Lung–kidney system ↗Cells produce carbon dioxide
Oxidative metabolism generates CO₂, which diffuses from tissues into capillary blood according to local gradients.
LUNGS + KIDNEYS / 03
One buffer pair.
Two organ controls.
Lungs alter carbon dioxide within minutes. Kidneys reclaim and generate bicarbonate and excrete fixed acid over longer times. Chemical buffering begins immediately but does not remove the primary load.
Carbonic anhydrase accelerates conversion
In red cells, CO₂ rapidly interconverts with carbonic acid, hydrogen ions, and bicarbonate.
Blood carries several forms
CO₂ travels dissolved, bound to proteins including hemoglobin, and predominantly through bicarbonate-related transport.
Lungs reverse the process
In pulmonary capillaries, bicarbonate-related carbon returns toward CO₂, diffuses into alveoli, and is exhaled.
Ventilation changes PaCO₂
Higher effective alveolar ventilation lowers PaCO₂; inadequate ventilation raises it when production is otherwise stable.
Kidneys reclaim filtered bicarbonate
Most filtered bicarbonate is recovered, especially in proximal tubule, preventing continuous loss of a major extracellular buffer.
Net acid excretion regenerates buffer
Hydrogen secretion, ammonium, and titratable urinary buffers allow fixed-acid excretion and addition of bicarbonate to blood.
Compensation has limits
Respiratory and metabolic processes can coexist; compensation rarely restores pH completely and never establishes the diagnosis alone.