Question

Explain in detail how the medullary osmotic gradient is generated and maintained and why the gradient is important for regulating water balance and urine volume

Answer 1

Urea recirculates in the inner medulla, building a stronger osmotic gradient. It enters the interstitial space from the collecting duct, following the concentration gradient created by the ADH-sensitive water reabsorption. Increased interstitial osmolarity pulls the remaining water from the descending structures.

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