In addition to maintaining fluid and electrolyte balance, the body must also maintain acid-base balance. This is the regulation of hydrogen ions in body fluids (pH balance).
In a water solution, an acid gives off hydrogen ions and a base picks them up. Hydrochloric acid is an example of an acid found in the body. It is secreted by the stomach and is necessary for the digestion of proteins. Ammonia is a base produced in the kidneys from amino acids.
Acidic substances run from pH 1 to 7, with the lowest numbers repre-senting the most acidic (which contain the most hydrogen ions). Alkaline substances run from pH 7 to 14, with the alkalinity increasing with the num-ber (as the number of hydrogen ions decreases). A pH of 7 is considered neutral. Blood plasma runs from pH 7.35 to 7.45. Intracellular fluid has a pH of 6.8. The kidneys play the primary role in maintaining the acid-base balance by selecting which ions to retain and which to excrete. For the most part, what a person eats affects the acidity not of the body but of the urine.
The body has buffer systems that regulate hydrogen ion content in body fluids. Such a system is a mixture of a weak acid and a strong base that reacts to protect the nature of the solution in which it exists. In a normal buffer system, the ratio of base to acid is 20:1. For example, when a strong acid is added to a buffered solution, the base takes up the hydrogen ions of the strong acid, thereby weakening it. When a strong base is added to a solution, the acid of the buffer system combines with this base and weakens it.
A mixture of carbonic acid and sodium bicarbonate forms the body’s main buffer system. Carbonic acid moves easily to buffer a strong alkali, and sodium bicarbonate moves easily to buffer a strong acid. Amounts are easily adjusted by the lungs and kidneys to suit needs. For example, the end products of metabolism are carbon dioxide and water, and together they can form carbonic acid. The hemoglobin in the blood carries carbon dioxide to the lungs, where the excess is excreted. If the amount of carbon dioxide is more concentrated than it should be, the medulla oblongata in the brain causes the breathing rate to increase. This increase, in turn, increases the rate at which the body rids itself of carbon dioxide. Excess sodium bicarbonate is excreted via the kidneys. The kidneys can excrete urine from pH 4.5 to pH 8. The pH of average urine is 6.
The healthy person eating a balanced diet does not normally have to think about acid-base balance. Upsets can occur in some disease conditions, however. Renal failure, uncontrolled diabetes mellitus, starvation, or severe diarrhea can cause acidosis. This is a condition in which the body is unable to balance the need for bases with the amount of acids it is retaining. Alkalosis can occur when the body has suffered a loss of hydrochloric acid from severe vomiting or has ingested too much alkali, such as too many antacid tablets.