the respiratory system brings in oxygen through the mouth or nose. humans breathe in oxygen, approximately 20 times each minute. as air is breathed in, it passes through the back of the nose where dust and gases are filtered. two tubes branch off from the windpipe and go to each lung. when you breathe out (exhale), carbon dioxide is release anatural by-product. how most creatures breathe is quiet similar except for that of birdss. technically speaking, even plants have respiratory system, but they exhale oxygen and inhale carbon dioxide. this is why plants are so important to our environment. they breathe in the harmful carbon dioxide that other mammals breathe out.