At the alveolar sacs in the respiratory system, which process occurs?

• A. Gas Exchange
• B. Filtration
• C. Sound Production
• D. Thermoregulation

Answer: (A)

Gas exchange happens at the alveolar sacs. The walls of the alveoli are extremely thin and lie next to a dense network of capillaries, creating a large surface area and a very short diffusion distance. This setup allows oxygen to move from the air in the alveoli into the blood, while carbon dioxide moves from the blood into the alveolar air to be breathed out. The movement is driven by partial pressure gradients: oxygen is higher in the alveolar air than in the blood, and carbon dioxide is higher in the blood than in the alveolar air. Surfactant helps keep the alveoli open and reduces surface tension, supporting efficient diffusion. Efficient exchange also relies on good ventilation-perfusion matching so that air reaching the alveoli is paired with adequate blood flow. Filtration, sound production, and thermoregulation are not processes that occur in the alveolar sacs—the first removes waste from the blood, the second involves the vocal apparatus, and the third is managed mainly by other tissues.