Clinical meaning
The body maintains blood pH within a very narrow range of 7.35 to 7.45, which is slightly alkaline. Even small deviations outside this range can impair enzyme function, alter electrolyte balance, and compromise cellular metabolism. Three primary regulatory systems maintain acid-base balance: the chemical buffer system (responds within seconds), the respiratory system (responds within minutes), and the renal system (responds within hours to days). The bicarbonate buffer system is the body's most important extracellular buffer, consisting of carbonic acid (H2CO3) and bicarbonate (HCO3-). The normal ratio of bicarbonate to carbonic acid is 20:1, and this ratio -- not the absolute values -- determines blood pH. The respiratory system regulates pH by adjusting the rate and depth of ventilation, thereby controlling CO2 elimination. When CO2 accumulates (hypoventilation), it combines with water to form carbonic acid, lowering pH (acidosis). When CO2 is blown off through hyperventilation, carbonic acid decreases and pH rises (alkalosis). The kidneys regulate acid-base balance by reabsorbing or excreting bicarbonate and hydrogen ions. The kidneys can regenerate bicarbonate and excrete hydrogen ions in the form of ammonium (NH4+) and titratable acids. An arterial blood gas (ABG) provides four critical values: pH (7.35-7.45), PaCO2 (35-45 mmHg, the respiratory component), HCO3- (22-26 mEq/L, the metabolic component), and PaO2 (80-100 mmHg, oxygenation status). The practical nurse must understand these values to recognize acid-base disturbances and report findings accurately. Compensation occurs when one system attempts to correct the imbalance caused by the other: respiratory compensation adjusts CO2 within minutes, while renal (metabolic) compensation adjusts HCO3- over hours to days. Full compensation returns pH to normal range; partial compensation moves pH toward normal but does not fully restore it.