Asthma occurs because the airways in the lungs overreact to various stimuli, resulting in narrowing with obstruction to air flow. This recurrently results in one or more of the following symptoms:
Tightness in the chest
Noises in the chest heard particularly during a prolonged forced expiration (wheezing). As a result of these symptoms, asthmatics may not tolerate exertion. They may be awakened frequently at night. More severe symptoms may result in requirements for urgent medical care and hospitalization. For a very few with particularly severe asthma, there is a risk of fatality.
Asthma affects the airways, which begin just below the throat as a single tube called the trachea. The trachea is situated immediately in front of the esophagus, the passageway that connects the throat with the stomach. The trachea divides into two slightly narrower tubes called the main bronchi (each one is called a bronchus). Each main bronchus then divides into progressively smaller tubes - the smallest are called bronchioles - to carry air to and from microscopic air spaces called alveoli. It is in the alveoli that the important work of the lung occurs, exchanging oxygen in the air for carbon dioxide in the blood. The airways (trachea, bronchi, bronchioles) are surrounded by a type of involuntary muscle known as smooth muscle.
The airways are lined with a mucus membrane that secretes a fine layer of mucus and fluid. This mucus washes the airways to remove any bacteria, dirt, or other foreign material that might get into our lungs. The overreaction or hyper-responsiveness of the airways results in bronchospasm, which is excessive contraction or spasm of the bronchial smooth muscle. The airways also become inflamed with swelling of the bronchial mucous membrane (mucosa) and secretion of excessive thick mucus that is difficult to expel. It is part of the evaluation process to identify the role of each of these physiologic components in asthma. This is important because bronchospasm (constriction of the muscle surrounding the airways) and inflammation respond to different medications.
Image of a normal bronchiole and an asthmatic bronchiole
The airway hyper-responsiveness leading to obstruction of the airways occurs from one or more of various stimuli that vary with the individual patient. These include:
Viral (but not bacterial) respiratory infections (common colds)
Inhaled irritants (cigarette smoke, wood burning stoves and fireplaces, strong odors, chemical fumes)
Inhaled allergens (pollens, dusts, molds, animal danders)
Occasional ingested substances (aspirin, sulfite preservatives, specific foods). Sometimes these exposures just act as triggers of brief symptoms with rapid relief once exposure ends. Sensitivity of the airway may be increased, however, following even brief exposure to one of these. This causes a longer period of asthmatic symptoms. More information should be provided to you for each of these that are judged to be important for your asthma.
The obstruction of the airways decreases the rate at which air can flow. This is felt as tightness in the chest and labored breathing (dyspnea). The obstruction and inflammation causes coughing. Obstruction to air flow can be measured with pulmonary function tests, which can detect even degrees of airway obstruction not yet causing symptoms. Pulmonary function measurements can be an extremely valuable tool for your physician to make decisions regarding treatment.
The increased mucus in the airways stimulates coughing as the body attempts to clear the airways. The unusually thick (viscous) mucus is difficult to expel, however, resulting in continued coughing that fails to adequately expel the mucus. General irritability of the airways also causes coughing. The coughing and mucus production may cause some physicians to diagnose bronchitis. However, the term "bronchitis" simply means inflammation of the airways, and asthma causes airway inflammation. Consequently, anti-asthmatic medication, and not antibiotics, are the appropriate treatment. (Of course an asthmatic can, on occasion, have an infectious bronchitis that does not respond to anti-asthmatic medication, but this is usually viral and usual antibiotics are still not generally of any value - although there are exceptions to this generality).
Narrowing of the airway causes noises when air passes through them with sufficient speed. This typical high-pitched noise is called wheezing. Mucus in the airway causes a rattling sound called coarse crackles. Complete obstruction of some airways can cause absorption of air from the alveoli (air sacks at the end of the airways in the lungs). This causes portions of the lung to appear more dense and cast more of a shadow on a chest x-ray (this is called atelectasis). The rattling sounds or increased shadows on the x-ray are often misinterpreted as indicating pneumonia. The inappropriate diagnoses of bronchitis and pneumonia cause much unnecessary use of antibiotics, which are ineffective both for asthma in general and for most of the infections, such as the common cold viruses, that trigger asthma.