Relative Constancy of the Body's pH Is Essential Because Metabolism Requires Enzymes That Operate at an Optimal pH

For optimal functioning of the cells constituting the animal, the ionic composition of body fluids is maintained within fairly narrow limits. Regulation of hydrogen ion (proton) con­centration is extremely important because it determines the acidity or alkalinity, or ρH_t of the body fluids.

When veterinarians use the terms acidosis and alkalosis, they are comparing the pH of an animal’s arterial blood with the normal value of 7.4. A pH below 7.4 is referred to as acidosis; a pH above 7.4 is referred to as alkalosis. The range of pH compatible with life is 6.85 to 7.8, but rarely are these extremes approached. The body buffers, lungs, and kidneys all defend the body from onslaught of hydrogen ions (H‘) from a variety of sources.

The largest daily load of H (protons) arises during the transport of carbon dioxide (CO₂) from the tissues to the lungs. If the lungs eliminate CO₂ as fast as it is produced in the tissues, there is no net H' gain by the body. However, the balance between CO₂ production and CO₂ elimination may be disturbed during exercise or in respiratory disease, thus threatening the acid-base homeostasis of the body.

Hydrogen ions also are a product of protein metabolism, which produces sulfuric and phosphoric acids, fat meta­bolism, and the incomplete oxidation of glucose to lactic acid. Although H* ions from these sources are few compared with those produced in CO₂ transport, the kidneys must eliminate them continually. In a disease state the H' load imposed on the body is frequently increased because of an increase in tissue breakdown (catabolism) or because the kidneys fail to eliminate H⁺. In less common situations, such as vomiting, H⁺ is lost from the body.

To understand how the body regulates pH and how acid­base disorders are diagnosed, it is necessary to first review acids, bases, and buffering.