Countercurrent mechanisms and ADH

Vivian Imbriotis | Jan. 4, 2026

A countercurrent system is a system in which the inflow runs parallel to, counter to, and in close proximity to the outflow for some distance.

Countercurrent multiplication allows the "single effect" of the ion pumps in the ascending loop of Henle to be magnified by concerntrating the filtrate and interstitium of the inner medulla.

Countercurrent exchange prevents vasa recta blood flow from disrupting the medullary osmotic gradient.

Urea recycling allows the concentrated urea in the distal collecting duct to diffuse into the medullary interstitium, further concentrating it.

The single effect

The ascending loop is actively excretes solutes (secondary active transport by NKCC2)

This increases the osmolality in the interstitium. (e.g. 300 \(\to\) 400mOsm)

The descending LOH is water-permiable, solute-impermiable; water flows out by osmosis to equilibrate with the interstitium.

The descending fluid is now more concerntrated than the ascending fluid.


Multiplication of the single effect by counter current flow

The concerntrated descending fluid is delivered to the deep part of ascending LOH (400mOsm).

The medulla remains concerntrated (400mOsm)

This allows more solute to be pumped out (the inner medulla is now 500mOsm)

More water moves out of the descending loop, concentrating it further

And so on in a positive feedback loop...


Osmolality values

Cortex: 300mOsm

Outer medulla: 800mOsm

Inner medulla: 1200mOsm


Proximal tubule: 300mOsm

Descending limb: 800mOsm

Hairpin turn: 1200mOsm

Ascending thin limb: 800mOsm

End of ascending thick limb: 100mOsm

Descending and ascending vasa recta are highly permiable vessels. The flow is low, ~10% of renal blood flow.

Water leaves descending vasa recta, concerntrating plasma and reducing flow rate as it penetrates deeper into the medulla. It diffuses to nearby ascending vasa recta and is carried back to the cortex.

Solutes diffuse out of ascending vasa recta, carrying concerntrated fluid from the inner medulla, and move into descending vasa recta.

This prevents washout of the medullary osmotic gradient.

Proximal collecting system is permeable to water (via ADH-sensitive aquaporins), not to urea.

Urea becomes very concentrated (~500mmol/L)

The inner medullary collecting duct is highly permiable to urea. About half the urea load diffuses into the inner medulla, contributing to the medullary osmotic gradient. Half is excreted.

Urea diffuses from the medulla to the ascending limb of LOH, and is then re-delivered to the cortical collecting duct.

Increased ADH

V2 receptors in collecting duct cells agonised

Increased aquaporin expression throughout the collecting duct.

Increased urea transporter expression in the inner medullary collecting ducts.

Urine becomes very concerntrated in the collecting system

Most urea is then reabsorbed in the distal collecting duct

Very high inner medullary osmolality results \(\to\) increased reabsorption in LOH.

V1 receptors in the vasa recta are agonized, decreasing medullary blood flow.