Respiration is defined as what?

• A. The exchange of O2 and CO2 on the cellular level
• B. The process of moving air in and out of the lungs
• C. The exchange of oxygen at the alveolar level
• D. The regulation of breathing by the brain

Answer: (A)

Respiration refers to how cells use oxygen to produce energy and, as a result, generate carbon dioxide as a waste product. This happens at the cellular level, inside the body's cells, not merely in the lungs or brain. The statement describing the exchange of O2 and CO2 at the cellular level captures this metabolic process, which is why it best fits the concept of respiration. By contrast, moving air in and out of the lungs is ventilation, the lungs’ mechanical process; exchanging oxygen at the alveolar level is external respiration, gas exchange between air and blood; and regulating breathing by the brain is about control, not the cellular use of oxygen. So the cellular-level exchange of O2 and CO2 is the best match.