|the specialized organs, collectively, concerned with external respiration: in humans and other mammals it includes the trachea, bronchi, bronchioles, lungs, and diaphragm|
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The system of organs and structures in which gas exchange takes place, consisting of the lungs and airways in air-breathing vertebrates, gills in fish and many invertebrates, the outer covering of the body in worms, and specialized air ducts in insects.
The organs in the body involved in respiration. Air enters the body through the nose and mouth and travels down the trachea, through the bronchial tubes, and finally into the lungs. Once in the lungs, the air is drawn into an enormous number of thin-walled sacs richly supplied with capillaries. The exchange of oxygen and carbon dioxide in the blood takes place in these tiny sacs.