The air passages—nasal cavity, pharynx, larynx, trachea, bronchi, and bronchioles—transmit air from the atmosphere to the alveoli (ventilation).
The bronchi divide dichotomously, becoming gradually smaller and more thin-walled as they progress away from the hilum toward the periphery. When the walls lose their cartilage, they are called bronchioles. Bronchioles are less than 2 mm in diameter, have smooth muscle walls, and terminate in the alveoli. The lining epithelium is ciliated columnar in the larger air passages and ciliated cuboidal in the distal bronchioles. Mucus-producing goblet cells are present, mainly in the larger bronchi. Scattered “small granule cells” are present in the bronchi on the basement membrane between epithelial cells; these are neuroendocrine cells that contain serotonin, bombesin, and other polypeptides. Small dome-shaped Clara cells in the terminal bronchioles secrete a protein that lines the small air passages.
Two units of lung parenchyma are recognized. The pulmonary lobule is represented by the structures derived from a small bronchiole, composed of 5–7 terminal bronchioles and the structures distal to them. The lobule is separated from other lobules by connective tissue.
The pulmonary acinus is represented by the structures arising from a single terminal bronchiole and consists of respiratory bronchioles and alveoli. Respiratory bronchioles are lined by simple cuboidal epithelium and participate in gas exchange. They lead into alveolar ducts. Alveolar sacs arise as saccular outpouchings from the alveolar ducts and respiratory bronchioles. The alveolar wall is 5–10 μm thick and covered by flat type I pneumocytes over 90% of the surface and by type II pneumocytes over the remainder. Type II pneumocytes are cuboidal cells with abundant cytoplasm that contains distinctive granules on electron microscopy. They produce surfactant and proliferate rapidly when there is alveolar injury.
The lung is encased by a layer of mesothelial cells, the visceral pleura, which becomes continuous with the internal lining of the chest wall (parietal pleura) at the lung hilum. The pleural cavity is lubricated by a small film of pleural fluid that permits movement of the lung in relation to the chest wall.
The lung has a dual blood supply. The bronchial arteriolar branches follow the bronchial tree and have a nutritive function. The pulmonary artery divides to produce a network of capillaries, the primary function of which is gas exchange.
Evaluation of the Integrity of Lung Structure
Lung structure may be assessed by several methods:
Physical examination of the chest.
Examination of sputum (coughed-up tracheal secretion) for the presence of specific microorganisms by culture and malignant cells by cytologic examination. Microbiologic interpretation requires care because sputum is almost invariably contaminated by saliva, which normally has a rich commensal flora. Less contaminated samples of sputum may be obtained by transtracheal ...