Final answer:
Animals use oxygen in the process of aerobic respiration, which is part of the carbon cycle where oxygen is converted to carbon dioxide, complementary to photosynthesis.
Step-by-step explanation:
Animals use oxygen in the process of aerobic respiration and produce more CO2 than what goes in. During this biological process, oxygen is essential for breaking down glucose to extract energy for cellular activities. The oxygen that is consumed is then converted to carbon dioxide as a waste product, which is exhaled by animals and utilized by plants during photosynthesis. Photosynthesis and aerobic respiration are complementary processes that play a crucial role in the carbon cycle, balancing the exchange of oxygen and carbon dioxide in the environment.