Describe the structure of alveolus which is known as tha breating organ.

The alveolus (singular form) or alveoli (plural form) are tiny sacs of organs that are attached to the ends of the alveolar duct. These tiny sacs of air are clumped together that form interconnected caves, working together as a unit. They are found inside the lungs and they make the exchange of air easy and possible. Alveoli units are responsible for putting oxygen into the air that we breathe in. Blood that passes through the heart and into the lungs are converted into oxygen which will be then transported by the blood all over the body systems.  

The walls of the alveoli are extremely thin to enable the quick exchange of gases Aside from this, the linings of alveoli units are fluid in nature enabling gases to dissolve.