Gas exchange occurs in the alveoli of the lungs, as well as in the capillaries that surround the alveoli, explains Teach PE. The exchange of gases in these two regions occurs through the process of diffusion, whereby oxygen and carbon dioxide move from higher-concentration areas to lower-concentration areas.
When an individual inhales oxygen from the environment, the oxygen enters the alveoli, causing the air in the alveoli to have a higher concentration of oxygen molecules than the air in the capillaries, notes Teach PE. Conversely, the air inside of the capillaries has a higher concentration of carbon dioxide molecules than the air inside of the alveoli following an inhalation. Via diffusion, some of the oxygen in the alveoli moves into the capillaries, while some of the carbon dioxide in the capillaries moves into the alveoli. Through the capillaries, the oxygen enters the person's bloodstream, explains WebMD. When the individual exhales, the carbon dioxide is released from the alveoli and into the environment.
Air breathed in from the environment comprises about 21 percent of oxygen and 0.04 percent carbon dioxide. The air a person exhales back into the environment is about 17 percent oxygen and 3 percent carbon dioxide, indicating that the oxygen content decreases while the dioxide content increases. This is because the body uses oxygen to create energy, while carbon dioxide is a waste product of the energy-creation process, notes Teach PE.