<P> Hair in the nostrils plays a protective role, trapping particulate matter such as dust . The cough reflex expels all irritants within the mucus membrane to the outside . The airways of the lungs contain rings of muscle . When the passageways are irritated by some allergen, these muscles can constrict . </P> <P> The respiratory tract is a common site for infections . </P> <P> Upper respiratory tract infections are probably the most common infections in the world . </P> <P> The respiratory system is very prone to developing infections in the lungs . Infants and older adults are more likely to develop infections in their lungs, because their lungs are not as strong in fighting off these infections . Most of these infections used to be fatal, but with new research and medicine, they are now treatable . With bacterial infections, antibiotics are prescribed, while viral infections are harder to treat, but still curable . </P>

What are the functions of the lower respiratory tract