Which process describes the exchange of CO2 for O2 between blood and tissues?

• A. Internal Respiration
• B. External Respiration
• C. Atmospheric Pressure
• D. Transpulmonary Pressure

Answer: (A)

Internal respiration is the process by which oxygen diffuses from the blood into tissues and carbon dioxide diffuses from tissues into the blood. This tissue-level exchange supplies oxygen for cellular metabolism and removes the CO2 produced by cells, with blood carrying it back to the lungs for exhalation. External respiration, by contrast, is the gas exchange at the lungs between alveolar air and blood. Atmospheric pressure is simply the surrounding air pressure, not the gas exchange step, and transpulmonary pressure is the pressure difference that helps keep the lungs inflated, not the tissue-level exchange.