Which transport mechanism causes the respiratory gases (O2, CO2) to move between the alveoli and pulmonary capillaries?

A. It is difficult to keep the alveoli open- surfactant reduces alveolar surface tension; without it, alveoli collapse (atelectasis).

B. Bronchioles relax- not directly related to surfactant.

C. The alveoli fill with water- surfactant prevents collapse, not water entry.

D. Air enters the intrapleural space- this would cause a pneumothorax, unrelated to surfactant function.

A. Bronchi: The bronchi are the main airways that branch off from the trachea and enter each lung. Structure #8 is shown as the large branches that split into the lungs, consistent with the location and appearance of the bronchi.

B. Trachea: The trachea is the main airway above the bronchi, running down the neck before it splits into the left and right bronchi. It is likely labeled as #7 in the diagram.

C. Alveoli: Alveoli are microscopic air sacs at the end of the bronchioles where gas exchange occurs. They are not visibly distinct in this diagram and are located much further down the airway branches than where #8 is labeled.

D. Bronchioles: Bronchioles are the smaller branches that come off the bronchi and lead to the alveoli. They would be located deeper into the lung structure and are smaller than the bronchi seen at label #8.