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Bronchial asthma is a chronic (long term) disease affecting the bronchial tubes (bronchi, breathing tubes, or airways) of the lungs. Normally when you breathe in (inhale), air is drawn through your mouth and nose into the trachea (windpipe) (see Figure 1). The trachea divides into two large breathing tubes called bronchi. These bronchi divide and divide again into ever smaller bronchi. Eventually, they divide into thousands of tiny bronchi called bronchioles. The bronchioles are connected to, or end in, tiny breathing sacs called alveoli.When you inhale, air passes down to the bronchioles and fills the air sacs (alveoli), permitting fresh oxygen to enter the body and allowing the waste gas, carbon dioxide, to leave the body when you breathe out (exhale).
FIGURE 1
The walls of the bronchial tubes are made up of different layers (see Figures 2A and 2B). The outer layer is composed of muscle and cartilage. The muscular tone of this layer determines how wide or narrow the bronchial tube passageway will be. The inner layer of the bronchial tubes is lined with mucous membranes that secrete mucus which coats the inside of these breathing tubes. Mucus functions to protect and clean the bronchial passageways.
FIGURE 2A
FIGURE 2B
Asthmatic symptoms are the result of constriction or narrowing of irritable bronchial tubes. This constriction is caused by bronchial tube muscle spasms and airway narrowing due to inflammation, which leads to swelling of the lining and increased mucous secretion. Narrowing of the airway causes resistance to the movement of air during breathing and often results in the symptoms of shortness of breath, chest tightness, wheezing and/or coughing.
Characteristically if you are asthmatic, your bronchial tubes are very sensitive and easily irritated. These sensitive airways can be thrown into spasm at times by a relatively minor stimulus (e.g. breathing in cold air, exposure to allergens, exercise, cigarette smoke, etc.).When you exhale, the lungs are compressed by the upward movement of the diaphragm and the obstructed bronchial tubes further narrow. This causes greater resistance to the outflow of air. A musical sound or wheeze is often heard as turbulent air passes out of the narrowed and partially obstructed bronchi.
Because it takes a longer amount of time to exhale through obstructed bronchi, some air remains trapped inside the airways and alveoli as you take your next breath. Over time, the remaining (residual) trapped air accumulates so that you feel difficulty breathing. In addition, the chest "tightness" you feel is due to increased resistance to air movement in the bronchial tubes. The decreased movement of your diaphragm muscle that has been pushed down by all the trapped air in your lungs also contributes to a feeling of shortness of breath. If the airway obstruction worsens, as in a severe asthmatic attack, poor air passage can result in reduced amounts of oxygen reaching the alveoli. Under these circumstances, your lungs can also begin to accumulate carbon dioxide waste which can, in the most extreme case, contribute to irregular heart beats, coma and death due to respiratory failure.
One of the major underlying causes of bronchial asthma is related to the mechanism of mast cell chemical mediator release (see Figure 3).
FIGURE 3
The unique, chemically-rich mast cells that line the mucous membranes of the bronchi may be triggered to release histamine or other chemical mediators (leukotriene, tryptase, etc.), which can cause bronchospasm and inflammation of the bronchial tubes. In allergic asthma, the allergy antibody, IgE, can cause the mast cells to release chemicals when exposed to appropriate allergen(s). In other circumstances, mast cells release their chemicals without an allergic trigger (e.g. following a cold or with exercise). If you are asthmatic, the effects of the mast cell mediator release are spasm (constriction) of the smooth muscle surrounding the airway, inflammation due to accumulation of various cells (eosinophils, neutrophils, lymphocytes, macrophages, etc.), edema (swelling) and increased mucus production within the bronchi. This process not only culminates in clinical asthmatic symptoms but also increases the hyper-responsiveness of the bronchial tubes to other irritant and/or allergen exposure(s).
Asthmatic symptoms that follow exposure to an allergen may occur immediately, within 15 30 minutes of exposure (immediate response), and may subside within an hour. In addition, you may have a delayed response (late phase reaction) that occurs 3 - 4 hours following the immediate response. This dual reaction is most often seen in more sensitive asthmatic individuals. The late phase reaction may last many hours or days and is thought to result from an inflammatory reaction. The late phase reaction may be associated with increased bronchial hyper-responsiveness or irritability of the airway, rendering you even more sensitive to a variety of inhaled irritants. As a result of increased hyper-responsiveness of the bronchi, the threshold for provoking asthmatic symptoms by various stimuli is lowered and may result in even more mediator release, inflammation and airway hyper-responsiveness. This can lead to a vicious cycle causing chronic symptoms, ultimately culminating in an acute attack of asthma unless the disease process is effectively treated (see Figure 4).
Figure 4
Finally, obstruction of the inner passageway of the bronchial tube may result from the thick, sticky mucus and mucous plugs that are characteristically produced during an asthmatic attack. If you are asthmatic, you may frequently complain of feeling "congested" from mucus accumulation, or you may often say that if the mucus could only be coughed up, your symptoms would be relieved. In a flare of asthmatic symptoms, however, as quickly as mucus is coughed up, new mucus is produced. Symptom relief is best achieved by treating the underlying bronchial inflammation and obstruction. In general, the earlier you treat asthmatic symptoms in an evolving attack, the less medication you need to bring it under control.
In summary, if you have asthma, you suffer from symptoms due to very sensitive bronchial tubes (see Table 1). Irritation of the bronchial tubes may trigger spasm or obstruction of the airways (see Table 2). Usually with the passage of time and/or the use of asthma medication, airway obstruction can ease up, and there is partial or total reversal of the bronchial tube obstruction. However, this is not true in all cases.
Partial or total reversibility of bronchial tube spasm is a primary characteristic of bronchial asthma early in the course of the disease.
Table 1
Table 2
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