Each lung us conical in shape with its base resting on the diaphragm and its apex extending superiorly to a point approximately 2.5 cm superior to each clavicle. Right lung is larger than the left lung. Each lobe is divided into lobules that are separated from each other by connective tissue but the separations are not visible as surface fissures. Because major blood vessels and bronchi do not cross the connective tissues, individual diseased lobules can be surgically removed. Primary bronchi divide into secondary bronchi as they enter their respective lungs.
Secondary bronchi give rise to tertiary bronchi which extend to the lobules.
The bronchial tree continues to branch several times, finally giving rise to bronchioles . Bronchioles also divide numerous times to become terminal bronchioles which then divide into respiratory bronchioles . Each respiratory bronchiole divides to form alveolar ducts that end as clusters of air sacs called alveoli . The bronchi are lined with pseudostratified ciliated columnar epithelium. Farther into the respiratory tree, the cartilage becomes more and more sparse and smaller and smooth muscle becomes more abundant.