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Chronic Obstructive Pulmonary Disease
(COPD, Chronic Obstructive Lung Disease, COLD)

What is COPD?

Chronic obstructive pulmonary disease (COPD) is comprised primarily of three related conditions - chronic bronchitis, chronic asthma, and emphysema. In each condition there is chronic obstruction of the flow of air through the airways and out of the lungs, and the obstruction generally is permanent and may be progressive over time.

While asthma features obstruction to the flow of air out of the lungs, usually, the obstruction is reversible. Between "attacks" of asthma the flow of air through the airways typically is normal. These patients do not have COPD. However, if asthma is left untreated, the chronic inflammation associated with this disease can cause the airway obstruction to become fixed. That is, between attacks, the asthmatic patient may then have abnormal air flow. This process is referred to as lung remodeling. These asthma patients with a fixed component of airway obstruction are also considered to have COPD.

Often patients with COPD are labeled by the symptoms they are having at the time of an exacerbation of their disease. For instance, if they present with mostly shortness of breath, they may be referred to as emphysema patients. While if they have mostly cough and mucus production, they are referred to as having chronic bronchitis. In reality, it is better to refer to these patients as having COPD since they can present with a variety of lung symptoms.

There is frequent overlap among COPD patients. Thus, patients with emphysema may have some of the characteristics of chronic bronchitis and chronic asthma and vice a versa.

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 like the branches of a tree 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 and the chest wall is enlarged. During exhalation, the same muscles relax and the chest wall springs back to its 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. Normal lung tissues look a lot like a normal sponge. Emphysema often looks like an old sponge with large irregular holes and loss of the spring and elasticity.

Source: MedicineNet.com
http://www.medicinenet.com/chronic_obstructive_pulmonary_disease_copd/article.htm