Step I Paradoxic aciduria

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Paradoxic aciduria - this is seriously killing me.

There's a question in Usmle Rx which basically asks what is the cause of paradoxic acuduria in a vomiting patient.

The answer was "hypokalemia".

The explanation: Basically the acidic urine occurs b/c the kidneys preserve Na (and H20) at the expense of K and H.

The point that confuses me is, is the Na that is reabsorbed exchanged for excretion of K+ or H+? Rx says Na is initially exchanged for K. As K is depleted in this manner, K is eventually exchanged for H to prevent worsening hypokalemia.

However other sources say Na is exchanged with H directly, and not with K.

So is it Na/K + K/H exchange? or Na/H + Na/K exchange?

Anyone know what I'm talking about? Is this being too nitpicky? Thanks!!

 

[Transposony]

1. Depletion of HCl causes a hypochloremic metabolic alkalosis and dehydration because of the loss of H+ and Cl- ions in addition to fluid.

2. Renal compensation for this loss of H+ ions is initially by preserving H+ at the expense of K+ leading to hypokalemia with hypochloremic, hypokalemic metabolic alkalosis.

3. If untreated, volume depletion results in renal Na+ retention partially in exchange for K+ excretion (Aldosterone).

Hypokalemia is not the mechanism for the acidotic urine. Increased H+ exchange for Na+ is the cause of paradoxic aciduria.