This medical illustration represents air sacs in adults. Each cell shown here constitutes a purple ball. They are gathered in clusters to form bags. Each sac is located at the end of a bronchiole that organizes itself like the branches of a tree. It is at the level of the pulmonary alveoli that the gas exchange between air and blood melts during breathing. In adults there are a large number of pulmonary alveoli which form the air sacs. Which gives it a great breathing capacity. Unlike adults, toddlers have a much smaller number of alveoli. They are more scattered and do not form bags.