Which of the following is NOT a function of mucous membranes?

• A. Lining
• B. Gas exchange
• C. Absorption
• D. Protection

Answer: (B)

Mucous membranes line cavities and passages that open to the outside and are specialized for protection, lubrication, absorption, and secretion. Gas exchange is not carried out by these membranes; it occurs across the very thin alveolar-capillary membranes in the lungs, where oxygen diffuses into blood and carbon dioxide diffuses out. The mucous membranes of the airways condition and humidify air, trap particles with mucus, and support immune defenses (the mucociliary escalator), while in the digestive tract they participate in absorption. Because gas exchange relies on the delicate alveolar walls rather than the mucous membranes, it is not a function of mucous membranes.