Alveoli. Computer graphic illustration showing the alveoli (round light blue structures), the microscopic air sacs of the lungs. They are seen wrapped by an intricate network of capillaries; it is in this network that gases, such as carbon dioxide and oxygen, are exchanged during respiration. Alveoli are found at the end of a bronchiole, one of the many small air passages in the lungs, and are arranged in groups that resemble bunches of grapes. About 300 million alveoli are found in each lung.