Which gas is essential for human respiration?

Oxygen is essential for human respiration because it plays a critical role in the process by which cells generate energy. Through a process known as cellular respiration, oxygen is used to convert glucose and other nutrients into adenosine triphosphate (ATP), the energy currency of cells. This process takes place in the mitochondria and is vital for sustaining life because ATP fuels various cellular functions such as muscle contraction, nerve impulse transmission, and overall metabolism.

While carbon dioxide is a byproduct of respiration that is expelled from the body, and nitrogen constitutes a significant portion of the air we breathe, it does not participate directly in the respiratory process needed for converting nutrients into usable energy. Hydrogen is not used in respiration in the same way oxygen is; rather, it is involved in biochemical reactions that occur in metabolic pathways but does not serve as a gas that humans require for breathing. Thus, oxygen is the gas that is indispensable for respiratory functions, making it the correct answer.