What Causes Asthma

Your airways work like a system of tubes. Air travels in through your mouth or nose and moves through the main pathway in your throat called the trachea. From there, it flows down into your lungs through two smaller tubes called the main bronchi and into even smaller tubes, called bronchioles, and finally, into tiny sacs called alveoli. In these tiny sacs, the oxygen you inhale is transferred into your bloodstream to be used by your body. Carbon dioxide, a waste product, is also drawn out from your bloodstream and removed when you exhale. When asthma causes your airways to be tight and narrow, you are unable to exhale fully, leaving trapped air in your lungs. This air left in your lungs then limits your ability to take a full breath.

How Airflow Gets Blocked

When your lungs overreact to triggers, the flow of air becomes blocked, and it becomes harder for you to breathe. This results in an asthma attack.

Your airways are made up of smooth muscle. When you have asthma, these muscles contract, making the airways narrow and small.
Inside the airway, fluid starts to build up. This is your body's natural response to constant irritation. Just as when you get a mosquito bite, the area is irritated and reacts with fluid buildup and swelling. With asthma, your airways are irritated, and fluid buildup in your lungs causes the airways to swell.
As airways swell, thick mucus forms, which is hard to clear out, plugging up the smaller parts of your airway.

Extra-sensitive Airways

When you have asthma, your airways are overly sensitive and easily irritated. They tend to tighten or constrict more than normal in reaction to an irritant. This extra sensitivity is called airway hyperresponsiveness. The amount that your airways tend to overrespond is directly related to the severity of your asthma. In general, the more sensitive your airways, the worse your asthma.

Airway Remodeling

In some people with asthma, long-term inflammation can lead to permanent changes in the airways. This is called airway remodeling. The actual structure of your airway walls can change, causing blockage that can't be completely reversed with treatment. It is important to work with your doctor for the right diagnosis and treatment with anti-inflammatory medicine to prevent airway remodeling from occurring.

Asthma and the Immune System

Asthma is a physical condition. It is important to know that asthma is not caused by emotional problems, although strong emotions can make your symptoms worse. When you come into contact with something to which you may be allergic — an allergen — you get asthma symptoms because your immune system reacts strongly to protect the lungs from "invaders."

Although asthma allergen triggers, such as pet dander and mold, may not really be able to harm you, your immune system behaves as though they were extremely dangerous. It overreacts to these triggers and this is what makes you cough, wheeze, produce mucus, and feel chest tightness and shortness of breath. If you have allergies, triggers may also make your eyes water and your throat itchy. In the following pages, you will look at your specific triggers, as well as ways that you can avoid them or reduce their impact.

There is still much to discover about why some people have this immune response. What's known about asthma is that many cells are involved in activating the swelling and narrowing of your airways. The main cells of the immune system are the white blood cells ( neutrophils, eosinophils, and T lymphocytes) which digest and destroy invaders. Macrophages and mast cells are other cells involved in the immune response. These cells travel through your body to reach every organ.

Written by Karen Serrano, MD

Emergency Medicine resident at the University of Wisconsin-Madison.

Reviewed by Lisa V. Suffian, MD

Instructor of Clinical Pediatrics in the Division of Allergy and Pulmonary Medicine at Saint Louis Children's Hospital, Washington University School of Medicine

Assistant Clinical Professor in the Department of Pediatrics at Cardinal Glennon Children's Hospital, Saint Louis University

Board certified in Allergy and Immunology

Last updated June 2008