MEHAU KULYK / SCIENCE PHOTO LIBRARY MEHAU KULYK / SCIENCE PHOTO LIBRARY
Lung alveoli. Computer digitised, historical illustration of tissue of the human lungs, showing the spongy appearance of alveoli. Alveoli are tiny balloon- like air sacs. Their walls of connective tissue contain smooth muscle and blood vessels. It is in the alveoli that exchange of gases takes place: inhaled oxygen enters the bloodstream and carbon- dioxide is breathed out. This spongy tissue is linked to a network of fine bronchiole tubes in the lungs which are linked to bronchi and then the trachea (windpipe). The lungs and alveoli are inflated and contracted by movement of the diaphragm and ribs.
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