Answer:
the alveoli
Step-by-step explanation:
Gas exchange occurs at the alveoli in the lungs and takes place by diffusion. The alveoli are surrounded by capillaries so oxygen and carbon dioxide diffuse between the air in the alveoli and the blood in the capillaries.
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As the blood moves through the capillaries in the alveoli, oxygen diffuses into it and carbon dioxide diffuses out of it
At the muscles, the opposite occurs as carbon dioxide enters the blood from the muscle while oxygen enters the muscle from the blood.
Capillaries surround the alveoli in the lungs. Both the capillaries and alveoli walls are very thin - just one cell thick. They are made of semi-permeable membranes which allow oxygen and carbon dioxide to pass through them.