The lung

$$H_2O + CO_2 \stackrel{Carbonic\ Anhydrase}{\leftrightarrow} H_2CO_3 \leftrightarrow HCO_3^- + H^+$$ (4)

The role of the lung in acid base physiology is in the main to eliminate CO . Increased hydrogen ion concentration in the cerebrospinal fluid stimulates the respiratory centre to increase the rate and depth of ventilation. This results in a drop in the paCO2 . In metabolic acidosis such increased ventilation will result in a compensatory respiratory alkalosis.

Chronic obstructive airways disease will result in hypo-ventilation. At first this mainly affects oxygen as carbon dioxide has a greater diffusing capacity than oxygen. But as the disease progresses there is accumulation of carbon dioxide and the picture is one of hypoxaemia and hypercapnia. There is buffering of the excess carbon dioxide by the formation of HCO and hydrogen ion, see equation 4. The body compensates by secreting more acid from the kidneys.

$$HCO\ensuremath{_3^-}\ falls\ by\ 7.5\ mmol = 1 kPa\ fall\ in\ P_{a_{CO_2}}$$ (5)

It has been estimated that for each 7.5 mmol change in HCO there will be 1 kPa change in paCO2 . This is a useful figure to remember when estimating mixed acid-base disturbances to see if the change in HCO is appropriate to the observed paCO2 .