Moslem Doctor
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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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| Leopard
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The answer is simple.Due to activation of RAA system and release of ADH ,both Na (aldosterone + Angiotensin) and water (ADH) are reabsorbed but Water is reabsorbed disproportionaly more than water leading to hyponatremia.
Best of luck
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| Moslem Doctor
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leopard i dont disagree with you in that , read my Question again plz , i meant that the cause of hyponatremia cant be angiotensin or aldosterone alone because simply they reabsorb water and Na proportionaly and this cant lead to hyponatremia but it lead to volume overload only (and it is a character in all these cases ) and in this volume overload , Na concentration is normal , so the only cause for hyponatremia in these cases must be ADH secretion which deliver free excess water without Na , so hyponatremia develops , this can be accepted explanation?
dr leopard iam really thankful for your continous help which you offer to my questions in this forum and iam sorry for delaying in thanking you in last post but i had exam , thanks again
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| ssrpk
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man, don't get confused....loepard has already explained it...disproportionate inc. in free water due to ADH!
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| Leopard
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Angiotensin acts directly only on PCT and causes Na absorption. Water follows.But this case is not with Aldosterone.It acts on Late DCT and Collecting duct which are are impermeable to water in the absence of ADH.
Angitensin ----------> release of ADH -------------> water absorption in the late DCT and CT.'
Best of luck
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| Moslem Doctor
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dear doctor leaopard again thank u , i dont want to appear stupid here but i still confuse , in these cases we know there is ADH so already Dct and CT are permeable to water , so also aldosterone will retain Na and water together so >>> it is like angiotensin II in proximal tubule so >>>> no change in Na concentration , so why there is change in its concentration in these cases : because ADH make free water reabsorped without Na via high meduallary osmolarity . this what u mean?
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| Leopard
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Sorry to say ,that you do not have clear concept in the Late DCT and Collecting duct.
Aldosterone acts on Late DCT and CT while tubular fluid is made exctremely hypo-osmotic in the TAL (Na/K/2Cl transporter = 100 mosmol/L) and Eearly DCT (Na/Cl tansporter = 50 mosmole/L).
While Na absorption under regulation of Aldosterone is from Later DCT and CT.'
Late DCT and CT are normally impermeable and they become permeable due to ADH. If there is complete absence of ADH tubular fluid may pass as urine with osmolarity of 50 mosmol/L ,if there is max ADH then fluid will pass with osmolrity of 1200 mosmole/L
There is a lot of difference between water absorption from PCT and CT
In PCT there is iso-osmotic absorption.But in CT it is not iso osmotic absorption as you are thinking.It is regulated by
- presence of ADH (level of permeability is proportional to ADH)
- Cortico medullary gradient (which is not available at PCT).
If it were iso -osmotic absortption as you are thinking then fluid leaving CT as urine must of of 300 mosmol/L as it is when it is leaving PCT.
In PCT water absorption is totally Na absorption dependent but here other factors are operative.
Best of luck
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| Ig F
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yes this is an example of dilutional hyponatremia..
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| star1
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In ECF volume overload situation Atrial Natriuretic Peptide ANP a hormone is secreted primarily from right atrium and this causes increase excretion of Na from the kidney, so this will cause hyponatremia in edema state!!!!!!!!!!!!!!!!!!!!!!!!!!!!
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| Moslem Doctor
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so why edema developed first?! u started from the end when there is already edema so how can u explain formation of edema , all say it is because retaining sodium and water
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| star1
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what is edema?.............presence of excess fluid in the intestitial compartment
Why edema occurs?........when the effective osmolarity of the ECF increases, body will respond in the for of increase water reasborption .. Why this? ..........just to maintain plasma osmolarity at a constant level.
Now edema has different causes with different mechanisms....
nephrotic syndrome .... what happened? Plasma protein decrease due to proteinuria and so plasma osmotic pressure decrease and this favour fluid migration to the intestitial compartment
renal failure ....... what happened? water is being absorbed in the intestine but kidney can't excrete the excess fuild so plasma water increases and plasma osmolarity decrease so again same as above
cardiac failure.......Heart can't pump enough blood to kidney so kidney respond by activating Renin-Antiotensiin-Aldosternose system and increase absortion of Na, water will more with sodium... then same as above
liver cell failure.... what happened? protein are not being formed so plasma osmolarity increase then same as above
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| star1
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For hyponatremia its same two thing
dilutional effect
ANP
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