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How do animals return carbon dioxide to the atmosphere?

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A labeled diagram of the periodic table entry for carbon, showing its atomic number 6, symbol C, electron configuration, atomic weight range, solid physical state, hexagonal crystal structure, and weakly acidic oxide properties. Color coding and icons indicate it is a nonmetal, solid at room temperature, with hexagonal structure and weak acidity. — carbon Carbon and its properties.

Animals return carbon dioxide to the atmosphere primarily as a by-product of respiration, a metabolic process that generates energy for life-sustaining activities.123

During cellular respiration, organisms break down food molecules, producing carbon dioxide as a waste product. Because this gas is soluble in body water, it diffuses from cells into the circulatory system or directly through cell membranes in simpler organisms.34

The mechanisms for releasing this carbon dioxide into the environment include:

Diffusion: Lower animals and some aquatic species release carbon dioxide directly through their cell membranes or thin vascular body surfaces into the surrounding water.4
Specialized Organs: In higher animals, carbon dioxide is transported to gills or lungs, where it diffuses out of the body into the external environment.4
Decomposition: Additionally, when animals die, decomposer organisms—such as bacteria and fungi—break down their remains, releasing the stored carbon back into the atmosphere as carbon dioxide.1