermeable to solutes. Water leaves into the hypertonic interstitium, so tubular fluid becomes progressively more concentrated. - Thick ascending limb (TAL): impermeable to water, actively reabsorbs Na+/K+/2Cl- via NKCC2 (the Na+/K+-ATPase on the basolateral side drives this). The fluid becomes dilute (hypotonic) while the interstitium becomes hypertonic. The TAL is called the "diluting segment." - Single effect: the TAL pumps solute out, creating a ~200 mOsm/L gradient between the tubule and the interstitium at any horizontal level. The countercurrent flow (opposite directions in adjacent limbs) multiplies this into a large vertical gradient from the cortex (~300 mOsm/L) to the inner medulla (~1200 mOsm/L). - Urea recycling: in the presence of ADH, UT-A1/A3 transporters in the inner medullary collecting duct let urea move into the interstitium. This contributes about 40-50% of inner medullary osmolality.

**Countercurrent exchanger (vasa recta)** - The vasa recta are a passive, hairpin-shaped capillary network that runs parallel to the loops of Henle. They preserve the medullary gradient by removing water and solute without washing out the gradient. The multiplier creates the gradient, and the exchanger maintains it. - Increased medullary blood flow (e.g., with vasodilators) can "wash out" the gradient.

**Role of ADH** - ADH (V2 receptor, Gs, cAMP) inserts AQP2 into the collecting duct's apical membrane. Water then moves out into the hypertonic medulla, producing concentrated urine. ADH also upregulates NKCC2 and UT-A1. - Without ADH (central or nephrogenic diabetes insipidus), the gradient still exists, but the collecting duct is water-impermeable and the urine is dilute.

**Clinical correlations** - Loop diuretics (furosemide, bumetanide, torsemide) block NKCC2 and abolish the medullary gradient. They produce isosthenuria (urine osmolality close to plasma) and impair both urine concentration and dilution. Adverse effects include ototoxicity, hypokalemia, metabolic alkalosis, hypocalcemia, hypomagnesemia, and hyperuricemia. Sulfa allergy is a caution (except ethacrynic acid). - Bartter syndrome: a loss-of-function defect in NKCC2, ROMK, or ClC-Kb. It mimics chronic loop diuretic use and presents with hypokalemic metabolic alkalosis, hypercalciuria, and normal to low BP. - Lithium and demeclocycline cause nephrogenic DI by blocking the ADH response. - Chronic polyuria/polydipsia (e.g., primary polydipsia) and prolonged water loading can wash out the medullary gradient. This is why a water deprivation test can show a suboptimal response in primary polydipsia. - Sickle cell disease and trait: the hypoxic, hypertonic medulla promotes sickling, causing papillary necrosis, hyposthenuria, and hematuria. - Osmolality values: the ascending limb fluid leaving the loop is hypotonic (~100-150 mOsm/L), regardless of the ADH state.

**High-yield pearls** - Multiplier = loop of Henle (active NaCl transport in the TAL). Exchanger = vasa recta (passive). - The TAL is impermeable to water. The descending limb is impermeable to solute. - The medullary interstitium's osmolality is maximal at the papillary tip.