Not all of what a person exhales is carbon dioxide. The gases exhaled by human beings are mainly nitrogen, oxygen, carbon dioxide, water vapor and so on. From the point of view of human respiratory structure, the gas between exhalation and inhalation is not exactly the same, the nose, pharynx, larynx is our upper respiratory tract, through which we exhale into the air, which contains a large amount of oxygen, through the transport to the alveoli, and then to the pulmonary capillaries, the oxygen is transported to the blood tissues and cells, and then discharged out of the body through the pulmonary capillaries after the cellular metabolism. From the point of view of air composition, the air is mainly nitrogen, oxygen, carbon dioxide, rare gases, water vapor, etc., the composition of a variety of components, meaning that we breathe after the composition of the gas can not be a single; and the human body’s water content is much larger than the water content of the air, so we exhale the gas increase is not only carbon dioxide, there is also water vapor, so the human body needs to be supplemented with water from time to time to achieve the balance of in and out. Therefore, the human body exhaled gas does have carbon dioxide, but not only the gas, but also nitrogen, oxygen, carbon dioxide, water vapor and so on.