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
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 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: