Asthma

Asthma is a disorder of the respiratory system in which the passages that enable air to pass into and out of the lungs periodically narrow, causing coughing, wheezing, and shortness of breath. This narrowing is typically temporary and reversible, but in severe attacks, asthma may result in death. Asthma most commonly refers to bronchial asthma, an inflammation of the airways, but the term is also used to refer to cardiac asthma, which develops when fluid builds up in the lungs as a complication of heart failure. This article focuses on bronchial asthma.

More than 17 million Americans suffer from asthma, with nearly 5 million cases occurring in children under age 18. In the United States, asthma causes nearly 5,500 deaths each year. Asthma occurs in males and females of all ages, ethnic groups, and socioeconomic levels. For reasons not completely understood, asthma is generally more common in poor urban neighborhoods, in cold climates, and in industrialized countries.

Among all Americans, the prevalence of asthma increased more than 60 percent between 1982 and 1994, especially among children. Deaths from asthma increased more than 55 percent from 1979 to 1992. Scientists suspect that increased exposure to second-hand cigarette smoke, growing populations in polluted city centers, and new housing that is poorly ventilated contribute to the increase in asthma cases.

Breathing

Every cell in the human body requires oxygen to function, and the lungs make that oxygen available. With every breath we take, air travels to the lungs through a series of tubes and airways. After passing through the mouth and throat, air moves through the larynx, commonly known as the voice box, and then through the trachea, or windpipe. The trachea divides into two branches, called the right bronchus and the left bronchus, that connect directly to the lungs. Air continues through the bronchi, which divide into smaller and smaller air passages in the lungs, called bronchioles. The bronchioles end in clusters of tiny air sacs, called alveoli, which are surrounded by tiny, thin-walled blood vessels called capillaries.

Here, deep in the lungs, oxygen diffuses through the alveoli walls and into the blood in the capillaries and gaseous waste products in the blood--mainly carbon dioxide--diffuse through the capillary walls and into the alveoli. But if something prevents the oxygen from reaching the alveoli, the body's cells do not receive a constant supply of vital oxygen, and carbon dioxide builds up to harmful levels in the blood.

Asthma Attacks

Asthma attacks occur when the bronchi and bronchioles become inflamed, reducing the space through which air can travel through the lungs. This causes the asthmatic to work harder to move air in and out of the lungs. Asthma attacks usually begin with mild chest pressure and a dry cough. As an attack intensifies, wheezing develops and increases in pitch; breathing becomes difficult; and coughing produces thick, stringy mucus. As the airway inflammation prevents some of the oxygen-rich air from reaching the alveoli, the cells of the body start to burn oxygen at a higher rate, actually increasing the body's demand for oxygen. The frequency of asthma attacks varies considerably among asthma suffers. Some people have daily attacks, while others can go months or even years without having an attack.

Inflammation of the airway occurs when an irritant--such as pet hair or cigarette smoke--comes into contact with the airway walls. Upon detecting the irritant as a harmful invader, the body's immune system sends special cells known as mast cells to the site of irritation, in this case the airway walls. The mast cells release histamine, a chemical that causes swelling and redness in a process called the inflammatory response. Histamine also causes bronchospasms, in which the muscles lining the airway walls contract repeatedly, causing the airways to narrow even more. In addition, cells that lubricate the airways with mucus--called goblet cells--overreact to the inflammatory response by secreting too much mucus. This mucus clogs the bronchioles, resulting in wheezing and coughing.

Causes

Asthma attacks are caused by airway hyperresponsiveness--that is, an overreaction of the bronchi and bronchioles to various environmental and physiological stimuli, known as triggers. The most common causes of asthma attacks are extremely small and lightweight particles transported through the air and inhaled into the lungs.