For proper cellular metabolism, the hydrogen ion concentration has to be maintained around pH 7.4 (range 7.37 to 7.45). About 20,000 mmoles of carbonic acid and non-volatile acids are added to the system everyday by metabolism. Even then, the pH of body fluids is kept fairly constants by the action of the buffer systems, Kidneys and lungs.
Conditions in which there is addition of acid or removal of alkali from the system are termed acidosis. In academia, the pH is measurably lowered. Acidosis may be metabolic or respiratory. The former is due to addition of acids or removal of bases. The latter is due to retention of carbon dioxide.