lung and airway disorders
Defense Mechanisms of the Respiratory System
Introduction
The average person who is moderately active during the daytime breathes about 20,000 liters (more than 5,000 gallons) of air every 24 hours. Inevitably, this air (which would weigh more than 20 kilograms [44 pounds]) contains potentially harmful particles and gases. Particles, such as dust and soot, mold, fungi, bacteria, and viruses deposit on airway and alveolar surfaces. Fortunately, the respiratory system has defense mechanisms to clean and protect itself. Only extremely small particles, less than 3 to 5 microns (0.000118 to 0.000196 inches) in diameter, penetrate to the deep lung. Cilia,tiny muscular, hair-like projections on the cells that line the airway, are one of the respiratory system's defense mechanisms. Cilia propel a liquid layer of mucus that covers the airways. Themucus layertraps pathogens (potentially infectious microorganisms) and other particles, preventing them from reaching the lungs. Cilia beat more than 1,000 times a minute, moving the mucus that lines the trachea upwards about 0.5 to 1 centimeter per minute (0.197 to 0.4 inch per minute). Pathogens and particles that are trapped on the mucus layer are coughed out or moved to the mouth and swallowed. a type of Components of the Immune System on the surface of alveoli, are another defense mechanism for the lungs. Because of the requirements of (See also Overview of the Respiratory System .) 