Respiration is...

• A. The act of breathing the exchange of carbon dioxide and oxygen in the lungs and within each cell
• B. The process of digestion
• C. The circulation of blood
• D. The production of energy in mitochondria

Answer: (A)

Respiration is the body’s process of taking in oxygen and expelling carbon dioxide, and it includes both breathing (gas exchange in the lungs) and the use of that oxygen by cells to produce energy. In cells, oxygen drives mitochondrial processes that generate ATP, with carbon dioxide produced as a waste product. The option that mentions both the act of breathing and the exchange of oxygen and carbon dioxide in the lungs and inside each cell captures this entire process, making it the best fit. Digestion is unrelated to gas exchange, circulation concerns the movement of blood but not the actual gas exchange or cellular energy production, and simply saying energy production in mitochondria omits the breathing component that is also part of respiration.