Either of the two main airways or branches (or tubules) to the lungs that branch off the lower end of the trachea to connect with the lungs. They have an outer fibrous coat with irregular placed plates of hyaline membrane, and an inner coat consisting of smooth muscle and a mucous membrane of columnar epithelial cells. There is no gas exchange in this part of the respiratory system.
See Alveoli, Bronchopulmonary dysplasia, Epithelium, Smooth (or involuntary) muscle