Gas exchange occurs in which vessels surrounding the alveoli?

Gas exchange occurs in which vessels surrounding the alveoli?

• A. Alveoli
• B. Bronchioles
• C. Alveolar sacs
• D. Pulmonary capillaries

Answer: (D)

Gas exchange relies on diffusion across the thin alveolar–capillary membrane, so the vessels surrounding each alveolus are the pulmonary capillaries. Oxygen moves from the air in the alveoli into the blood because the alveolar air has a higher partial pressure of O2 than the blood, while carbon dioxide moves from the blood into the alveolar air because its partial pressure is higher in the blood. The barrier—alveolar epithelium, the shared basement membranes, and the capillary endothelium—is extremely thin and exposed to a large surface area, which makes diffusion efficient. The other structures listed are airways or air sacs themselves, but the actual vessels performing gas exchange around the alveoli are the pulmonary capillaries.

Gas exchange relies on diffusion across the thin alveolar–capillary membrane, so the vessels surrounding each alveolus are the pulmonary capillaries. Oxygen moves from the air in the alveoli into the blood because the alveolar air has a higher partial pressure of O2 than the blood, while carbon dioxide moves from the blood into the alveolar air because its partial pressure is higher in the blood. The barrier—alveolar epithelium, the shared basement membranes, and the capillary endothelium—is extremely thin and exposed to a large surface area, which makes diffusion efficient. The other structures listed are airways or air sacs themselves, but the actual vessels performing gas exchange around the alveoli are the pulmonary capillaries.