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In the alveoli Alveolus (also called air sac) is the capillary-rich sac in the lungs where the exchange of oxygen and carbon dioxide takes place by diffusion.
Lungs ensure that efficient gas exchange takes place between the air and the person's blood. The lungs are a pair of lobed s â¦ tructures made up of a series of highly branched tubules called bronchioles, which end in tiny air sacs called the alveoli (the alveolar membrane is the site of gas exchange).  Lungs allow us to take in oxygen from the external air and allow us to get rid of excess carbon dioxide. alveolar = air sac   simple squamous epithelium; single layer of flattened cells with disc-shaped central nuclei and sparse cytoplasm; the simplest of the epithelium.