Because carbon dioxide is used by the body to regulate pH. If the body is acid, the brain tells the body to breathe faster to get rid of the acidic carbon dioxide. The same principle applies to breath hold. The urine pH may be acid. But if the acidosis is caused by excess potassium in the system, the kidneys will dump potassium and retain hydrogen. So the urine is alkaline while the body fluids are acid.
Alkalosis is the opposite.
A person with alkalosis will generally have a respiratory rate of 13 or slower and a breath hold of 65 seconds or more. There is also an acidosis caused by a glucogenic or fast oxidizer imbalance. Fast oxidation produces excess carbon dioxide and thus alkalosis. Slow oxidizers are the opposite. The anaerobic person generally has alkaline urine, high urine surface tension and acid saliva among other signs.