IB Sports, Exercise and Health Science Practice Exam 2026 - Complete Prep Resource

Gas exchange happens at the interface where air in the lungs meets the blood in the capillaries, across a very thin barrier with a large surface area. The alveoli provide this environment best because they are tiny, sac-like structures densely packed in the lungs and surrounded by a thick network of pulmonary capillaries. This setup creates a huge surface area and an extremely short diffusion distance, enabling quick movement of gases. The thin walls of the alveolar epithelium and the adjacent capillary endothelium form the respiratory membrane, and surfactant keeps the alveoli from collapsing so the surface area is maintained for ongoing exchange. Diffusion follows partial pressure gradients: oxygen moves from the higher concentration in alveolar air to the lower concentration in blood, while carbon dioxide moves from higher concentration in blood to lower concentration in alveolar air to be exhaled.

Other parts like the trachea, bronchi, and nasal cavity serve to filter, warm, and moisten air and to conduct it toward the lungs; they lack the thin, moist, highly vascularized membranes and large surface area necessary for gas diffusion, so gas exchange does not occur there.