Lung

The lung consists of airways and structures for gas exchange.

The trachea divides into a primary bronchi for each lung. The smaller intrapulmonary airways are devoid of cartilage and glands in their walls.

• Primary (Muscular) Bronchioles (not present in this specimen)
• Epithelium changes from pseudostratified columnar to simple, ciliated columnar epithelium as they decrease in diameter.
• Clara Cells - dome-shaped secretory cells with short microvilli. (Do not have cilia.)
• Smooth Muscle - variable amounts present.
• Terminal Bronchioles - conducting airways.
• Epithelium changes from simple, ciliated columnar epithelium to cuboidal epithelium.
• Clara Cells - become more prominent.
• Smooth Muscle - variable amounts present.
• Respiratory Bronchioles (#1, #2 and #3) - mixture of conducting epithelium and alveoli where respiration occurs.
• Epithelium is a mixture of simple columnar and simple cuboidal epithelia.
• Clara Cells - become more prominent.
• Smooth Muscle - only small amounts are present.

• Alveolar Ducts (#1 and #2)- arise from respiratory bronchioles and are passageways lined with alveoli and occasional regions of simple cuboidal epithelium.
• Alveolar Sacs (#1 and #2) - spherical-like spaces with openings into multiple alveoli.
• Alveoli - blind ending sacs that are the final termination of the airways.
• Simple Epithelium (#1, #2, #3, #4 and #5) - contains two types of pneumocytes resting on a thin basal lamina.
• Type I - flattened, simple squamous epithelial cells.
• Gas exchange occurs through these cells.
• Type I - dome-shaped, cuboidal epithelial cells that project into the lumen.
• Secrete surfactant that covers the alveolar surface and reduces surface tension.
• Macrophages (or Dust Cells) (#1, #2 and #3) - large, dark cells within alveoli that engulf dust particles, bacteria, and other pathogens.
• Capillaries - dense network surround alveoli. They are usually seen in cross-section and can be identified by finding a red blood cell within a lumen next to an endothelial cell.
• Smooth Muscle - often found at the ends of alveolar walls. (Not easily seen in this specimen.)