26.7 : 폐포와 폐포관

Within a pulmonary lobule, the terminal bronchioles branch into microscopic respiratory bronchioles, which conduct air to the respiratory zone of the lungs.

These bronchioles further divide into several ducts lined with simple squamous epithelium, known as the alveolar ducts, which end into grape-like structures called the alveolar sacs.

Alveoli are the dilated epithelial structures within these alveolar sacs.

Every alveolus is covered by an extensive capillary network with encircling elastic fibers to stabilize the positions of the alveoli and respiratory bronchioles.

An alveolar wall has numerous squamous pulmonary epithelial cells — type-I pneumocytes or type-I alveolar cells — which facilitate gas exchange.

Alveolar walls also have cuboidal epithelial cells called type-II pneumocytes, type-II alveolar cells, or septal cells. These cells secrete alveolar fluid, mainly composed of a surfactant, which prevents alveolar collapse.

There are alveolar macrophages or dust cells in the alveolar walls that aid in removing inhaled pathogens and microscopic dust.

Each lung holds an estimated 150 million alveoli, giving the lungs a spongy texture.