The primary organ involved in gas exchange in fishes is the gills. Fishes take in water through their mouths and force it past their gills, where dissolved oxygen is absorbed by the blood circulating within the gills.

Aquatic animals have a faster rate of breathing because the amount of dissolved oxygen in water is lower compared to the oxygen content in the air, requiring them to extract oxygen more rapidly from their aquatic environment.

Terrestrial animals breathe oxygen from the atmosphere, while aquatic animals use the oxygen dissolved in water.

At night, when photosynthesis is not occurring, carbon dioxide elimination is the major exchange activity. During the day, carbon dioxide generated during respiration is used up for photosynthesis, resulting in oxygen release as the major gas exchange activity.

Plants exchange gases through stomata, and diffusion occurs in the large inter-cellular spaces. Carbon dioxide and oxygen are exchanged based on environmental conditions and the needs of the plant.