How does oxygen and carbon dioxide remain nearly constant in the atmosphere?

Oxygen and carbon dioxide remain nearly constant in the atmosphere because a cycle operates in nature wherein these two gases are exchanged.

Plants take carbon dioixide CO2 from atmosphere to perform photosynthesis and release out oxygen O2 which is inhaled by man and animals in turn giving out carbon dioxide which plant use for photosynthesis. Thus this maintains a nearly constant level of oxygen and carbon dioixide in the atmosphere.