RENAL PHYSIOLOGY • STEP 1

Countercurrent Multiplier

Build the corticomedullary gradient cycle by cycle. Change the pumps, blood flow, ADH, and urea—then watch urine concentration respond.

Run the multiplier

flow cycles

mOsm/kg gradient

NKCC2 pump strength 100%Vasa recta washout 15%ADH 70% Urea recycling adds inner-medullary osmoles Give loop diuretic: NKCC2 blockade

Press Next cycle: TAL salt extrusion creates the single effect.

Single effect

Water-impermeable thick ascending limb actively reabsorbs NaCl via apical NKCC2, basolateral Na⁺/K⁺-ATPase, and ClC-K/barttin. A transverse gradient of about 200 mOsm/kg can be maintained.

Water equilibrates

Thin descending limb has AQP1 and high water permeability. Water exits until descending-limb fluid approaches interstitial osmolality; NaCl is not actively pumped here.

Fresh fluid advances

Continuous flow pushes new ~300 mOsm/kg fluid into the descending limb and concentrated fluid around the bend, while dilute fluid exits the TAL.

Small becomes tall

Repeating pump → equilibrate → flow stacks small horizontal differences into a large vertical cortex-to-papilla gradient: ~300 to as high as ~1200 mOsm/kg.

Multiplier ≠ exchanger: the loop of Henle creates the gradient; slow hairpin flow in the vasa recta preserves it by passive countercurrent exchange.

Inner medulla

NaCl dominates the outer-medullary gradient. During antidiuresis, ADH increases UT-A1/UT-A3-mediated urea permeability in the inner medullary collecting duct; recycled urea supplies a major fraction of inner-medullary osmolality. ADH also inserts AQP2 in principal cells so the collecting duct can use—not create—the gradient.

LOOP DIURETIC

Blocks NKCC2

Reduces medullary hypertonicity, increases distal Na⁺ delivery, and impairs maximal urine concentration. It also abolishes the lumen-positive voltage that supports paracellular Ca²⁺ and Mg²⁺ reabsorption.

VASA RECTA

Preserves, not produces

Slow flow minimizes solute washout. Excess medullary blood flow removes NaCl and urea, flattening the gradient.

LOW ADH

Gradient can still exist

The TAL still creates dilute tubular fluid, but the collecting duct remains relatively water-impermeable. Result: high-volume, low-osmolality urine.

HIGH ADH

Needs an intact gradient

AQP2 alone cannot maximally concentrate urine if NKCC2 function, medullary architecture, vasa recta exchange, or urea recycling is impaired.

Checkpoint

Which structure is the countercurrent multiplier?

The loop multiplies; the vasa recta exchanges; the collecting duct exploits the gradient under ADH.