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Pneumocytes, or alveolar cells, are cells lining the alveoli (or air sacs) of the lungs. Two types of pneumocytes exist: type I pneumocytes and type II pneumocytes.

Type I pneumocytes[edit]

Main article: Type I pneumocyte

Type I pneumocytes (or squamous alveolar cells) cover approximately 90–95% of the alveolar surface, and are involved in the gas exchange between the alveoli and blood.

Type II pneumocytes[edit]

Main article: Type II pneumocyte

Type II pneumocytes cover a minority of the alveolar surface. Their function is to secrete surfactant, which decreases surface tension within the alveoli. They are also capable of cellular division, giving rise to more Type I pneumocytes when the lung tissue is damaged.

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