Mannitol and Tubular fluid Osmolarity

[Kobebucsfan]

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FA says Mannitol increases tubular fluid osmolarity --> more urine flow



i thought osmolarity would be low? Mannitol inhibits water reabsorption, so lot of water in the tubular fluid, shouldnt that decrease osmolarity ?

 

[zhopv10]

The idea is that it (mannitol) increases the osmolarity initially/through the tubules thus pulling more water in. Mannitol is not reabsorbed so it maintains a higher level of solute in the tubule. Whether or not the osmolarity of the filtrate by the end is lower or higher doesn't matter as much but the volume of water lost will be greater from the osmotic pressure.


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[zhopv10]

If I remember right it can be used with increased intracranial pressure for the ability to pull out extra water too...can't remember if that's true or not tho.


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[Jabbed]

FA says Mannitol increases tubular fluid osmolarity --> more urine flow



i thought osmolarity would be low? Mannitol inhibits water reabsorption, so lot of water in the tubular fluid, shouldnt that decrease osmolarity ?

Mannitol itself is contributing to the increased tubular osmolarity. You are correct in saying that there will be a greater volume of water remaining in the tubular fluid.

 

[Phloston]

With high intracranial pressure, you elevate the bed, do permissive hypocapnia at 26-33 pCO2 (normal is 33-44), plus or minus mannitol. Steroids can be used only if there's genuine cerebral edema (e.g., from intracerebral malignancy).

Mannitol itself increases osmolarity because it's a solute that will stay confined to the renal tubules.

Increased straight proximal tubule / thin descending limb osmolarity --> increased free water movement from interstitium into renal tubules --> increased urine flow.

Don't use in cardiac failure because of transient possibility of causing increased vascular volume --> increased preload --> increased cardiac workload.