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 If either bicarbonate or carbonic acid is increased or de-creased so that the 20:1 ratio is no longer maintained, acid–base imbalance results.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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