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Answer to Question #3960 in Biochemistry for sharon

Question #3960
An urine specimen is found to contain high level of ammounium ions (NH4+) is this an example of blood acidosis or alkalosis? explain
Expert's answer
Answer: Acidosis.Explanation: Free hydrogen ions are not filtered by the kidney.
Instead, they are secreted into the tubular fluid. Because free
hydrogens cannot be excreted in the urine easily, there are excreted
with weak acids such as H2PO4- which function as urinary buffers. Now
in the presence of an increased acid load, the phosphate ions are used
up and the kidney then increases its production of ammonium. That's
why new bicarbonate ions are produced during ammoniagenesis. The
generation of new bicarbonate ions conceptually is akin to increased
hydrogen excretion. Therefore the real function of ammoniagenesis is
not as an urinary buffer of hydrogen ions as is commonly inaccurately
described. The real function of ammoniagensis is to increase the
generation of new bicarbonate ions. Ammonium before it is excreted is
first re-absorbed in the thick ascending limb, circulated in the
medullary interstitium and then pumped back in the collected tubule as
ammonia. In the collecting tubule, ammonia then takes up hydrogen ions
secreted into the lumen by intercalated cells, to form ammonium. In
order for ammoniagenesis to be effective in the generation of new
HCO3-, the NH4+ produced must be excreted in the urine. NH4+ secreted
in the proximal tubule is in equilibrium with a small quantity of NH3.
The urine does not become maximally acidified until the collecting
tubule where secretion of hydrogen ions by intercalated cells
significantly reduce the urine pH. Therefore the kidney prevents loss
of ammonium by reabsorbing NH4+ in the thick ascending limb and
pumping it into the collecting duct where the urine pH is very low,
facilitating ammonia in its protonated form. This process is enhanced
during periods of acidosis when hydrogen secretion by the intercalated
cells is significantly increased.

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