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
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.