So why do we have bronchiole muscles anyway?

In a recent post at MyAsthmaCentral.com I provide the answer about asthma you may never have thought of before, and neither until someone post it to me in an email.


Asthma: The appendix of the Lungs

by Rick Frea Monday, July 12, 2010 @MyAsthmaCentral.com

It's time for a change of pace and some fun trivia.

An asthma attack is caused when exposure to your asthma triggers causes the smooth muscles lining the air passages of your lungs to spasm and constrict. So why then do we have smooth muscles in our lungs anyway?

This was a question posed to me recently, and even though I've studied asthma extensively over the years, had never thought of this basic question. Thus, the answer eluded me until I did a little research.

For starters, here are some basic lung definitions. First, the air passages in your lungs are called bronchioles, and the smooth muscles lining them are bronchial muscles. I'm sure you knew that, I'm just saying.

When these muscles spasm it's called bronchospasm. This causes the bronchioles to constrict, which is duly called bronchoconstriction. (To view a good picture of this click here.)

Medicines like Albuterol and Xopenex are bronchodilators, and thus relax your bronchioles by stopping them from spasming.

Logic would have it that if we didn't have bronchial muscles, asthma wouldn't exist. So, then, to repeat our question: why did God line our lungs with muscles in the first place? It can't be just to have the ability to produce bronchospasm, can it?

The best article I found was in The American Journal of Respiratory Care Medicine published January 23, 2004 titled, "Airway Smooth Muscle: The Appendix of the Lungs," by Wayne Mitzner.

According to Mitzner, it has been "further suggested that perhaps airway smooth muscle was a vestigial remnant of its common embryologic origin with the gastrointestinal system, having no modern function."

In the gastrointestinal system smooth muscles lining your intestines are used to cause peristalsis, which is, according to Dictionary.com, "a progressive wave of relaxation of these muscles in order to create a wave that moves food through the intestines.

That in mind, Mitzner notes there have been some theories as to the use of bronchial smooth muscles:

1. To assist exhalation: This theory postulates contraction of these smooth muscles might assist with exhalation. Yet Mitzner concludes that, "However, the magnitude of such an effect, if it exists, must be quite small and of little physiologic significance. Gas exchange in the lung can be performed quite well with fully relaxed airway smooth muscle."

2. To aid expectoration (spitting up phlegm): Again, peristalsis might help move secretions from the lower airway to the upper airway to help you cough it up. Yet, evidence shows that cilia (small hair like structures in your lungs) work like an escalator to bring up mucus, and then you cough it up. There is no evidence that smooth muscles have any use in coughing up junk from your lungs.

3. Protecting the lungs: Believe it or not, your lungs are supposed to be sterile. Contracting of bronchial smooth muscle might prevent substances from entering the lungs. However, to maintain life, an organism must breath. And, to draw in air, lung muscles must be relaxed. If contracted for an extended time to "protect the lungs" one would not ventilate, and would therefore die. Thus, this theory holds little merit.

4. Other: Okay, so there have been many too difficult to explain reasons for bronchial smooth muscle. None, however, have been substantiated.

So, Mitzner writes that since most of these theories have pretty much been disproved, or at least remain as unpopular as asthma itself, there appears to be no modern reason at all for bronchial smooth muscle.

In a sense, bronchial smooth muscle is as useless as the appendix, hence his title: "Appendix of the Lungs." Bronchial muscles, like the appendix, may once upon a time have had a function, but all they do now is cause trouble for us humans.

Body hair serves no purpose, although at least body hair doesn't cause sickness.

Bronchial muscles are therefore like the appendix, or maybe even wisdom teeth, which also serve no modern purpose other than to cause us to bite our cheeks unless we have a dentist yank them out, which may also cause grief.

So, if you've ever wondered why there's smooth muscle in your lungs other than to cause asthma, now you know the rest of the story.