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Old 12-10-2011
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Kidney Why Hypokalemia with Diuretics?

Why do diuretics that block Na absorption cause hypokalemia and alkalosis?
As Na load in the tubule increases should it not actually inhibit the RAAS system leading to less aldosterone secretion?

Is it bcos as the Na load is increaased the water also increases,thereby decreasing Na concn, tricking the kidney into thinking its hypovolemia and hence the compensation will be to secrete more Aldosterone.

Pls pls pls someone clarify this doubt:sorry:
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Old 12-10-2011
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If there is No reabsorption in the proximal tubule..there will e load on the distal tubule and it will try to reabsorb Na+..and because of Na+ reabsorption..K+ will be lost in the diatal tubule collecting tubule..in the principal cells in think..and in the intercalated cells there will be reabsorption of HCO3- ions which will lead to alkolosis..
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Old 12-10-2011
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Originally Posted by usmle step 1 View Post
Simple when diuretics blocks sodium reabsorbtion above the collecting duct it causes greater sodium load in tubular fluid and this will reach collecting tubules where this sodium is absorbed in exchange of potassium( this is aldosterone mediated effect at principal cells) so sodium is absorbed and potassium secreted ! Greater potassium secretion so hypokalemia
Yeah but why is the Aldosterone secretion high, Aldosterone rises when RAAS is active and that is activated by decreased Na load in the tubules. In diuretics the Na load is high so why should aldo increase???

My best guess is that aldosterone will increase due to Sympathetic stimulation cos of volume loss.
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Old 12-10-2011
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can more readers pls try clarifying this
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Old 12-10-2011
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as i remember from kaplan:

an increase in sodium load in the distal tubules[example, diuretics]
will lead to increase in potassium excretion[causing hypokalemia]

how?

since sodium reabsorption in distal tubules is selective(meaning only sodium will only enter the cell, LEAVING its partner, chloride inside the lumen)
-->
this will cause a GREATER negative potential(chloride has a negative charge) inside the lumen, promoting and inviting the potassium(which has a positive charge) inside the cell to go outside the cell and stay in the lumen.

Thus, diuretics cause hypokalemia.

Correlate: Potassium sparing drugs(example, SAT[amiloride&Triamterene]) act by blocking THAT sodium channel. Thus, decreasing negative potential -> decreased potassium excretion -> Hyperkalemia

Last edited by neoeinstein; 12-10-2011 at 10:28 PM. Reason: spelling
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Old 12-10-2011
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That makes sense neo,thanks.
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Old 12-11-2011
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The concept is fairly simple..
Since less Na is absorbed from the proximal tubule, the load of sodium is high in the urine when it reaches the distal tubule. Now there is a channel for sodium absorption here, so it tries to absorb as much Na as possible and in return there is a loss of K leading to hypokalemia and also a loss of Hydrogen ions leading to alkalosis!
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Acid-Base-, Pharmacology-, Physiology-, Renal-

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