Cellular respiration refers to the process by which:

• A. Blood cells deliver oxygen to the lungs
• B. Exchange of O2 and CO2 between cells and blood, with O2 entering cells
• C. Oxygen diffuses from arteries into tissues only
• D. Cells generate energy without oxygen

Answer: (B)

Cellular respiration is the process by which cells use oxygen to generate energy (ATP) from nutrients, primarily inside the mitochondria. This requires oxygen to move from the blood into cells and for carbon dioxide produced by metabolism to be carried away by the blood. The statement describing the exchange of O2 and CO2 between blood and tissues, with O2 entering cells, best captures this relationship: blood delivers oxygen to tissues, the oxygen enters cells to fuel energy production, and CO2 is produced as a waste product and returned to the blood for removal. The other ideas describe related but distinct steps—ventilation and oxygen delivery to the lungs, diffusion from arteries into tissues without emphasizing cellular use, or energy production without oxygen (anaerobic metabolism).