Lung anatomy. Coloured historical illustration of the human lungs, trachea and bronchiole network. At upper centre, the trachea (windpipe) introduces air from the larynx of the throat into the respiratory system. It 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. 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 and ribs.
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