BUFFER
SYSTEMS
Buffer systems prevent
major changes in the pH of body fluids by removing or releasing H+; they can act quickly
to prevent exces-sive changes in H+ concentration. Hydrogen
ions are buffered by both intracellular and extracellular buffers. The body’s
major ex-tracellular buffer system is the bicarbonate-carbonic acid buffer
system. This is the system that is assessed when arterial blood gases are
measured. Normally, there are 20 parts of bicarbonate (HCO3−) to one part of
carbonic acid (H2CO3). If this ratio is al-tered, the pH
will change. It is the ratio of HCO3− to H2CO3
that is important in maintaining pH, not absolute values. Carbon dioxide (CO2)
is a potential acid; when dissolved in water, it be-comes carbonic acid (CO2+ H2O = H2CO3).
Thus, when CO2 is increased, the carbonic acid content is also
increased, and vice versa. If either bicarbonate or carbonic acid is increased or
de-creased so that the 20:1 ratio is no longer maintained, acid–base imbalance
results.
Less important buffer systems in the ECF include
the inor-ganic phosphates and the plasma proteins. Intracellular buffers
include proteins, organic and inorganic phosphates, and, in red blood cells,
hemoglobin.
The kidneys regulate the bicarbonate level in the ECF; they can
regenerate bicarbonate ions as well as reabsorb them from the renal tubular
cells. In respiratory acidosis and most cases of meta-bolic acidosis, the
kidneys excrete hydrogen ions and conserve bicarbonate ions to help restore
balance. In respiratory and meta-bolic alkalosis, the kidneys retain hydrogen
ions and excrete bi-carbonate ions to help restore balance. The kidneys
obviously cannot compensate for the metabolic acidosis created by renal
failure. Renal compensation for imbalances is relatively slow (a matter of
hours or days).
The lungs, under the
control of the medulla, control the CO2 and thus the carbonic acid
content of the ECF. They do so by ad-justing ventilation in response to the
amount of CO2 in the blood. A rise in the partial pressure of CO2
in arterial blood (PaCO2) is a powerful stimulant to respiration. Of
course, the partial pressure of oxygen in arterial blood (PaO2) also
influences respiration. Its effect, however, is not as marked as that produced
by the PaCO2.
In metabolic acidosis, the respiratory rate
increases, causing greater elimination of CO2 (to reduce the acid
load). In metabolic alkalosis, the respiratory rate decreases, causing CO2
to be retained (to increase the acid load).
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