Explanation

Oxygen $(O_{2})$ is utilised by the living entities to indirectly break down the nutrients like glucose, to derive energy for performing various activities, etc. $CO_{2}$ (carbon dioxide) which is a harmful gas, releases during the catabolic reactions. It is therefore, evident that $O_{2}$ has to be continuously provided to the cells and $CO_{2}$ produced by the cells have to be released out