Question about Acid-Base Status and Osmolality

[monsoon338]

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Hi everyone, I had a question regarding two different things.

1) Diarrhea. I see that it can cause both a Metabolic Acidosis AND Metabolic Alkalosis. Acidosis is due to bicarb loss. That is one effect of the diarrhea. The other effect of diarrhea is you lose fluids (water) from your system as well. They system then increases aldosterone to maintain normal fluid levels with the side of an effect of alkalosis due to the effect of aldosterone.

So what exactly does diarrhea create? An Acidosis or alkalosis?

2) Hypovolemia

There are two types of hypovolemia in relation to hypernatremia - hypernatremic hypovolemia and hyponatremic hypovolemia. In Step Up to Step 2, the causes are the same however! Diarrhea, diuretics, renal failure, low aldosterone. These are causes to BOTH hypernatremic hypovolemia and hyponatremic hypovolemia. I don't understand how sodium can be high in one and low in the other with the same exact cause.

Can anyone help please?

 

[AndyRSC]

1. You had the mechanisms that occur in diarrhea completely right. Loss of H+ via kidneys, loss of K+ in stool, and increased CO2 excretion by ventilation do contribute to the pH balance, but not to the degree that bicarb loss does. Thus, diarrhea causes an overall metabolic acidosis.

2. The –natremias resulting from hypovolemia have different underlying mechanisms. Hyponatremia is usually caused by free water retention due to increased ADH. The hypovolemic hyponatremias specifically are caused by salt and water loss from the body, albeit more salt than water. They may be classified into renal causes (thiazide diuretics, Addison's disease, chronic renal disease, cerebral salt wasting, Legionellosis, etc.), where salt-wasting occurs directly from kidneys, and extrarenal causes (vomiting, diarrhea, third-spacing, large aspirations, etc.), where the large volumes of fluids are withdrawn from the body (or main spaces) and ADH mediates the hyponatremia.

Hypernatremic hypovolemia usually occurs due to loss of hypotonic fluid or impaired access to water. Again, this can be classified as renal losses (osmotic or loop diuretics, diabetes insipidus), or extrarenal losses (vomiting, osmotic diarrhea, fistula, fever, exercise, etc.).

To answer your question more directly, then, diarrhea can cause both, but depending on whether diarrhea is isotonic or hypotonic, and whether it's replenished with sodium-containing solution or just regular water, will determine whether the body becomes hyponatremic or hypernatremic. Of the diuretics, thiazide diuretics reduce the diluting capacity of the nephron, causing a hyponatremia, whereas loop diuretics decrease the urine concentrating ability, leading to osmotic diuresis and hypernatremia. Renal failure leads to a hyponatremia, via salt-wasting or impaired ability to secrete free water. Lastly, low aldosterone would also cause a hyponatremia, again by way of salt-wasting.