Terminal Bronchioles

• Lined by simple ciliated columnar epithelium without goblet cells

• No cartilages, glands

Respiratory Bronchioles

• Branches of terminal bronchioles• Give rise to alveolar sacs, alveolar ducts, and

alveoli• Not sites of actual gas exchange• Lining epithelium: simple cuboidal, non-

ciliated epithelium• Some Clara cells

Alveolar Ducts

• Branch of from the walls of respiratory bronchioles

• Conical, thin-walled tubes• Lined by simple squamous epithelium• Give rise to numerous alveolar sacs and alveoli

that the openings practically occupy the entire wall of the duct

Alveolar Sacs and Alveoli

• Sites of actual exchange of gases between blood and inspired air

• Arise from either respiratory bronchioles or alveolar ducts

• Thin-walled polyhedral sacs• Open on one side – allow entry of air from the

respiratory bronchioles or alveolar ducts

Alveolar Sacs and Alveoli

• Around 300 million• Packed so closely that each alveolus does not

have a separate wall• Adjacent alveoli share a common wall

(interalveolar septum)

Interalveolar Septum

• Consists of thin supporting layer of connective tissue covered on either side by simple squamous epithelia

• Alveolar pores – allow direct communication between the cavities of adjacent alveoli

Interalveolar Cells

• Type I (pulmonary epithelial cell, small alveolar cell, pneumonocyte type I cell)

• Type II (great alveolar cell, pneumonocyte type II cell)

Type I Interalveolar Cell

• Less numerous but cover a majority of the alveolar surface area (> 95%)

• Squamous or flattened cells• Form the structure of the alveolar wall

Type II Interalveolar Cell

• Secrete pulmonary surfactant – reduces alveolar surface tension, thus prevents collapse of alveoli at the end of expiration

• Cuboidal or round cells

Pulmonary Alveolar Macrophages

• “Dust cells” • Most numerous cells in alveoli• Float freely in alveoli• Phagocytes

Blood-Air Barrier

• Ultra-thin structure separating the blood in a pulmonary capillary from the air in the alveolus

• Consists of:– Pulmonary epithelial cell– Basal lamina of the alveolar epithelium– Basal lamina of the capillary endothelium– Capillary endothelial cell