Internal respiration involves diffusion between blood and tissues. Which statement is true?

• A. Oxygen diffuses into tissues from blood and carbon dioxide diffuses from tissues into blood
• B. Oxygen diffuses into tissues from blood
• C. Carbon dioxide diffuses from blood into tissues
• D. Gas exchange occurs in the lungs only

Answer: (A)

Internal respiration relies on diffusion driven by partial pressure differences between blood and tissue cells. In tissues, cells consume oxygen and produce carbon dioxide, so the tissue partial pressures become lower for O2 and higher for CO2 than in the blood. Oxygen thus diffuses from the blood into tissues, while carbon dioxide diffuses from tissues into the blood. This is exactly why the statement is true: oxygen moves into tissues from blood and carbon dioxide moves into blood from tissues.

This is distinct from gas exchange in the lungs (external respiration), where oxygen diffuses from alveolar air into blood and carbon dioxide diffuses from blood into alveolar air.