How does blood gain oxygen and release carbon dioxide in the lungs?

In the lungs, blood gains oxygen and releases carbon dioxide through a process of diffusion across the thin, membranous barrier between the alveoli and the pulmonary capillaries. This movement is driven by differences in the partial pressures of these gases.12

Oxygen Uptake

When blood reaches the lungs, the partial pressure of oxygen in the alveoli is higher than in the venous blood. Consequently, oxygen diffuses through the plasma and into the red blood cells, where it binds to hemoglobin. The oxygenation of hemoglobin in the lungs also triggers a change in its molecular configuration, which facilitates the release of carbon dioxide.314

Carbon Dioxide Release

Carbon dioxide is transported in the blood primarily as bicarbonate ions, as carbamate bound to hemoglobin, and in small amounts dissolved in plasma. In the lungs, where the partial pressure of carbon dioxide is lower than in the blood, these reactions reverse. Bicarbonate is converted back into free carbon dioxide, which then diffuses out of the blood and into the alveoli to be exhaled.43