Composition of Alveolar Gases
Under normal breathing conditions the composition of alveolar gases is largely determined by two processes.

1

Ventilation: the periodic partial replacement of alveolar air with fresh ambient air during inspiration.  

2

Blood flow: CO2 diffuses out of the blood as it passes through the pulmonary capillaries while O2 from the alveoli diffuses into the blood stream from the alveoli.  

The tidal volume is around 500ml. This volume is added to the 2500ml of the Functional Residual Capacity (FRC) with every inspiration. As a result, there is a minimal variation (~17%) in the concentration of the gas mixture with every inspiration. Thus the FRC acts as a buffer against rapid and drastic changes in the alveolar air composition.