For efficient pulmonary gas exchange, which statement best describes the necessary process?

For efficient pulmonary gas exchange, which statement best describes the necessary process?

• A. Oxygen and carbon dioxide must be able to freely diffuse across the alveolar-capillary membrane
• B. There must be low quantities of pulmonary surfactant to allow for full alveolar expansion
• C. The pulmonary capillaries must be completely constricted and the alveoli collapsed
• D. The percentage of inhaled carbon dioxide must exceed the percentage of inhaled oxygen

Answer: (A)

Gas exchange hinges on diffusion across the thin alveolar-capillary barrier. Oxygen moves from the alveolar air, where its partial pressure is higher, into the blood, where it is lower. Carbon dioxide moves from the blood, where its partial pressure is higher, into the alveolar air to be exhaled, all driven by their partial pressure gradients. Adequate surfactant and open alveoli support this process by maintaining surface area and preventing collapse, but the fundamental mechanism is diffusion across the membrane. The other statements describe conditions that would hinder diffusion—insufficient surfactant, collapsed alveoli, or relying on inhaled gas percentages instead of partial pressures.

Gas exchange hinges on diffusion across the thin alveolar-capillary barrier. Oxygen moves from the alveolar air, where its partial pressure is higher, into the blood, where it is lower. Carbon dioxide moves from the blood, where its partial pressure is higher, into the alveolar air to be exhaled, all driven by their partial pressure gradients. Adequate surfactant and open alveoli support this process by maintaining surface area and preventing collapse, but the fundamental mechanism is diffusion across the membrane. The other statements describe conditions that would hinder diffusion—insufficient surfactant, collapsed alveoli, or relying on inhaled gas percentages instead of partial pressures.