SAMANTHA ELMHURST / SCIENCE PHOTO LIBRARY SAMANTHA ELMHURST / SCIENCE PHOTO LIBRARY
Alveoli, Illustration of alveoli (air sacs) and cross-sectioned alveolar ducts, which connect the alveoli to the bronchioles. The alveoli are the site of gaseous exchange in the lungs. Carbon dioxide from de-oxygenated red blood cells (blue) passes from the capillaries, across the alveolar epithelium, into the alveoli, and is exhaled from the lungs. Inhaled oxygen dissolves on the mucosal lining of the epithelium and passes into the capillaries where it is taken up by the red blood cells. This oxygenated blood (red) is then carried around the body.
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