Carbon dioxide buffering system

• When CO2 enters the blood it is broken down by the enzyme carbonic anhydrase into carbonic acid. Carbonic acid then immediately dissociates into bicarbonate and hydrogen ions. The blood carries these molecules to the lungs where CO2 is reformed and exhaled out of the body.
  
  

Acidity level in the blood

• The acidity level of the blood is determined by measuring pH. pH is an inverse log of hydrogen ion concentration. The greater the number of hydrogen ions the lower the pH, and acid level, of the blood. According to Guyton's "Textbook of Medical Physiology" normal blood pH is in a range of 7.3 to 7.5.
  
  

Additional buffering systems

• Acid-base balance in the body is regulated by the buffering system. In addition to bicarbonate, McKardle, Katch, and Katch's "Exercise Physiology" also identify additional carbon dioxide buffering systems. These include blood phosphate and proteins as well as increased kidney function. However, respiration is a powerful driver of CO2 buffering; as carbon dioxide increases, breathing rate also increases to help eliminate excess CO2.
  
  

Increases in carbon dioxide through exercise

• As a person exercises their metabolism increases to meet the demands of the activity. One by-product of this higher metabolism is CO2. The body normally has little problem eliminating the CO2 produced during exercise. However, if exercise intensity continues to increase the blood will become acidic and exercise exhaustion will set in.
  
  

Disease states that increase carbon dioxide

• Aside from exercise and self-induced hyperventilation, there are several disease states that will increase carbon dioxide levels in the blood. These include diarrhea, vomiting, uremia from kidney failure, and diabetes mellitus. All of these conditions may lead to what is called metabolic acidosis.
  
  

Training effects on carbon dioxide transport and buffering

• Exercise training does seem to confer an increase in the buffering capacity in the trained athlete. Hug et al. found a higher rate of pH return to normal levels in highly trained professional cyclists. However, McKardle et al. also believe the higher capacity in trained individuals could also be due to increased motivational factors and a greater tolerance for the discomfort associated with acidic blood pH levels.