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Oxygen is the gas primarily responsible for respiration in living organisms. During the process of aerobic respiration, organisms take in oxygen to help convert glucose into energy. This occurs in the mitochondria of cells, where oxygen is utilized to produce adenosine triphosphate (ATP), the energy currency of the cell.

The importance of oxygen is underscored by its role in the electron transport chain, which is essential for producing the majority of ATP in aerobic respiration. Without oxygen, the efficiency of energy production decreases significantly, leading to reliance on less efficient anaerobic processes.

While carbon dioxide, nitrogen, and hydrogen are also involved in various biological processes, they do not play the primary role in respiration. Carbon dioxide is actually a byproduct of the respiration process, while nitrogen is mostly inert and does not participate directly in respiration. Hydrogen can be involved in cellular processes, but it is not a primary gas for respiration.