EVAN OTO / SCIENCE PHOTO LIBRARY EVAN OTO / SCIENCE PHOTO LIBRARY
An illustration of pulmonary alveoli, hollow cavities at the terminal ends of the respiratory tree where gas exchange occurs with the blood. Two types of pneumocytes make up the alveolar wall. Type I cells are squamous epithelial cells that have a flattened surface area to facilitate gas exchange. Type II cells are cuboidal epithelial cells which secrete surfactant, a substance which increases gas exchange capability. Alveolar dust cells are macrophages which destroy any foreign substances and pathogens which enter the lungs.
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