What gas do animals predominantly take in for respiration?

Animals predominantly take in oxygen (O2) for respiration because it is essential for cellular processes that produce energy. During respiration, oxygen is utilized in the mitochondria of cells to help convert glucose into ATP (adenosine triphosphate), the energy currency of the cell. This process releases carbon dioxide (CO2) as a byproduct, which animals then exhale.

The respiratory system in animals, whether it's through lungs, gills, or other structures, is specifically adapted to facilitate the uptake of oxygen from the environment. Oxygen plays a critical role in aerobic respiration, which is a more efficient way to extract energy from food compared to anaerobic processes that do not use oxygen.

In contrast, while carbon dioxide is a byproduct of respiration, nitrogen is largely inert and does not participate in metabolic processes crucial for energy production. Argon, on the other hand, comprises a small fraction of the atmosphere and does not play any significant role in respiration. Thus, oxygen is the key gas that animals rely on for their survival.