When human breath(inhale), the air
reaches the lungs, from there oxygen enters the bloodstream,
through the blood the oxygen reaches the muscle cell.
While passing through the blood,
the oxygen and hemoglobin are bonded. When it reaches the muscle
cell, the bond become loos and then the oxygen molecule will get
mixed with the muscle cell.
From the muscle cell oxygen
molecule will go inside mitochondria, to perform process like
electron transportation, for producing ATP.
At the same time carbondioxide
present in the muscle cell will enter the bloodstream, in the form
of bicarbonate ions.
The bloodstream carrying the
carbonate ions reaches the lungs, from there it get converted into
CO2, from the lungs it get exhaled or breath out.