As a response to any cause of a drop on arterial pO2 , the lungs will be driven to increase the alveolar ventilation and drop the paCO2 . Thus the common complaints of pulmonary embolism, pneumonia and pulmonary oedema will result in a fall in pO2 with a consequent fall in paCO2 as alvelolar ventilation is increased in an attempt to increase the pO2 . Thus there will be a respiratory alkalosis. Eventually the effects of the changes in pH on ventilation overcome the effects of the fall in pO2 and a steady state is established.
Some conditions may increase the barrier to diffusion in the alveoli to such an extent that the paCO2 will in fact rise. The first situation where the pO2 falls and the paCO2 also falls is termed type I respiratory failure and the second type where the pO2 falls and the paCO2 rises is type II respiratory failure.
This effect of alveolar ventilation on paCO2 can be used therapeutically on people on a ventilator in whom a drop in paCO2 is deemed advantageous, for instance in the situation of a patient with head injury.