the primary purpose of the respiratory system is to bring oxygen to the lungs and remove any unnecessary gases. without oxygen , mammals could not breathe, more or turn food into energy. all animals need oxygen to live. this is true whether it is humans who breathe through their mounths, fish who breathe through their gills or reptiles who breathe through their skin. in each case, there is an exachange of gases in the body. the respiratory system in mammals has a passageway that goes from the mouth to the lungs for breathing. it is most commonly called the windpipe.