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Understanding asthma: the respiratory system

Some knowledge of the respiratory system is helpful to understand what happens when a person has an asthma attack or experiences even mild asthma symptoms.

The average adult breathes in about 10,000 litres of air each day. It is important to our health that this air is as pure as possible, so the air we breathe is cleansed as it passes through the respiratory system - the body's own complex filtering system.

We take in air through the nose and mouth, which make up the upper respiratory system. The insides of the nose, mouth and throat, called muscosal surfaces, are warm and moist. When we inhale via the nose the air is warmed to body temperature and moistened so it will not dry out the insides of the lungs. Dust particles in the air stick to the muscosal surfaces and do not reach the lungs. By the time the air reaches the back of the throat, virtually all particles down to ten micrometres have been filtered out.

The air we've breathed then passes from the throat, through the larynx and into the trachea. The trachea divides into two main tubes, called the bronchi, which lead to each lung. Each bronchus branches many times into progressively smaller airways. The bronchi continue to warm, moisten and clean the air as it passes through on its way into the lungs. Millions of microscopic, hair-like projections, called cilia, line the bronchi. The cilia constantly flush mucus up the bronchi to the throat, where it is automatically swallowed. The average person swallows up to 100 ml of mucus a day, yet is unaware of the process. It usually isn't until excess mucus is present, say, during a bad cold, that people even become conscious of its production.

The two bronchi subdivide many times into smaller tubes called the bronchioles. By the time the air reaches the bronchioles, all particles down to around one micrometre have been removed. The bronchioles finally end in tiny, balloon-like sacs called alveoli. There are millions of these tiny sacs - in fact, if one person's alveoli were spread out, they would cover an area the size of a tennis court.

The alveoli have very thin walls through which the oxygen in the air we breathe passes into the bloodstream. At the same time, carbon dioxide, which is the gas we exhale, passes from the bloodstream back into the alveoli.