Sources of acid or alkali

By far the greatest production of chemical which threatens acid-base homeostasis is that of carbon dioxide in metabolism. Quantitatively, this amounts to 25 Moles of acid per day. This acid is blown off by the lungs (Table 5.1). 

The food ingested by mouth in everyday life is usually neutral but it yields non-respiratory acids and alkalis as a result of metabolism. In normal circumstances it is unusual for the production of acid exactly to balance that of alkali. The one which is produced to excess is excreted in the urine. 

During starvation, body fats replace dietary carbohydrates as a major source of energy. Metabolism of these fats yields organic acids such as acetoacetic acid. This causes a mild metabolic acidosis. In strenuous exercise, particularly in a trained athlete, fats are preferentially metabolized also, in exercise sufliciently strenuous to exceed the anaerobic threshold, lactic acid is released into the extracellular fluid in all subjects, both the trained athlete and the normal person. Fever and trauma such as extensive bums or major surgery result in tissue breakdown this catabolism results in the release of organic acids into the extracellular fluid. 

Meat-sulphur-containing amino acids yield sulphuric acid Vegetables yield alkali Excreted via the kidneys 70 m Moles per day 

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