Medullary osmotic gradient

Final concentration of urine in the collecting ducts rely on the medullary osmotic gradient

 

Mechanism

Medullary osmotic gradient is produced by

• Active sodium transport by thick ascending limb from lumen into interstitium
• Low blood flow countercurrent mechanism
  * (of vasa recta)
• Recycling of urea
  * Between medullary collecting ducts and deep portions of loop of Henle

Cortex

Rich blood supply (peritubular capillaries)

--> Solutes reabsorbed from lumen do not accumulate in interstitium

Medulla

Lower blood supply

--> Solutes can accumulate in interstitium

Countercurrent exchange system of the vasa recta

--> Preservation of osmotic gradient

Active sodium transport causes the hyperosmolality

Recycling of urea

Diffusion (i.e. resorption) of urea out of lumen into interstitium contribute to medulla osmolality

• Urea contribute about half of medullary osmolality
• Sodium chloride contribute the other half

 

ADH

ADH increases water permeability in cortical AND medullary collecting duct

ADH also increases urea resorption by

• Increasing urea concentration in lumen
• Increasing permeability in the inner medullary collecting duct
  * By increasing urea uniporter

--> Allows recycling of urea

--> Increase medullary osmotic gradient

Inner medulla

Inner medullary collecting ducts has finite water permeability

When ADH is absent

--> Osmotic gradient in MCD (between lumen and interstitium) is large

--> Increased water reabsorption in MCD
* In contrast to cortical and outer medullary collecting ducts

Thus,

Interstitial osmolality in inner medulla is reduced by

• Increased water reabsorption
• Decreased urea reabsorption

--> Kidney's ability to concentrate urine is reduced

NB:

• Osmolality can be reduced to half of the level when ADH is maximal