The Controller and Acid-Base Physiology: An Introduction to a Complex Process

Definitions

An acid is a substance that can donate a hydrogen ion (proton). A base is a substance that can accept a hydrogen ion. The production of carbon dioxide resulting from the metabolism of carbohydrates leads, as already noted, to the formation of carbonic acid, which dissociates into a hydrogen ion and molecule of bicarbonate.

Normal metabolism, primarily of carbohydrates and fats, leads to the production of approximately 10,000 to 15,000 mmol of carbon dioxide every day. We sometimes refer to carbonic acid as a volatile acid because we can eliminate it by breathing more, essentially driving the reaction above to the left by increasing alveolar ventilation, thereby decreasing PCO₂. In contrast, the body makes a relatively small amount of fixed acids, mostly phosphates and sulfates resulting from the metabolism of protein, which must be eliminated via the kidneys. Every day, approximately 50 to 100 meq of fixed acids are produced as a consequence of the body’s metabolism. Absent the respiratory system and its ability to eliminate carbon dioxide, the body would quickly be overwhelmed by acid. Absent functioning kidneys, the body would also be overwhelmed by acid, but it would take days rather than minutes for that to occur.

A buffer is a molecule that is able to accept or release hydrogen ions so that changes in the free hydrogen ion concentration and, hence, the pH, are minimized. The primary intracellular buffers are proteins, phosphates, and hemoglobin in red blood cells (RBCs). Serum albumin and bone also serve as important sites for the buffering of acid. Sodium and potassium ions on the surface of the bone may exchange with protons, or bone mineral may be dissolved, a process that leads to the release of buffers into the extracellular fluid. The primary source of buffer in the extracellular fluid is bicarbonate, largely because of the ability of the body to eliminate carbon dioxide and drive the carbonic acid equilibrium to the left.

A process that leads to a disturbance in the balance of acids or bases in the body is noted with the suffix -osis. Thus, a process leading to the excess production of acid is an acidosis. A process leading to excess production of base is an alkalosis. If the process that alters the relative concentration of acids and bases alters the pH of the blood, the suffix -emia is used to signify the effect on arterial hydrogen ion concentration. If the pH of the blood is higher than normal (i.e., more basic, with a pH > 7.44), an alkalemia is present; if the pH is lower than normal (increased concentration of hydrogen ions, with a pH < 7.36), an acidemia exists. It is important to remember that the presence of a process leading to a change in the quantities of acid and base in the system does not necessarily imply a change in the pH of the blood. The buffering capacity of the body and its ability to initiate secondary processes to counter or compensate for the initial challenge to the acid-base equilibrium may result in a normal pH despite the primary physiological disorder.