Physiological Acid Production

  • Large amounts of free hydrogen ions, that is acids, are generated and added to the extracellular fluid (ECF) by both normal and pathophysiological metabolic processes. Over time, generation of these acids would begin to dangerously decrease the extracellular fluid pH. Consequently, to maintain the normal ECF pH of 7.4, the body must possess mechanisms to eliminate these generated acids. Two basic chemical forms of acids can be generated by physiological and pathological processes, termed "Volatile Acids" and "Fixed Acids". As outlined below, the mechanisms responsible for eliminating these different forms of acids are distinct.
Volatile Acid
  • Volatile acids are defined as those acids which can be converted into a gaseous form and can thus be eliminated by the lungs. The primary volatile acid of the body is carbon dioxide which is produced in substantial amounts by the processes of cellular respiration.
  • A detailed explanation of CO2 transport is provided on the Carbon Dioxide Transport page. Briefly, metabolically generated CO2 diffuses into the venous blood and is largely converted by erythrocyte carbonic anhydrase to carbonic acid (H2CO3), that then releases a free hydrogen ion into solution and leaves bicarbonate (HCO3-). The bicarbonate travels through the circulation to the lungs where local carbonic anhydrase can reverse the reaction, regenerating CO2, that can be "breathed off" by the lungs.
Fixed Acids
  • Fixed Acids are defined as acids that cannot be easily converted into a gaseous form and thus cannot be eliminated through exhalation. A large variety of fixed acids are generated by both physiological and pathophysiological mechanisms, and their elimination from the body is ultimately dependent on urinary excretion by the kidneys.
  • Normal metabolism of proteins and phospholipids results in generation of sulfuric acid while bouts of strenuous exercise can result in the generation of substantial lactic acid.
  • In contexts of diabetic ketoacidosis large amounts of beta-hydroxybutyric acid and acetoacetic acid can be generated. Additionally, metabolism of certain pharmacological agents can be metabolized into strong acids. For example, hepatic metabolism of aspirin can be produce salicylic acid.