Which structure is the primary site of gas exchange in the respiratory system?

Which structure is the primary site of gas exchange in the respiratory system?

• A. Alveoli
• B. Trachea
• C. Epiglottis
• D. Bronchi

Answer: (A)

Gas exchange happens where air meets blood, in the tiny air sacs called alveoli. Their walls form an extremely thin respiratory membrane and there is a vast surface area, because each lung contains millions of alveoli. This setup lets oxygen diffuse from the alveolar air into the blood and carbon dioxide diffuse from the blood into the alveolar air, driven by matching partial pressure gradients. Surfactant keeps the alveoli open so this exchange can occur efficiently. The trachea and bronchi are mainly conduits that transport air to the lungs, and their thicker walls and lack of close contact with blood vessels prevent efficient gas exchange. The epiglottis is a flap that protects the airway from food entry; it doesn’t participate in gas exchange.

Gas exchange happens where air meets blood, in the tiny air sacs called alveoli. Their walls form an extremely thin respiratory membrane and there is a vast surface area, because each lung contains millions of alveoli. This setup lets oxygen diffuse from the alveolar air into the blood and carbon dioxide diffuse from the blood into the alveolar air, driven by matching partial pressure gradients. Surfactant keeps the alveoli open so this exchange can occur efficiently.

The trachea and bronchi are mainly conduits that transport air to the lungs, and their thicker walls and lack of close contact with blood vessels prevent efficient gas exchange. The epiglottis is a flap that protects the airway from food entry; it doesn’t participate in gas exchange.