How does carbon dioxide move from the blood into the alveoli?

Carbon dioxide moves from the blood into the alveoli primarily through the process of diffusion, which is driven by the difference in partial pressures of the gas between the pulmonary capillaries and the alveolar air spaces.12

Because the partial pressure of carbon dioxide is lower in the alveoli than in the blood arriving at the lungs, the gas naturally moves down its pressure gradient across the thin membranous barrier separating the capillaries from the alveoli.31

While most carbon dioxide is transported in the blood as bicarbonate or bound to hemoglobin as carbamate, it must exist in a free, dissolved form to cross these biological membranes. When blood reaches the lungs, a reverse sequence of chemical reactions occurs: bicarbonate ions are converted back into carbon dioxide, and oxygenation of hemoglobin facilitates the release of bound carbon dioxide. Once converted to its free form, the carbon dioxide diffuses into the alveoli to be expelled from the body during exhalation.32