(Solved) Explain the roles of mucus and cilia in the respiratory system.

The cilia (tiny hairs) found in the lining of the nostrils and trachea entrap dust and soot particles from the inhaled air. In the trachea their sweeping movements, move these particles to the pharynx for expectoration. [Smoking will eventually kill the cilia causing scar tissue to form in their place, thus depriving the body of a way to clean and clear inhaled dust and pose a health hazard to the air passages and the lungs]

The fact that at the alveoli an area of our body the size of a tennis court is separated from the outside air by a very narrow barrier imposes demands on the respiratory tract.

Outside air:

- varies in temperature. At the alveolar surface it must be at body temperature

- varies from very dry to very humid. At the alveolar surface it must be saturated with water vapour

- contains dust and debris. These must not reach the alveolar wall

- contains micro-organisms, which must be filtered out of the inspired air and disposed of before they reach the alveoli, enter the blood and cause possible problems.

It is easy to see that the temperature and humidity of inspired air will increase as it passes down a long series of tubes lined with a moist mucosa at body temperature. The mechanisms for filtering are not so obvious.

Mucus - The respiratory tract, from nasal cavities to the smallest bronchi, is lined by a layer of sticky mucus, secreted by the epithelium assisted by small ducted glands. Particles which hit the side wall of the tract are trapped in this mucus.

Cilia - Once the particles have been sidelined by the mucus they have to be removed, as does the mucous. This is carried out by cilia on the epithelial cells which move the mucous continually up or down the tract towards the nose and mouth. (Those in the nose beat downwards, those in the trachea and below upwards). The mucus and its trapped particles and bacteria are then swallowed, taking them to the sterilising vat of the stomach.

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Explain the roles of mucus and cilia in the respiratory system.

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