Métabolisme acido-basique

Acid–base homeostasis is the homeostatic regulation of the pH of the body's extracellular fluid (ECF). The proper balance between the acids and bases (i.e. the pH) in the ECF is crucial for the normal physiology of the body—and for cellular metabolism. The pH of the intracellular fluid and the extracellular fluid need to be maintained at a constant level. The three dimensional structures of many extracellular proteins, such as the plasma proteins and membrane proteins of the body's cells, are very sensitive to the extracellular pH. Stringent mechanisms therefore exist to maintain the pH within very narrow limits. Outside the acceptable range of pH, proteins are denatured (i.e. their 3D structure is disrupted), causing enzymes and ion channels (among others) to malfunction. An acid–base imbalance is known as acidemia when the pH is acidic, or alkalemia when the pH is alkaline. In humans and many other animals, acid–base homeostasis is maintained by multiple mechanisms involved in three lines of defense: Chemical: The first lines of defense are immediate, consisting of the various chemical buffers which minimize pH changes that would otherwise occur in their absence. These buffers include the bicarbonate buffer system, the phosphate buffer system, and the protein buffer system. Respiratory component: The second line of defense is rapid consisting of the control the carbonic acid (H2CO3) concentration in the ECF by changing the rate and depth of breathing by hyperventilation or hypoventilation. This blows off or retains carbon dioxide (and thus carbonic acid) in the blood plasma as required. Metabolic component: The third line of defense is slow, best measured by the base excess, and mostly depends on the renal system which can add or remove bicarbonate ions (HCO3-) to or from the ECF. Bicarbonate ions are derived from metabolic carbon dioxide which is enzymatically converted to carbonic acid in the renal tubular cells. There, carbonic acid spontaneously dissociates into hydrogen ions and bicarbonate ions.