Internal respiration occurs in which site?

• A. Capillaries
• B. Lungs
• C. Trachea
• D. Bronchioles

Answer: (A)

Internal respiration is the gas exchange between blood and body tissues, and it occurs mainly in the systemic capillaries. Here, oxygen diffuses from the blood into tissue cells while carbon dioxide diffuses from the tissues into the blood to be carried away. Capillaries are the right site because their walls are extremely thin and they form a dense network right next to every cell, keeping the diffusion distance short and providing lots of surface area for exchange. This is distinct from external respiration, which happens in the lungs at the alveolar level, where gases move between air and blood. The trachea and bronchioles merely conduct air and do not participate in gas exchange.