Internal respiration refers to which process?

• A. Gas exchange in the lungs.
• B. Exchange of O2 and CO2 at the cellular level within organs of the body.
• C. Air movement during exhalation.
• D. Oxygen binding to hemoglobin.

Answer: (B)

Internal respiration is the gas exchange between the blood and the body's tissues. It involves oxygen moving from the blood into cells to support metabolism and carbon dioxide moving from cells into the blood to be carried away and eventually exhaled. This tissue-level exchange is driven by partial pressure differences and is distinct from external respiration, which is the alveolar gas exchange in the lungs, and from processes like ventilation or oxygen binding to hemoglobin, which relate to transport rather than the actual exchange at the tissue level.