Which structure of the respiratory system plays the main role in carry out the system’s primary function?
trachea
pharynx
alveoli
nose

alveoli

Explanation:

Alveoli are the basic functional units of respiratory system which play actual role of exchange of gases. Alveoli are tiny balloon like air sacs which exist as a single cell not as a group of cells. They are responsible for exchange of gases like oxygen and carbon dioxide in our body. As soon as we inhale with the help of nose, a lot of air full of oxygen first reaches pharynx and then trachea. From trachea it ultimately reaches alveoli. Since alveoli have air full of oxygen so the partial pressure is more at alveolar end while capillaries nearby alveoli have blood which is full of carbon dioxide so the partial pressure of carbon dioxide is more in capillaries. Now, a gradient is set up as a result of which alveoli and blood capillaries exchange oxygen and carbon dioxide passively. Oxygen molecules from alveoli tend to diffuse to capillaries while carbon dioxide molecules from capillaries tend to move towards alveoli. The movement/exchange of gases continues until an equilibrium condition is reached. After which the carbon dioxide is exhaled out of out body with the help of same respiratory tract mentioned above.

more words for this could be abiotic and biotic

abiotic: living organisms within an ecosystem

biotic: non-living organisms within an ecosystem