Gas exchange in the lungs is facilitated by which factor?

• A. Adequate amounts of surfactant.
• B. Pulmonary capillary constriction.
• C. Surfactant-destroying organisms.
• D. Water or blood within the alveoli.

Answer: (A)

Gas exchange depends on alveoli that remain open and have a very thin, moist surface for diffusion. Surfactant lowers surface tension inside the alveoli, which prevents collapse during exhalation and reduces the work of breathing. By keeping many alveoli inflated and preserving a large surface area, surfactant makes diffusion of oxygen into the blood and carbon dioxide out of it more efficient.

If there isn’t enough surfactant, small alveoli can collapse (atelectasis), surface area drops, and gas exchange suffers. Conversely, factors that restrict blood flow to the lungs, destroy surfactant, or flood the alveoli with water or blood impair gas exchange even if ventilation is present.