MEHAU KULYK / SCIENCE PHOTO LIBRARY MEHAU KULYK / SCIENCE PHOTO LIBRARY
Bronchiole network, composite image. The trachea (windpipe, not seen) branches into two bronchi, which enter the pair of lungs. Within the lungs the bronchi divide into an internal network of smaller and finer bronchioles (seen here). These bronchiole tubes end in tiny balloon-like air sacs called alveoli. Exchange of gases occurs in the alveoli; inhaled oxygen enters the blood and carbon-dioxide is breathed out. For this respiration process, the lungs are inflated and contracted by movement of the diaphragm (not seen) and ribs (thick white lines).
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