So, I am a bit unsure about the effects of vomiting and diarrhea and serum sodium and serum osmolality. A couple of sources say that the fluid loss from diarrhea is isotonic (Na and H2O are lost in equal molar amounts), which would just cause a reduction in the ECF volume, but I have also seen diarrhea and vomiting listed as causes of hypertonic volume loss (More Na is lost relative to H2O) which would cause an increase in the ICF and a decrease in the ECF. I was wondering which of these is actually the case or if there are situations when it is both true?
GI loss of sodium causes what?
- Thread starter CBG23
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Fluid losses from the GI tract can be from multiple causes and thus the fluids can have multiple compositions. In terms of sodium i like to group them very broadly as per Harrison's into non-secretory (lactulose, sorbitol, carb malabsorption, viral gastroenteritis) and secretory (laxative abuse, cholera, VIPoma, villous adenoma, carcinoid).
If you look at the non-secretory diarrheas - you can see why they are also called "osmotic diarrheas". Osmoles sitting in the GI tract pulling in water and trapping it there, thus you lose more free water than you do sodium --> hypernatremia.
On the other hand secretory diarrheas involve the secretion of sodium into the GI tract. Water will follow of course but at best you'll be losing isotonic fluid and not changing serum sodium concentration (by itself anyway, the patient may have replaced the losses with water and you could get a little hyponatremic).
If you look at the non-secretory diarrheas - you can see why they are also called "osmotic diarrheas". Osmoles sitting in the GI tract pulling in water and trapping it there, thus you lose more free water than you do sodium --> hypernatremia.
On the other hand secretory diarrheas involve the secretion of sodium into the GI tract. Water will follow of course but at best you'll be losing isotonic fluid and not changing serum sodium concentration (by itself anyway, the patient may have replaced the losses with water and you could get a little hyponatremic).
