Answer:
Alveoli
Explanation:
Alveoli are the smallest passageway in the respiratory system, they have the shape of tiny balloons or air sacs, and are only one cell thick to allow its main function to happen, an easy passage of oxygen and carbon dioxide between alveoli and blood vessels (capillaries).
I annexed an image of alveoli sos you can see how they are shaped and how they work.
I hope you find this information useful and interesting! Good luck!
