In respiration, what is the primary function of the respiratory membrane (alveolar-capillary membrane) in the lungs?

• A. To produce surfactant that decreases alveolar surface tension
• B. To warm and humidify inspired air before it reaches the bronchioles
• C. To actively pump $O_2$ into the pulmonary capillaries using ATP-driven transporters
• D. To allow diffusion of $O_2$ from alveoli into blood and ${\mathrm{CO}}_2$ from blood into alveoli

Answer: (D)

Verified step by step guidance

1. Understand that the respiratory membrane, also called the alveolar-capillary membrane, is the thin barrier between the air in the alveoli and the blood in the pulmonary capillaries.
2. Recognize that the primary role of this membrane is to facilitate gas exchange, specifically the movement of oxygen (O\_2) and carbon dioxide (CO\_2) between the alveoli and the blood.
3. Recall that gas exchange occurs by diffusion, a passive process where gases move from areas of higher partial pressure to areas of lower partial pressure across the respiratory membrane.
4. Note that the respiratory membrane does not produce surfactant (which is produced by alveolar cells) nor does it actively pump gases using ATP; instead, it provides a thin, permeable surface for efficient diffusion.
5. Conclude that the primary function of the respiratory membrane is to allow diffusion of O\_2 from the alveoli into the blood and CO\_2 from the blood into the alveoli, enabling effective respiration.