Sweating-101

[Phloston]

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Is sweat:

normally isotonic?

hypertonic in CF?

Cheers,

 

[thehundredthone]

Hypotonic normally. Isotonic in CF.

 

[Phloston]

Hypotonic normally. Isotonic in CF.

Where did you read that?

 

[Suncrusher]

found a paper that mentioned sweat was hypotonic normally and everything else i read backs that up. Robbins says that in CF sweat is hypertonic...but lung/pancreas/etc mucus is isotonic because (unlike with skin apparently) water simply reequilibrates across the membrane, with the side effect of dehydrating the lumenal mucus.

 

[Phloston]

found a paper that mentioned sweat was hypotonic normally and everything else i read backs that up.

Okay, cool.

Robbins says that in CF sweat is hypertonic...

I knew it....

I had encountered a question in USMLE Rx back in March/April that had said that sweating due to CF is the equivalent of using a loop diuretic in terms of volume change. P.57 of Goljan's RR says loops are hypertonic losses.

but lung/pancreas/etc mucus is isotonic because (unlike with skin apparently) water simply reequilibrates across the membrane, with the side effect of dehydrating the lumenal mucus.

Yeah, UWorld says that the inspissation of luminal contents is due to isotonic dehydration.


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So normal sweat = hypotonic
CF sweat = hypertonic
CF-induced inspissated contents = isotonic

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I just realized: if fluid movement is isotonic, it's because the water moves with the Na+, and the process is dependent on the sodium (e.g. aldosterone-induced reabsorption of water or ENaC upregulation in the pancreas secondary to defective CFTR). However, in sweat, since the concentration of Cl- is increased in CF, the process must be instead dependent on chloride, where the anionic losses matched by sodium must be proportionally greater (i.e. increased molarity of NaCl) than the corresponding water losses, thereby inducing hypertonicity.

 

[thehundredthone]

Isotonic, hypotonic even.

http://www.ncbi.nlm.nih.gov/pubmed/6543847

In 27 CF patients sweat osmolality was 285.86 +/- 41.25 mmol/kg (mean +/- SD), the mean value - 2 SD being 203.36 mmol/kg. In 266 control subjects sweat osmolality was 107.38 +/- 29.49 mmol/kg (mean +/- SD), the mean value + 2 SD being 166.36 mmol/kg.

http://adc.bmj.com/content/82/5/420/T2.expansion.html

In any case the osmolality/Cl content of the sweat being higher than normal is what matters, not the absolute value.

 

[Suncrusher]

osmolality != tonicity. Plasma has all sorts of stuff that contributes to extra osmolality

 

[thehundredthone]

Quoting the already presented source. Let's go by Na and Cl.

Na (mmol/L) - Control: 23 (12) CF: 88 (15)
Cl (mmol/L) - Control: 14 (8) CF: 99 (18)

Looks hypotonic to me.

Here's more:
http://www.ncbi.nlm.nih.gov/pubmed/2182749

Disclaimer: If I've completed misunderstood this discussion I'm sorry. I'm on autopilot, Step 1 in 10.5hrs, sleep in .5hrs.

 

[Suncrusher]

Disclaimer: If I've completed misunderstood this discussion I'm sorry. I'm on autopilot, Step 1 in 10.5hrs, sleep in .5hrs.

I don't necessarily think you're wrong (if you are, I'm too stupid to figure out how) I'm just trying to figure out where the confusion and discrepancy between sources is coming from so that I know which answer to put for step 1. I looked on OMIM and a lot of the papers simply refer to "increased sodium/chloride concentration" or "increased tonicity" rather than outright calling it hypertonic, so it could be that it is indeed still hypotonic in CF and Robbins was just lazy (calling it hypertonic when it is actually only hypertonic in relation to normal sweat)

P.S. Good luck on step1! 👍 Get off SDN lol