why there is hyponatremia in these cases?

[Moslem Doctor]

in oedema forming diseases ( nephrotic syndrome , renal failure , cardiac failure ,liver cell failure ) there is increasing in total body sodium due to activation of rening-angiotensin system , which results in increasing sodium and water retention especially from proximal tubule , but why hyponatremia develope?! i find this strange and guyton also says that increasing angiotensin II cant be responsible for developing hyponatremia as water and sodium be reabsorbed proportionaly , so what is the cause for developing hyponatremia in this disease ? can we say it is ADH which deliver excess free water ?

thanks

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[browncow]

low sodium

 

[pleaseletmepass]

As a result of low effective circulating volume, the body compensates by increasing aldosterone and ADH release. Aldosterone increases sodium reabsorption in the late distal tubule and collecting ducts, with water following passively. In addition, ADH increases water reabsorption in these same segments. The net result is that more water is reabsorbed relative to sodium, thus hyponatremia.

 

[Hallm_7]

ADH is secreted because of two mechanisms:
1. CSF Na+ chemoreceptors in the 3rd ventricle
2. Baroreceptors detecting a drop in BP

In heart failure there is a drop in blood pressure stimulating the barorectors to signal to the brain to release ADH. It would stand to reason that the CSF chemoreceptors would then inhibit ADH release because of decreasing osmolality. However, the baroreceptors' input signalling an increase in ADH release trumps the chemoreceptor input signalling a drop in ADH release.