The lower respiratory tract is protected by all except competition with the normal flora of the lungs.
The windpipe (trachea) and the bronchi, bronchioles, and alveoli found inside the lungs are among the principal channels and structures of the lower respiratory tract. Each bronchus splits into secondary and tertiary bronchi deep within the lungs, which continue to branch to smaller airways known as the bronchioles. The alveoli are air sacs that form the end of the bronchioles. Alveolar sacs are formed when alveoli are grouped together. A network of capillaries carrying blood that has travelled through veins from various parts of the body perform gas exchange on the surface of each alveolus.
Hence, lower respiratory track is only used for gaseous exchange .
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