Question

How does the structure of the alveoli allow for its function of gas exchange?
Alveolar sacs have space to hold mucus that makes gas exchange easier.
Alveolar sacs rely on valves to ensure that air flows only in one direction.
Alveolar sacs are located within muscles that contract, which increases gas exchange.
Alveolar sacs are surrounded by thin capillaries where gases can be exchanged.

Answer 1

Correct answer: Alveolar sacs are surrounded by thin capillaries where gases can be exchanged.

Why: Each alveolus has very thin walls (a single layer of epithelial cells) and is closely apposed to a network of pulmonary capillaries. This creates a large surface area and a very short diffusion distance for O2 and CO2, allowing rapid gas exchange. The alveolar lining is moist and contains surfactant to reduce surface tension and keep the alveoli open, further aiding efficient gas transfer.

Why the other choices are wrong:
- Mucus in the alveoli would block diffusion; mucus is in larger airways to trap particles, not to aid gas exchange.
- There are no one‑way valves at the alveoli; airflow is driven by pressure changes from the respiratory muscles.
- Alveoli are not located within contracting muscles; the diaphragm and intercostal muscles change lung volume, but alveoli themselves are thin air sacs, not muscle tissue.