Alveoli are small air sacs in the lungs that absorb the oxygen that we breathe in and remove all the carbon dioxide from the bloodstream.
The walls of the alveoli are only one cell thick, and blood flows on the other side of them. Gases pass through the walls of the alveoli and into the bloodstream. However, although the walls of the alveoli are so thin, blood is still unable to get into the alveoli. Alveoli are like rubber bands. They are able to stretch wide open to receive the air we breathe in and then collapse and push air out. Each lung is made up of millions of alveoli, and this makes it possible for some areas of the lungs to work even when there has been significant damage to other areas.
Alveoli may become damaged through contact with toxins that have been inhaled through smoking or other environmental pollutants. When alveoli become damaged, they lose their elasticity. This is know as emphysema. When a person has emphysema and is exhaling, the air becomes trapped in the spaces. When the airflow in and out of the alveoli encounters any kind of interference, the person will experience shortness of breath.