Chronic Obstructive Pulmonary Disease (COPD)
aka Chronic Obstructive Lung Disease (COLD)
Medical Revising Author: George Schiffman, MD
Medical Editor: William C. Shiel Jr., MD, FACP, FACR
What is COPD?
Chronic obstructive pulmonary disease (COPD) is
comprised primarily of two related diseases - chronic bronchitis and emphysema.
In both diseases, there is chronic obstruction of the flow of air through the
airways and out of the lungs, and the obstruction generally is permanent and
progressive over time.
Asthma also is a pulmonary disease in which there is
obstruction to the flow of air out of the lungs, but unlike chronic bronchitis
and emphysema, the
obstruction in asthma usually is reversible. Between "attacks" of asthma the
flow of air through the airways usually is good.
There are exceptions, however. In some patients with
COPD the obstruction can be partially reversed by medications that enlarge or
dilate the airways
(bronchodilators) as with asthma. Conversely, some patients with asthma can
develop permanent airway obstruction if chronic inflammation of the airways leads to scarring and
narrowing of the airways. This process is referred to as lung remodeling. These
asthma patients with a fixed component of airway
obstruction are also considered to have COPD.
There also is frequent overlap among COPD patients. Thus, patients with
emphysema may have some of the characteristics of chronic bronchitis. Similarly,
patients with chronic bronchitis also may have some of the characteristics of
emphysema.
How does the normal lung work?
The lung is the organ for gas exchange; it
transfers oxygen from the air into the blood and carbon dioxide (a waste product of the body) from the blood into
the air. To accomplish gas exchange the lung has two components; airways and
alveoli. The airways are branching, tubular passages that allow air to move in
and out of the lungs. The wider segments of the airways are the trachea and the
two bronchi (going to either the right or left lung). The smaller segments are
called bronchioles. At the ends of the bronchioles are the alveoli, thin-walled
sacs. (The airways and alveoli can be conceptualized as bunches of grapes with
the airways analogous to the stems and the alveoli analogous to the grapes.)
Small blood vessels (capillaries) run in the walls of the alveoli, and it is
across the thin walls of the alveoli where gas exchange between air and blood
takes place.
Breathing involves inspiration followed by exhalation. During inspiration,
muscles of the
diaphragm and the
rib cage contract and
expand the size of the chest (as well as the airways and alveoli) causing negative pressure within the
airways and alveoli. As a result, air is sucked through the airways and into the
alveoli. During exhalation, the same muscles relax to their resting positions,
shrinking the chest and creating positive pressure within the airways and
alveoli. As a result, air is expelled from the lungs.
The walls of the bronchioles are weak and have a
tendency to collapse, especially while exhaling. Normally, the bronchioles are
kept open by the elasticity of the lung. Elasticity of the lung is supplied by
elastic fibers which surround the airways and line the walls of the alveoli.
When lung tissue
is destroyed, as it is in patients with COPD who have emphysema, there is loss
of elasticity and the bronchioles can collapse and obstruct the flow of air.
Next: What is chronic bronchitis? »
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