Basically it is done through the process of respiration & .
The above figure is an oxygen cycle which shows the both the processes
Respiration-
Respiration is a process in which the ##O_2## molecule gets inhaled by the animal body. Then it is taken to the Alveolar space inside the lungs.
From there it is taken by the blood capillaries and then the oxygenated blood is circulated throughout the animal body.
After this the Deoxygenated blood / ##CO_2## rich blood is collected from all the parts of the body and is back transported to the alveolar space via blood capillaries.
After this the ##CO_2## is exhaled.
Photosynthesis-
The ##CO_2## is taken up by the open pores on the surface of leaf known as stomata (singu. stoma) guarded by the guard cells.
The plant body also takes up many other things such as water and sunlight for preparation of its food.This could be expressed as follows-
##6CO_2## + ##12H_2O## ##->## ##C_6H_12O_2## + ##6H_2O## + ##6O_2##
After this the oxygen molecule is back released in and again it is inhaled by us. This cycle repeats and carbon and oxygen cycle through the atmosphere.
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