Humans exhale oxygen, carbon dioxide and nitrogen. Humans inhale these same gases, though in different proportions to exhalation; that is, oxygen is exchanged for carbon dioxide during breathing.
When inhaling, humans take in approximately 21 percent oxygen, 0.04 percent carbon dioxide and 79 percent nitrogen. On exhalation, humans give off approximately 16 percent oxygen, 4 percent carbon dioxide and 79 percent nitrogen, according to the BBC; only the amount of nitrogen remains constant in the exchange. Exhaled air also contains water vapor. Red blood cells travel by vein to the lung; they give of carbon dioxide and take in oxygen.