How is blood oxygenated in the lungs?

Blood becomes oxygenated in the alveoli as oxygen diffuses from air to blood and carbon dioxide diffuses from blood to air across a thin respiratory membrane.

Deoxygenated blood arrives via pulmonary arteries. The large surface area of ~300 million alveoli, their thin walls, and moist lining enable rapid diffusion. O₂ binds hemoglobin; CO₂ leaves mainly as dissolved gas and bicarbonate converted back to CO₂ by carbonic anhydrase. Oxygenated blood returns through pulmonary veins to the left atrium.