How is oxygen cycled in human metabolism?
Humans INHALE dioxygen, and EXHALE carbon dioxide.
When you light a wood-burning stove, you open the damper to increase air flow, and get the fire going. The outflow gas include combustion products, i.e. soot, carbon monoxide, water, and carbon dioxide.
Humans need to inhale dioxygen gas to combust sugars, and likewise the oxidation products are carbon dioxide and water.