How carbon dioxide travels

Every cell in the body makes carbon dioxide, the waste gas you have to clear out when you breathe. The curious part is that not all of that gas travels through the blood as a gas.

It travels in three ways. Some of it travels dissolved, just as it is. Some of it sticks to hemoglobin, the protein that fills red blood cells, and forms what is called carbaminohemoglobin. But most of it is transformed: inside the red blood cell there is an enzyme, carbonic anhydrase, that joins carbon dioxide to water and forms carbonic acid. That acid splits almost instantly into two pieces, and one of them is bicarbonate, which is the main form in which the gas makes the trip.

Think of a traveler who changes clothes for the trip and takes them off on arrival. And once it reaches the lung the whole reaction runs backward: the bicarbonate turns back into carbon dioxide, the gas leaves the red blood cell and passes into the alveolus, the little air sac of the lung, so you can breathe it out.