Internal respiration involves:

• A. Diffusion of oxygen into the alveoli
• B. Exchange of gases between blood and body cells
• C. Transfer of carbon dioxide to the lungs
• D. Balancing oxygen and carbon dioxide in the air

Answer: (B)

Internal respiration is fundamentally the process of gas exchange that occurs at the cellular level within the body's tissues. It specifically refers to the exchange of oxygen from the blood into the body's cells and the simultaneous transfer of carbon dioxide from the cells into the blood. This process is critical for cellular metabolism, as cells require oxygen for energy production and need to eliminate carbon dioxide, a waste product of that process.

In contrast, external respiration relates to the exchange of gases in the lungs, particularly the movement of oxygen into and carbon dioxide out of the alveoli and blood. Therefore, while the other options mention processes related to respiration, they do not accurately define internal respiration as it pertains to the exchange of gases specifically between the blood and body cells. Thus, recognizing that internal respiration is centered on cellular gas exchange clarifies why this answer is the most appropriate choice.