Video - Bronchioles and alveoli histology

The bronchioles are the smallest air passages in the lungs and they end in tiny sacs called alveoli. The wall of each bronchiole has a layer of smooth muscle that can contract to narrow the airway. Distally are the smallest bronchioles called terminal bronchioles, which give rise to the respiratory bronchioles that end up with alveolar ducts. Distal to the alveolar ducts are alveolar sacs and their individual alveoli. The alveoli are where most of the gas exchange takes place between the lungs and the bloodstream. Each alveolus is a microscopic sac lined with a single layer of flattened epithelial cells. There are tiny blood vessels (capillaries) surrounding each alveolus, and it is through these capillaries that oxygen and carbon dioxide are exchanged. Within the alveoli are clusters of cells called type I and type II pneumocytes. The type I cells make up about 90% of the epithelial layer and their primary function is to keep the alveoli from collapsing. The type II cells are much less numerous and their primary function is to produce surfactant. Surfactant is a substance that reduces the surface tension of the fluid that covers the alveoli. This allows the alveoli to stay inflated and prevents them from collapsing.