The exchange of oxygen, carbon dioxide and other gases from air to bloodstream only occurs in the alveoli of the lungs, the tiny air sacs at the end of the bronchial tree (see LUNGS). That portion of each breath taken into the lungs which does not take part in gas exchange is known as dead space. Anatomical dead space describes air in the airways up to the terminal BRONCHIOLES. Physiological dead space also includes gas in damaged alveoli unable to take part in gas exchange because of structural abnormalities or disease.