The lungs contain conducting portions (bronchi that lead to terminal bronchioles) through which air passes and respiratory portions (respiratory bronchioles that lead to the alveoli) where gas exchange occurs. Study the functional organization of lung tissue on the diagram and corresponding image, noting how bronchioles branch into terminal bronchioles, which lead into the respiratory bronchioles, alveolar ducts, and alveoli. Also, note the intrapulmonary blood circulation around the alveoli.

• Examine this section of bronchus. Using the table below as a guide, compare and contrast the structure of the bronchus with that of the bronchioles and alveoli in these slides of lung.

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 basophils, precipitating immediate bronchioconstriction and labored breathing. Various drugs are helpful in minimizing the severity of the attacks.

Elastic fibers in the lung.