Asthma is a condition in which the airways narrow—usually reversibly—in response to certain stimuli.
Coughing, wheezing, and shortness of breath that occur in response to specific triggers are the most common symptoms.
Doctors confirm the diagnosis of asthma by doing breathing (pulmonary function) tests.
To prevent attacks, people should avoid substances that trigger asthma and should take drugs that help keep airways open.
During an asthma attack, people need to take a drug that quickly opens the airways.
Asthma affects more than 25 million people in the United States, and it is becoming more common. The reason for the increase in asthma is not known.
Although it is one of the most common chronic diseases of childhood, adults can also develop asthma, even at an old age. Asthma affects more than 6 million children (see Asthma in Children) in the United States and occurs more frequently in boys before puberty and in girls after puberty. Asthma can eventually resolve in children.
Asthma also occurs more frequently in non-Hispanic blacks and Puerto Ricans. Although the number of people affected by asthma has increased, the number of deaths has decreased.
The most important characteristic of asthma is narrowing of the airways that can be reversed. The airways of the lungs (the bronchi) are basically tubes with muscular walls (see Overview of the Respiratory System). Cells lining the bronchi have microscopic structures, called receptors. These receptors sense the presence of specific substances and stimulate the underlying muscles to contract or relax, thus altering the flow of air. There are many types of receptors, but two main types of receptors are important in asthma:
Beta-adrenergic receptors respond to chemicals such as epinephrine and make the muscles relax, thereby widening (dilating) the airways and increasing airflow.
Cholinergic receptors respond to a chemical called acetylcholine and make the muscles contract, thereby decreasing airflow.