The term 'breathing organs' refers to the organs that are responsible for taking in oxygen and expelling carbon dioxide. The two most common breathing organs are LUNGS and GILLS. LUNGS are found in mammals, birds, and some reptiles, and they are responsible for exchanging oxygen and carbon dioxide through inhalation and exhalation. GILLS are found in fish and other aquatic organisms, and they are responsible for exchanging oxygen and carbon dioxide through diffusion.