Which statement correctly describes internal respiration?

Which statement correctly describes internal respiration?

• A. Internal respiration is the exchange between blood and tissue cells
• B. Internal respiration is the exchange between lung air and blood
• C. Internal respiration is gas exchange within mitochondria
• D. Internal respiration occurs only in the heart

Answer: (A)

Internal respiration is the gas exchange that happens between the circulating blood and the body's tissue cells. Oxygen moves from the blood into cells to fuel metabolism, and carbon dioxide moves from cells into the blood to be carried away and expelled. This tissue-level exchange is what keeps cells supplied with oxygen and lets them dispose of CO2. The statement that describes this exact process—exchange between blood and tissue cells—is the right one. The other ideas describe different steps or processes: gas exchange between lung air and blood is external respiration, gas exchange inside mitochondria is cellular respiration (the metabolic use of oxygen) rather than a diffusion exchange between blood and tissues, and internal respiration isn’t limited to the heart but occurs throughout the body’s tissues.

Internal respiration is the gas exchange that happens between the circulating blood and the body's tissue cells. Oxygen moves from the blood into cells to fuel metabolism, and carbon dioxide moves from cells into the blood to be carried away and expelled. This tissue-level exchange is what keeps cells supplied with oxygen and lets them dispose of CO2.

The statement that describes this exact process—exchange between blood and tissue cells—is the right one. The other ideas describe different steps or processes: gas exchange between lung air and blood is external respiration, gas exchange inside mitochondria is cellular respiration (the metabolic use of oxygen) rather than a diffusion exchange between blood and tissues, and internal respiration isn’t limited to the heart but occurs throughout the body’s tissues.