A great many metabolic processes result in the production of acids and these must be eliminated from the body if the reaction of the tissue and blood is to remain within the normal range. The route of disposal of acids depends upon whether or not they are capable of being oxidized completely to carbon dioxide and water. Carbonic acid within the body is eliminated as carbon dioxide by ventilation. Other acids such as aceto-acetic or sulphuric, which are derived from oxidation of the fatty acids or sulphur containing proteins, respectively, are excreted by the kidneys. At the site of their production in the tissues and during their carriage in the blood, all acids increase the hydrogen ion concentration. Its extent is minimized by the stabilizing power of the blood and tissues. Carbonic acid is produced by metabolic reactions far greater than any other acid. A greater part is converted into carbonic acid in the red cells. The hydrogen ions of carbonic acid are taken up by the haemoglobin in the red cells while the bicarbonate ion moves out from the red cells to the plasma in exchange for chloride ions (chloride shift). Most of the carbonic acid added to the blood, therefore, appears not as acid, but as bicarbonate ion. When the blood passes through the lungs and the haemoglobin is re-oxygenated, the process is reversed and the carbon dioxide formed is expelled by ventilation. In health, a small amount of inorganic acid produced each day and being nonvolatile requires to be excreted by the kidneys. The excreting power of the kidneys is the limiting factor in the body to rid itself of ions derived from inorganic acid and organic acids other than carbonic acid.”