Acid base regulation basics, pulmonary regulation and renal handling of acid-base balance.

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pH is an indicator of acidity. The body’s blood pH is strictly regulated within a narrow range between 7.35 and 7.45. This is because even a minor change in acidity may have devastating effects on protein stability and biochemical processes.
Normal cellular metabolism constantly produces and excretes carbon dioxide into the blood. Carbon dioxide combines with water to make carbonic acid which dissociates into hydrogen ions and bicarbonate. This equilibrium is central to understand acid-base regulation. CONTINUED carbon dioxide production by all cells of the body drives the equilibrium to the right to generate more hydrogen ions. Because pH is basically a function of hydrogen ion concentration, more hydrogen means higher acidity and lower pH. Normal metabolism, therefore, constantly makes the blood more acidic. The body must react to keep the blood pH within the normal limits. This is achieved by 2 mechanisms:
- Elimination of carbon dioxide through exhalation. The amount of carbon dioxide exhaled by the lungs is regulated in response to changes in acidity. A decrease in pH is sensed by central or arterial chemoreceptors and leads to deeper, faster breathing; more carbon dioxide is exhaled, less hydrogen is made, blood acidity decreases and blood pH returns to normal. Pulmonary regulation is fast, usually effective within minutes to hours.
- Excretion of hydrogen ions and reabsorption of bicarbonate through the kidneys. The kidneys control blood pH by adjusting the amount of excreted acids and reabsorbed bicarbonate. Renal regulation is slower; it usually takes days to respond to pH disturbances.
Pathologic changes may cause acid-base disturbances. Acidosis refers to a process that causes increased acidity, while alkalosis refers to one that causes increased alkalinity. It’s not uncommon for a patient to have several processes going on at once, some of them in opposite directions. The resulting plasma pH may be normal; too acidic, called acidemia; or too basic, called alkalemia.
Acidosis may result from inadequate function of the lungs which causes arterial carbon dioxide to accumulate. This is RESPIRATORY acidosis. On the other hand, METABOLIC acidosis may result from excessive production of metabolic acids, decreased ability of the kidneys to excrete acids, ingestion of acids, or loss of alkali. Metabolic acidosis is characterized by primary decrease in plasma bicarbonate.

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