Total Body Sodium And Volume Depletion

[lemonade90]

1-Why are there signs of volume depletion when Total Body Na goes down?
From what I can gather the ECF decreases and the ICF doesn't change. Is it because there is decreased hydrostatic pressure in the capillaries (due to decreased ECF) so less fluid is going out into the interstitial compartment and then less is going in the ICF compartment? This is what Goljan seems to be saying (pages 55-56 of revised 3rd edition), but I would like to clarify.

2-Additionally is a disturbance in TB Na+ the same thing as an isotonic fluid disorder (Goljan 55-56)?

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[Fuzuli]

Serum sodium concentration: Analogous to hematocrit
Total body sodium: Analogous to RBC mass

Just like hematocrit is a reflection of total RBC mass, depending on the volume of the blood; serum sodium concentration is a reflection of total body sodium, depending on ECF water volume. Therefore, this relation can be expressed as:

Serum Na+ concentration = ECF Na+ / ECF volume

where a decrease in serum [Na+] is called as hyponatremia. Therefore, hyponatremia can be seen both in conditions with:

(a) Increased ECF water (ECF Na+ ↑ / ECF volume ↑&#8593: This is seen in CHF, cirrhosis, and nephrotic syndrome (i.e. hypervolemic hyponatremia)
(b) Decreased ECF water (ECF Na+ ↓↓ / ECF volume &#8595: This is seen in diuretic use, Addison's disease, and diarrhea (i.e. hypovolemic hyponatremia)

As you can see, changes in ECF volume are related to ECF Na+ content (total body sodium). But changes in ECF volume may not reflect serum [Na+], as hyponatremia can be seen both in hypervolemic and hypovolemic conditions; and a patient with volume overload can be hyponatremic (ECF Na+ ↑ / ECF volume ↑↑ - most often seen) or hypernatremic (ECF Na+ ↑↑ / ECF volume ↑ - rare; seen in conditions with infusion of hypertonic fluid).

The next step is based on Starling forces: In hypervolemic hyponatremia, since ECF volume is increased, hydrostatic pressure inside the blood vessels will increase → exudation of fluid into the interstitium → edema. This won't be seen in hypovolemic hyponatremia.

 

[rickjamesbtch]

1-Why are there signs of volume depletion when Total Body Na goes down?
From what I can gather the ECF decreases and the ICF doesn't change. Is it because there is decreased hydrostatic pressure in the capillaries (due to decreased ECF) so less fluid is going out into the interstitial compartment and then less is going in the ICF compartment? This is what Goljan seems to be saying (pages 55-56 of revised 3rd edition), but I would like to clarify.

2-Additionally is a disturbance in TB Na+ the same thing as an isotonic fluid disorder (Goljan 55-56)?

Just remember this and you'll never be confused. Water follows Na. For whatever reason, if Na is being lost in the urine then intravascular volume is being lost as well. When losing volume the intravascular space will be the first space to experiences volume loss. Since the intravascular volume is lost the hydrostatic pressure in the vessel is now increased pulling more water from the cell into the vessel. This is how fluid loss leads to dehydration.

 

[Bernoull]

Just remember this and you'll never be confused. Water follows Na. For whatever reason, if Na is being lost in the urine then intravascular volume is being lost as well. When losing volume the intravascular space will be the first space to experiences volume loss. Since the intravascular volume is lost the hydrostatic pressure in the vessel is now increased pulling more water from the cell into the vessel. This is how fluid loss leads to dehydration.

I'm not sure if you meant oncotic pressure as opposed to hydrostatic pressure? One would expect hydrostatic pressure to fall along with reduced instravascular volume, and assuming an intact renal system, proteins should be retained hence oncotic pressure increases...

 

[rickjamesbtch]

Ya I meant oncotic pressure, sorry for the confusion. Thanx man..!!

 

[Bernoull]

Ya I meant oncotic pressure, sorry for the confusion. Thanx man..!!

anytime, I figured it was a typo..

great explanation tho..