[Water-electrolyte and acid-base disorders. V. Acid-base balance].

Disturbances of acid-base homeostasis are frequently associated with many commonly observed disease states. Recognition of abnormal plasma acid-base composition may often direct the clinician to the diagnosis of a specific disease. Simply stated, external hydrogen ion balance exits when the net hydrogen ion production from cellular metabolism equals the net hydrogen ion excretion from the body. Net cellular production of hydrogen ions consist of two components: carbonic acid, which is excreted by the lungs as CO2, and fixed acids, which require renal excretion. The net amount of fixed acid production is determined by cellular metabolism, dietary intake and gut absorption of available base equivalents. The immediate defense of pH is provided by the buffering systems, which also act as transporters of the acid from sites of production to sites of elimination. CO2 exchange by the lungs is responsible for the bulk of acid excretion: more than 99.5 percent of the normal daily acid load and 100 percent of CO2 produced by metabolism are eliminated by this route. On the other hand, the kidney is the only organ capable of excreting an appreciable quantity of fixed acid as titratable acids an ammonium, with the regeneration of bicarbonate buffer. Acid-base; buffering systems; acid-base regulation.