Defense Mechanisms
The average person who is moderately active during the daytime breathes about 20,000 liters of air every 24 hours. Inevitably, this air (which would weigh more than 20 kilograms) contains toxic particles and gases. Particles, such as dust and soot, mold, fungi, bacteria, and viruses, all deposit on airway and alveolar surfaces. Fortunately, the respiratory system has defense mechanisms to protect it.
One such defense mechanism involves tiny muscular projections (cilia), which line the airways. The airways are covered by a liquid layer that is propelled by cilia. These tiny muscles beat more than 1,000 times a minute, moving the mucus that lines the trachea about 0.5 to 1 centimeter per minute. Particles and pathogens trapped on this mucus layer are cleared to the mouth and swallowed.
Because of the requirements of gas exchange, alveoli are not protected by mucus and cilia--mucus is too thick and would impair movement of oxygen and carbon dioxide. Instead, the body has another defense system. Mobile cells on the alveolar surface called phagocytes seek out deposited particles, bind to them, ingest them, kill any that are living, and digest them. Phagocytes in the lungs are called alveolar macrophages. When the lung is exposed to serious threats, white blood cells in the circulation, such as neutrophils, can be recruited to help. For example, when the person inhales a great deal of dust or is fighting a respiratory infection, more macrophages are produced and neutrophils are recruited.
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