Which molecules diffuse across respiratory surfaces during gas exchange?

• A. Oxygen and Carbon Dioxide
• B. Glucose and Fatty Acids
• C. Sodium and Potassium
• D. Amino Acids

Answer: (A)

Gas exchange relies on diffusion of small, readily soluble molecules down their partial pressure gradients across a very thin, large-area respiratory surface. Oxygen moves from the air in the alveoli, where its partial pressure is higher, into the blood of the surrounding capillaries, while carbon dioxide moves from the blood, where its partial pressure is higher, into the alveolar air to be exhaled. The other substances listed are nutrients or ions that require specific transport mechanisms and are not gases that diffuse across the respiratory surface to participate in gas exchange. Therefore, only oxygen and carbon dioxide diffuse efficiently across these surfaces.