Red blood cells turn carbon dioxide into a traveling form
As blood passes through working tissues, carbon dioxide diffuses into red blood cells. There, an enzyme called carbonic anhydrase speeds its reaction with water, producing carbonic acid. That quickly separates into bicarbonate ions and hydrogen ions.
Bicarbonate dissolves readily in blood plasma, so it can carry carbon from tissues without requiring all of it to remain as dissolved gas. To keep electrical charges balanced as bicarbonate leaves a red blood cell, chloride ions move in. Meanwhile, hemoglobin helps bind some of the hydrogen ions, limiting the change in blood acidity.
The lungs reverse the route
In lung capillaries, the exchange runs the other way. Bicarbonate moves back into red blood cells, where it combines with hydrogen ions to form carbonic acid. Carbonic anhydrase rapidly converts that into carbon dioxide and water. The carbon dioxide then diffuses into the air spaces of the lungs and is breathed out.
This reversible chemistry lets blood carry carbon dioxide efficiently while helping control its acidity. It also means that each exhale releases carbon that began as a byproduct of cells using fuel—after the blood temporarily packed much of it into a form that travels easily.
