Internal respiration is the gas exchange between which structures?

• A. The body's cells and systemic capillaries
• B. The alveoli and pulmonary capillaries
• C. The mitochondria and cytoplasm
• D. The bronchi and bronchioles

Answer: (A)

Internal respiration refers to the gas exchange at the tissue level, between the blood in the body's systemic capillaries and the cells themselves. Oxygen moves from the blood into cells to support metabolism, and carbon dioxide produced by cells diffuses into the blood to be carried away. This exchange happens across the capillary walls and into the cell membranes, driven by differences in partial pressures of O2 and CO2.

This is different from external respiration, which occurs in the lungs where alveolar air and pulmonary capillaries exchange gases so blood becomes oxygenated and CO2 is released for exhalation. The other options describe processes that aren’t about gas exchange between blood and tissues: the mitochondria and cytoplasm are involved in intracellular respiration, and the bronchi and bronchioles are airways that don’t participate in gas exchange.