Question on Na+ and ADH response

[Daitong]

Hi all,

I read that when we have increased serum Na+, we have increased thirst and ADH secretion (to retain water and prevent hypernatremia); however, when total body Na+ is elevated, the body DECREASES ADH secretion.

I am confused on the latter portion as it seems to me that serum Na+ and total body Na+ are NOT the same (and don’t serve as a proxy for one another (?), so I was wondering if ya'll could clarify how I should think about the ADH response to Na+ in both these situations.


Ty in advance!

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[worldbeater]

I am confused on the latter portion as it seems to me that serum Na+ and total body Na+ are NOT the same (and don’t serve as a proxy for one another (?),

Yeah they are two separate things, like for the low volume (low BP) profile its low Na, low Cl, low K, high total body Na (which means in the cell I believe), high BP..they oppose each other

So if serum Na is increased, I could see why total Na would be decreased.

 

[P0ke]

Hi all,

I read that when we have increased serum Na+, we have increased thirst and ADH secretion (to retain water and prevent hypernatremia); however, when total body Na+ is elevated, the body DECREASES ADH secretion.

I am confused on the latter portion as it seems to me that serum Na+ and total body Na+ are NOT the same (and don’t serve as a proxy for one another (?), so I was wondering if ya'll could clarify how I should think about the ADH response to Na+ in both these situations.


Ty in advance!

It might be that if you have a high [Na] concentration in your blood, but a low total blood volume, flow through the PCT is reduced, meaning they have more time to absorb more sodium. By the time the fluid reaches the DCT, there is low [Na] in the fluid, and this is detected by the macula densa cells, which trigger renin release, which increases aldosterone.

If total blood Na is high, then that indicates there is a high blood volume. This increase in pressure could decrease firing of baroreceptors that stimulate renin release. Also, there is a higher rate of flow through the PCT with high blood pressure, so less Na is absorbed by the PCT cells. Macula densa cells in the DCT detect more Na, meaning less renin release and less aldosterone. This is probably a negative feedback mechanism to prevent your blood volume from getting too high.

I'm not sure if this is right, I just looked at my renal phys. notes from last semester briefly.