Explain, with the help of a diagram, the structure of human lungs, and how exchange of gases occurs in the lungs.

Human lungs consist of small tubes, bronchioles and tiny air sacs known as alveoli (about 300 million in each lung).



Exchange of gases in lungs:
The air breathed in reaches the air sacs that are surrounded by blood vessels. These blood vessels transfer the oxygen present in the air to the blood. The carbon dioxide present in the blood passes to the air sacs.