Answer: Alveoli
Step-by-step explanation:
The respiratory system of the human beings consists of nose, pharynx, trachea and alveoli. The nose inhales air inside.
This air moves in towards lungs. The whole respiratory system is covered with mucous membrane which does not allows the entry of harmful microorganism and dust particles.
This filtered air reaches the alveoli and gaseous exchange takes place in between oxygen and carbon dioxide. The carbon dioxide is exhaled out and oxygen is transferred to cells of the body.