Lung -- organ in which bronchi become highly branched and in which respiratory exchange occurs (Fig. 17-6).

Study the functional organization of lung tissue as shown in Fig. 17-6 and on the drawings here, noting especially the extra- and intrapulmonary airways and the intrapulmonary blood circulation around the alveoli.

Examine an H&E-stained section of the lung (slide 97).

• Locate an uncut edge showing visceral pleura covered by a simple squamous epithelium, the mesothelium (Fig. 17-18).
• Deeper in the lung, identify the small bronchioles (Fig. 17-9), which lack any cartilage framework and usually run parallel to small arteries or arterioles. (This slide does not include larger branches of bronchi, Figs. 17-7 and 17-8.)
• Note the bronchioles’ respiratory epithelium and the thin layer smooth muscle in their lamina propria layer.

Compare and contrast the structure of the bronchioles in this slide with that of the trachea.

Clinical note: Asthma involves hyperirritability of the respiratory passages, expressed as contraction of the bronchial smooth muscle, edema of the mucosa, and increased mucus secretion. It may be transient, following an upper respiratory tract infection, but more commonly the sensitization has an immunological basis and the symptoms are episodic. Re-exposure to an airborne antigen such as pollen causes release of histamine from mast cells and eosinophils, precipitating immediate bronchioconstriction and labored breathing. Various drugs are helpful in minimizing the severity of the attacks.

Elastic fibers in the lung.