Gas exchange in the lungs is facilitated by which of the following?

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

Answer: (C)

Surfactant lowers surface tension inside the alveoli, preventing collapse at the end of exhalation and keeping the alveoli stable and open. This stability preserves surface area and makes it easier for oxygen to diffuse into the blood and carbon dioxide to diffuse out across the thin alveolar-capillary membrane. Adequate amounts of surfactant thus maximize the conditions needed for efficient gas exchange. Without enough surfactant, alveoli can collapse (atelectasis), reducing surface area and slowing diffusion. Other options would either not promote diffusion (water in the alveoli or blood in the space) or would impair gas exchange (surfactant destruction or capillary constriction).