The alveoli are surrounded by tiny blood vessels, called capillaries. The alveoli and capillaries both have very thin walls, which allow the oxygen to pass from the alveoli to the blood.

What is the alveoli surrounded by?

At the very end of the air passages are clusters of small air filled sacs called alveoli (singular: alveolus). These air sacs are surrounded by tiny blood vessels called capillaries (Image 1).

Where is alveoli located?

Tiny air sacs at the end of the bronchioles (tiny branches of air tubes in the lungs). The alveoli are where the lungs and the blood exchange oxygen and carbon dioxide during the process of breathing in and breathing out.