Our airway walls are made up of cartilage, smooth muscle (one of the three muscle types), mucus-producing glands, connective tissue, and epithelium (the type of tissue that covers all outer and inner body surfaces, including the lining of vessels and other small cavities). Because the relative amounts of these components changein healthy lungsas you move into the lungs along the airway branches, the large, medium, and small airways are distinctive in composition as well as in size.

Mucus

Connective tissue (the fibroelastic framework) kind of holds the overall structure together. Mucusthe layer of sticky fluid covering the lining of epithelial cellsis made up of water, large molecules of sugar and protein, and salts. It is technically called sputum. Mucus is produced by goblet cells in the lining of medium and large airways, and by mucous glands deep in the airway wall. (In a healthy nonsmoker, the proportion of goblet cells greatly decreases as the airways become smaller.) The mucus-producing goblet cells are interspersed among other epithelial cells with moving, hair-like projections called cilia. These two types of cells form a protective alliance that keeps the airways sterile. Mucus traps foreign particlesincluding bacteriathat enter the airways, then the cilia push both mucus and particles up to the mouth to be swallowed or coughed out.

Excessive amounts of mucussuch as the lungs produce during respiratory infectionsare an irritant once they reach the larger airways. Its presence there stimulates the cough reflex, which expels it. (When a cough is stimulated, the respiratory muscles contract forcefully while the vocal cords stay tightly shut, building up very high pressure in the lungs. Then the vocal cords fly open and air blasts out of the lungs, sometimes as fast as 70 mph.)

Chronic bronchitis overwhelms the cough reflex in several ways. To begin with, it paralyzes the cilia. As they stop working, mucus starts accumulating in smaller and smaller airways (Figure 2.5). It combines with the inflamed airway lining to increase expiratory airflow resistance substantially. This high resistance prevents air from blasting out of the lungs at high speeds. The cough is still stimulated appropriately, but it has become too weak to dislodge the mucus filling the upper airways.

Muscle

Cartilagewhich helps support all but the smallest airwaysforms unclosed rings around them. Its presence decreases substantially in each airway generation until it disappears in the smaller airways. A layer of smooth muscle lies between the epithelium and cartilage, forming a continuous muscular ring encircling the airways. In healthy lungs, how much it contracts basically determines the size of the airway opening.

How this airway muscle behaves results primarily from the balance between the autonomic nervous systems two opposing parts. (This balance also controls the mucus-producing glands.) The autonomic nervous system continuously controls our automatic bodily functions, constantly adjusting them to maintain our interior environment with a minimum of fluctuation. Its two divisionsthe sympathetic and the parasympatheticusually work in opposite directions.

The nervous system acts via mediators, chemicals that it produces and releases. Mediators "talk" to muscles via receptors, specialized areas that all muscles have on their surface. Each type of mediator has its own type of receptor. That receptor can't be activated by any other mediator. Because muscles have more than one type of receptor, they can respond to more than one kind of mediator.

When airway muscle receptors sense the presence of the specific chemical mediators they respond to, the machinery that contracts the muscle is either started or turned off, depending on the mediator. Two types of airway muscle receptors cause the muscle to contract. One is activated by acetylcholine (cholinergic receptors) released by the parasympathetic nervous system. The other type recognizes adrenaline (adrenergic alpha-receptors) released primarily by the adrenal glands (part of the sympathetic nervous system). Receptors that turn off the contraction machinery, allowing the airway muscle to relax, also recognize adrenaline but are called adrenergic beta-receptors. Adrenalines relaxing (or bronchodilating) effect predominates because there are many more beta-receptors than alpha-receptors.

So when the healthy respiratory system is at restmeaning the absence of conditions (such as exercise) temporarily increasing the body's demands on itthe opening size of each airway is the end result of the balance between the cholinergic and adrenergic alpha-receptors on one side, and the adrenergic beta-receptors on the other.

But when the respiratory system is affected by COPD, pathological factors further influence the airway opening. In emphysema, the smallest airways shut down during expiration. In chronic bronchitis, the mucus glands can more than quadruple in size, substantially narrowing medium and large airways with their sticky secretions. And goblet cells begin appearing in even the smallest airways. Because of their small size, these airways are very vulnerable to complete blockage by mucus. Once that blockage occurs, air in the surrounding alveoli remains trapped there. These air sacs become distended, and the walls eventually rupture. This is why chronic bronchitis and emphysema are typically found together. If emphysema has not already developed separately from the chronic bronchitis, it will eventually be produced by it.