MAVO Year 4 · Gas exchange and excretion · Section 14.6

Gas exchange in animals

Lungs

Mammals, birds and reptiles use lungs to take up oxygen from air. Their lung structure differs between groups. Mammals have alveoli; birds have a highly efficient flow-through system involving lungs and air sacs. Reptiles are generally ectothermic and often have lower metabolic demands than similarly sized active endotherms, but their lung size and structure vary widely rather than simply being smaller in every species.

Amphibians: skin and lungs

Many amphibian larvae, including tadpoles, use gills. During metamorphosis, many develop lungs. Adult amphibians also exchange gases through their thin, moist, vascular skin. Oxygen diffuses towards blood and carbon dioxide away. The relative importance of lungs and skin differs between species; some salamanders lack lungs altogether.

Fish gills

Most fish exchange gases using gills rather than lungs. In many bony fish, water enters the mouth and passes over thin gill surfaces rich in capillaries before leaving under the gill covers. Oxygen moves from water into blood and carbon dioxide in the opposite direction. The large area and short diffusion distance support exchange. There are exceptions, including lungfish, so it is inaccurate to say no fish has lungs.

Insect tracheal systems

Insects have branching air tubes called tracheae, opening to the outside through spiracles. Finer branches deliver gases close to tissues, including muscles. Oxygen normally reaches these tissues directly rather than being transported mainly by the circulatory fluid. Body movements can help ventilate the tubes, and spiracle control helps balance gas exchange and water loss.