One of the by-products of food metabolism is CO2, carbon dioxide. However, in order to eliminate all of the carbon dioxide that is generated from normal metabolism, the lungs would need a respiration rate far above normal breathing. Holding this constantly accelerated rate would indeed be very difficult. Therefore, other mechanisms comes into play for handling the excess. Hydrogen ions are equivalent to acidity.
Acids are a normal by-product of metabolism.
The body has the mechanisms in place to eliminate these acids. But poor dietary habits, prolonged stress, lack of exercise, and toxicity exposures can lead to liver and kidney malfunction, and the acids in the body do not always get eliminated as they should. When the body has an excess of acid it can't get rid of, it gets stored for later removal. Therefore, in order to be in balance, it knows that for every molecule of acid that gets stored in the tissues, an equal molecule of bicarb or base needs to be put into the blood because one day it will need to escort the acid out of the body. Below or above this range means symptoms and disease.