Internal pH depends on renal-pulmonary cooperation

• Normal metabolism continually produces acid

• Carbonic acid/bicarbonate is the main buffer in the blood (pKa = 6.1)
  

• The lungs eliminate acid as carbon dioxide, maintaining a normal PCO2 of about 40 mm Hg

  Increasing the pCO2 drives the equilibrium toward increased production of H+, lowering the pH. "Breathing off" CO2 (reducing the pCO2) pulls the equilibrium away from H+, raising the pH. Thus CO2 is essentially equivalent to acid in the body.

• The kidneys remove H+ (distal tubule) and conserve bicarbonate (proximal tubule), maintaining a normal bicarbonate of about 24 meq/l

• This cooperative relationship maintains an internal pH very close to 7.4

Identify expiration of carbon dioxide, renal carbonic anhydrase, and renal proton excretion (most likely a result of Na+/H+ exchange) in the "cooperative relationship" animated diagram.