The former has approximately 21% oxygen and less than 1% carbon dioxide. Once the air enters our lungs, gaseous exchange takes place. The air that we exhale contains about 4% carbon dioxide and 18% oxygen. Therefore, although the oxygen content is lower in expired air, it can still work to revive a person. The latter is composed of water vapor (75%), nitrogen (20%) and other gases (5%). Exhaled air is actually the only source of breathable air for animals on land. Without it, they would die within minutes. This is because they cannot get rid of the carbon dioxide that builds up inside their bodies. For humans to survive, they need oxygen and they need it fast. Our cells use oxygen to burn food with energy cells cannot live without oxygen. So as we breathe in oxygen, we are also breathing out carbon dioxide. This helps us release built-up energy stores as heat during physical activity or maintain body temperature at a constant level when we are not moving. Without this gas exchange taking place, our blood would become saturated with carbon dioxide and we would asphyxiate. This would be similar to being stuck in a car with the windows shut - there would be no way to get fresh air in or waste gas out. Exhaled air is therefore important for life because it allows us to remove carbon dioxide from our bodies while getting enough oxygen to stay alive.