Loss of bicarbonate from the gastrointestinal tract
Pancreatic secretions contain bicarbonate, but this is usually reabsorbed by the end of the ileum. Most diarrhoeal illnesses will not lead to acidosis unless there is impaired ileal reabsorption. The classic example is cholera.
Diarrhoeal losses are also rich in potassium and hypokalaemia is a common feature.
The kidneys should respond by increasing acid excretion. The extra H+ appears in the urine as NH4+.
Most laboratories will not measure NH4+ directly, but it can be assessed by looking at the urine net charge:
Urine net charge = urine [Na+] + [K+] - [Cl-]
In normal urine the major cations are Na+, K+ and NH4+. The major antions are Cl- and HCO3-. The assumption is that NH4+ is excreted with Cl- and that if there is increased amounts of NH4+ in the urine, the net charge will be negative.
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Therefore if |
[Cl-] > [Na+] + [K+] Þ High [NH4+] = normal renal response [Cl-] < [Na+] + [K+] Þ Either a low [NH4+] or excretion of NH4+ with an anion other than Cl-. |