Final answer:
External respiration is the exchange of gases between the atmosphere and lungs, while internal respiration is the exchange of gases at the tissue level.
Step-by-step explanation:
Gas exchange occurs in two sites in the body: in the lungs and at the tissues. External respiration is the exchange of gases between the atmospheric air and the lungs, while internal respiration is the exchange of gases between the blood and the body tissues. In external respiration, oxygen is taken in and carbon dioxide is released in the alveoli of the lungs. In internal respiration, oxygen is released from the blood into the tissues, while carbon dioxide from the tissues is picked up by the blood.