3. Physiology
        3.12. Renal
            3.12.6. Renal handling of sodium, chloride and water Medullary osmotic gradient

Medullary osmotic gradient

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



Medullary osmotic gradient is produced by


Rich blood supply (peritubular capillaries)

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


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



ADH increases water permeability in cortical AND medullary collecting duct

ADH also increases urea resorption by

--> 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


Interstitial osmolality in inner medulla is reduced by

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




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