Alveolus video. The alveolus is an anatomical structure that has the form of a hollow cavity found in the lung. Alveoli (plural) are tiny air sacs at the end of the bronchioles which are the 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. Oxygen breathed in from the air passes through the alveoli and into the blood and travels to the tissues throughout the body. Carbon dioxide travels in the blood from the body's tissues and passes through the alveoli to be breathed out. Alveoli videos from VFX experts Polymime to download royalty free.