Alveoli
Located at the end of the terminal bronchioles within the lungs, the alveoli are small cup shaped air sacs responsible for gas exchange.
Alveolus : individual cup-shaped outpouching Alveolar Sac : 2 or more alveoli that share a common opening/duct.
The alveolar walls consist of 2 types of alveolar epithelial cells: Type I Alveolar Cells : simple squamous epithelial, form a continuous lining of the alveolar wall. These are the main sites of gas exchange. Type II Alveolar Cells: fewer, found between type I cells, mainly secrete alveolar fluid which keeps the surface between the cells and air moist. Surfactant : included in the alveolar fluid, a complex mixture which lowers the surface tension of alveolar fluid; reducing the tendency of alveoli to collapse Alveolar Macrophages: phagocytes that remove fine dust particles located within the alveolar walls On the outer surface of the alveoli is a network of blood capillaries. This allows for the exchange of O2 and CO2 between the blood & lungs. Gas exchange takes place by diffusion across the alveolar and capillary walls, which together are called the Respiratory Membrane . This layer consists of 4 layers from alveolar air space to blood plasma:
1. Layer of type I and II cells with alveolar macrophages = Alveolar Wall. 2. Epithelial Basement Membrane (under the alveolar wall) 3. Capillary Basement Membrane 4. Capillary Endothelium
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