part_of part of part of organ respiration organ breathing organ Organ that functions in gaseous exchange between an organism and its environment. In plants, microorganisms, and many small animals, air or water makes direct contact with the organism's cells or tissue fluids, and the processes of diffusion supply the organism with dioxygen (O2) and remove carbon dioxide (CO2). In larger animals the efficiency of gaseous exchange is improved by specialized respiratory organs, such as lungs and gills, which are ventilated by breathing mechanisms. organ of apparatus respiratorius apparatus respiratorius organ respiratory organ organ of respiratory system gas exchange organ respiratory system organ respiratory system