What happens when you take a breath?
After the air you inhale passes through the windpipe and bronchioles, it ends up in the millions of tiny sacs called alveoli. Here, gas exchange takes place, so oxygen passes out of the air into the blood, and carbon dioxide passes out of the blood into the alveoli. The carbon dioxide you exhale is a waste product of energy production, and the oxygen you inhaled has been used up by the body.
So far, so natural. Surely people don’t mess it up?