Question

how is the level of oxygen maintained in air?

Answer 1

Oxygen is a life - supporting gas, so it is necessary to maintain its level composition in air.

The level and composition of Oxygen, the life supporting gas, is maintained in air because of the existence of a continuous Oxygen-CO2 cycle, a gaseous cycle.

When we exhale CO2 rich air into the atmosphere, trees inhale it and use it in photosynthesis. Then, the trees exhale oxygen in the air which is used by us.

And, hence the process continues.

Answer 2

O2 is taken in body
CO2 is released from body
The CO2 is taken by plants and then the plants releases O2.
Then O2 is taken in body......Same process
cycle continues...