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

Medullary osmotic gradient

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

 

Mechanism

Medullary osmotic gradient is produced by

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

 

ADH

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

Thus,

Interstitial osmolality in inner medulla is reduced by

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

NB:

 

 



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