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JMT
09-18-2007, 04:31 PM
In acidosis, there is a K+/ H+ exchange in the cells, H+ moves inside the cells and K+ moves out of the cell leading to hyperkalemia. Hyperkalemia should increase the GFR/Filtered Load of K+ and therefore an increase in K+ excretion. However there is decreased K+ excretion in acute acidosis instead. The Kaplan video explains the increase K+ excretion in chronic acidosis is due to the increase GFR from the hyperkalemia. If that's the case, then acute acidosis should have an increase excretion of K+ as well because they are both hyperkalemic, but it's the opposite, there is a decrease in K+ excretion in aute acidosis. Can anyone explain this step by step?

In alkalosis, again there is a K+/H+ exchange in the cells, but this time H+ moves out of the cell while K+ moves in. Kaplan says that intracellular K+ would be increased in Acute alkalosis and Intracellular K+ would be decreased in Chronic Alkalosis. So if K+ moves into the cell during alkalosis, why would intracellular K+ be decreased in chronic Alkalosis?

Also in Alkalosis, since K+ moves into the cell, there should be hypokalemia in both acute and chronic alkalosis, so How would K+ excretion be increased in both acute and chronic alkalosis if GRF for K+ is decreased from hypokalemia?

If someone can answer these 3 questions giving the steps involved, that would be wonderful.

Dr. X
09-21-2007, 10:40 AM
in my opinion, i would say its difficult to say when theres an acidosis, there will be such and such.. and if its alkalosis, there will be such and such. its important know why there is an acidosis to begin with and that could explain the hypo/hyper kalemia situations as both are possible. The generalization of saying in acidosis, there will be hyperkalemia is only a generalization. lol.. not much of wisdom there.

Whenever you see the word acidosis/alkalosis in a same context of "Chronic" i would always always always think.. "compensation" (probably opposite). Our body really does wonders.

1) just few thoughts: if theres lack of aldosterone - less sodium, more K+, more H+ (acidosis and hyperkalemia). Hyperkalemia to begin with can inhibit H+ excretion hence acidosis. For chronic, remember K+ is capable of absorbing or excreting 1-100% of dietary K in the DCT/CT.. when there's a need, it does its job.

2) chronic - K+ shift compensated.

3) when we measure K+.. we detect it by measuring it off the ECF. The driving force for excretion or reaborption actually relies on the metabolic changes in ICF. so the general idea is that when there is an alkalosis - H+ goes out, K+ comes in. The cell thinks it got lots of K+ inside the cell so time to increase K+ excretion.

To get you thinking.. apply the third concept that i said to your 1 question now.. and it should definitely help. Hope i didnt complicate it.

JMT
09-22-2007, 08:53 AM
The explanation from my friend:

"In acidosis the collecting will increase its secretion of H+. This will decrease the negative charge in the lumen (see figure on page 419) of the notes) and as a consequence decrease the secretion of K+. Thus in the initial stage of an acidosis the hyperkalemia is associated with a decrease secrtion of K+ which would aggravate the hyperkalemia. With a chronic acidosis there is a diuresis that originates at the level of the proximal tubule. This is thought to be due to the inhibition of the Na pump by the acidosis. The secretion of K into the colloecting duct is also flow dependent thus the diuresis reverses the situation and an elevated K secretion results.
Alkalosis is usually associated with a hypokalemia for several reasons. Acutely the potassium may enter the cell in exchange for H+ as stated in the Kaplan notes. This may be a significant contributor the the hypokalemia and there can be an increase in intracellular K+. However, overall there is a tendency for a loss of body K+ in an alkalosis maily because of the origin of the alkalosis. Major causes of alkalosis include:- vomiting, a fluid with a high conc. of K+; diuretics, which wash out K+; excess minerocorticoids which increase the secretion of K+. There is also an increased negative charge in the collecting duct due to the reduced H+ secretion which promotes secretion of K+. The overall loss of K+ long term is the reason that the intracellular level falls chronically."

Water
09-30-2007, 10:35 PM
Acidocis => Hyperkalemia
Acute response: Excrete H => increase cation in filtrate
=> decrease the other cations excretion such as Potassium
Chronic response: Hyperkalemia due to acidocis causes aldosterone release therefore there is an increase of k excretion by intercalated cells


Alkalocis => Hypokalemia
Acute response: excrete more bicarbonate (anion), cation excretion increases such as K
Chronic: continue bicarbonate excretion, and continue excretion of K (by this time aldosterone level may drop so excretion is not as much)







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