Hint: Air contains numerous different gases, not just oxygen making it extremely evident that we don't breathe in just oxygen, yet in addition, different gases.
Complete step by step answer: We breathe in air that contains oxygen and other gases such as nitrogen, argon, carbon dioxide and methane, as well as a host of other things that are not good for your health. Oxygen only makes 21% percent of air. The oxygen present in the air goes to the lungs, where the exchange of gases takes place. In the lungs, the oxygen is exchanged with the carbon dioxide that comes to the blood from our body tissues. Gases are exchanged by the process of diffusion and thus, we breathe out carbon dioxide into the atmosphere. The carbon dioxide is then utilized by plants in the photosynthetic process, which leads to the production of glucose and oxygen as the byproduct. The oxygen produced by plants goes to the atmosphere and gets mixed with air. The carbon we exhale out as carbon dioxide originates from the carbon in the food. Each living cell of our body completes the energy delivering cycle of breath where glucose is scorched to give carbon dioxide and water.
Note: Both oxygen and carbon dioxide is carried by a red-colour pigment present in the blood called haemoglobin. It is present in the red blood cells, which are biconcave in structure and are involved in the gases exchange process in animals. Gases exchange occurs in alveoli found in the lungs.