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Across the alveolar-capillary interface, oxygen moves from alveoli into blood while carbon dioxide moves from blood into alveoli. Both follow partial pressure gradients, and diffusion is described by Fick’s Law. In healthy lungs, oxygen generally reaches equilibrium before blood completes its passage through the capillary, making its transfer perfusion-limited. Carbon monoxide remains diffusion-limited and is used to assess DLCO. DLCO is reduced in emphysema, fibrosis, and anemia, and increased in polycythemia and exercise. Carbon dioxide diffuses much faster than oxygen, so its exchange is typically preserved even in early disease. The full lesson develops these principles and considers how changes in gradients or membrane thickness affect gas transfer.