May 1, 2007

SWALLOWING: It's Not As Easy As it Looks


By Dr. Howard Glicksman

The ability to safely swallow food and drink without letting it go into the lungs is of vital importance for survival.  After all, it really is of no use to have a sophisticated gastro-intestinal system that can digest and absorb the nutrients to keep us alive if one cannot safely get the food and water there in the first place.

If you look at the accompanying figure, which represents a side view of the head and neck, you will immediately see that if food and drink are passed by the tongue into the pharynx on their way to the esophagus, they are very likely to end up going down the trachea to the lungs below instead. 

(taken from www.dysphagiaonline.com)

Last time we highlighted the respiratory system which provides the body with oxygen (O2) while getting rid of deadly carbon dioxide (CO2).  This system depends on the ability for these gases (O2 & CO2) to travel to and from the alveoli for efficient gas exchange to take place.  Having a Big Mac, super-sized fries, and a large Coke, or alternatively, a filet mignon, baby veggies and Merlot, in the way, will result in problems breathing.  Food and drink located in the lungs, no matter its taste and expense will result in acute respiratory failure and is an equal opportunity killer.

So how is it that we are able to safely swallow food and drink to provide ourselves with nutrients and water for our survival without compromising the respiratory system?

Gross Anatomy is only the Beginning
The presence of the structures for swallowing represents only a static or still picture and is wholly insufficient to explain the development of swallowing.  It’s like looking at the parts of a car without considering how they came together and by what mechanism are able to perform the function of locomotion.   The human genome may provide the information necessary for the body to make the parts we see in the diagram above but it has yet to be determined how the body knows where the parts should go and how they developed the coordinated action that provides for the protection of the airway during swallowing.  Lower order life forms having a similar system does not by definition explain how they got them in the first place.   

The Phases of Swallowing
Most authors talk about there being four phases of swallowing.  The first two are voluntary, meaning that we have conscious control over them. They are the initial phase where the food and drink are placed in the mouth through the lips, the jaws close and saliva begins to be secreted and mixed with it.  The second of these voluntary phases is the oral phase where the food and drink is processed by chewing and mixing with saliva and the tongue begins to prepare it in a size that can be swallowed (bolus). This bolus is then moved to the back of the tongue as the front part of the tongue lifts to the hard palate and then moves backwards to push the bolus into the pharynx  to be swallowed.

The third and fourth phases of swallowing are involuntary, in that they occur automatically and without our control once they are initiated.  The last phase, known as the esophageal phase, takes place when the food enters the esophagus and by peristalsis, that is by worm-like muscle wave contraction, is carried down to the stomach.  The third or pharyngeal phase, and the phase that this article will concentrate on, is the one where the bolus passes from the back of the pharynx and is deposited safely in the esophagus without any of its contents going down the trachea, thereby preventing aspiration.

Phase #3
Before I begin to explain what happens in the pharyngeal phase and how the body is able to protect its airway from receiving deadly food and drink, I’d like you to look at the diagram of the anatomy again and ask yourself what would have to take place for this to be achieved.  Then gently hold the front part of your neck with your thumb and fingers just below the jaw and swallow a few times to see what happens. 

Phase #3A
When the bolus of food and drink touches the pharynx it triggers sensory receptors that go to the swallowing center of the brain and sets off the involuntary actions of swallowing while at the same time inhibiting the respiratory center so we don’t breathe in at the same time thereby helping to prevent aspiration.

Phase #3B
It is evident that when a bolus of food and drink is pushed by the tongue into the pharynx that this action is likely to send some of it, not only downwards, but also upwards into the nasal part of the pharynx (nasopharynx).  Indeed,  I’m sure all of us at one time or another have experienced this very embarrassing situation when we are beginning to swallow and have been made to laugh unexpectedly. 

As swallowing begins several different muscles immediately contract and begin to move the soft palate and the back part of the upper pharynx closer together to close off the passageway to the nose.  At this point if you happen to laugh, in exerting too much upward pressure before the nasopharynx can be closed off, some of what you are trying to swallow may sneak up there and come out your nose.

Phase #3C
Once the bolus has safely been directed downward toward the esophagus the constrictor muscles of the pharynx contract (like a boa constrictor).  There are three sequentially positioned constrictor muscles from top to bottom, called the superior, middle, and inferior constrictors, which push the bolus down toward the entrance to the esophagus.  This coordinated constricting action is like the peristalsis that takes place throughout the gastrointestinal tract.

Phase #3D
Now comes the tricky part.  The bolus has been prevented from going north into the nose by muscular closure of the nasopharynx coordinated by the swallowing center in the brain.  Now it is hurtling its way south by peristaltic action toward the esophagus.  If you take another look at the diagram above, you’ll see that the bolus can go in either of two directions. 

It can go toward the muscular opening of the esophagus and/or it can go down through the larynx (voice box) on its way through the trachea to the lungs.  Aspiration does not automatically cause problems since the body is able to handle small amounts of it.  But recurrent and significant aspiration of food and drink into the lungs is likely to result in disease, namely pneumonia: dysfunction; namely respiratory failure: and death; from asphyxiation. 

There are basically three separate actions that take place at this point in order to protect the airway from aspiration.  First; muscles contract to allow the larynx, which is the gateway to the lungs, to close at various levels.  Second; as you can note by feeling your neck during swallowing, other muscles move the larynx not only up, but also forward, in order to hide it under the floor of the mouth and the base of the tongue while being protected by the epiglottis. This action combined with other muscular activity  results in the third event;  the upper esophagus  opening  up to allow the bolus to enter on its way to providing nutrition and water for your life.

Of course, if one hasn’t chewed properly and somehow allows too big of a bolus to enter this region, it may totally block up the passageway and get stuck.  This typically happens to people in a restaurant where they usually are too busy talking, and not watching how they’re eating, sometimes aggravated by alcohol use, and they inadvertently obstruct their airway.  Without immediate attention, such as the Heimlich maneuver, these people are at risk of immediately choking to death.  

What Can Go Wrong?
Now that you understand how swallowing works and how important it is to protect the airway from aspiration of foreign bodies into the lungs, I suspect you may be able to come up with some ideas of how things can malfunction.  Patients with problems experienced in this 3rd phase of swallowing comprise a large portion of people who suffer from what is called oropharyngeal dysphagia: difficulty in swallowing related to the oral and pharyngeal phases. 

They usually eat very slowly, cough after swallowing, have problems managing their own secretions, are always clearing their throat, are prone to choking, have weight loss because of diminished oral intake, and are prone to recurrent bronchitis and  pneumonia.

Reviewing the mechanics involved in swallowing that adequately provides for airway protection one can see where some problems can arise; 

Who Gets These Problems?
Anyone who has had head and neck surgery or radiation treatment to this region, usually related to cancer, runs the risk of affecting the structures of swallowing resulting in some sort of oropharyngeal dysphagia.  But the largest portion of people who develop this problem are those who have underlying neuromuscular disorders that affect the brain, the peripheral nerves, and muscle function and coordination.  Conditions such as:

Reflection
It is common today to encounter people who would look at the anatomy of the head and neck, see the high risk for aspiration, and would conclude that it couldn’t have been intelligently designed because that’s not the way they would have done it.  Of course, one has to take into account the importance of the faculty of smell and its direct connection to the faculty of taste to see why the parts for breathing and eating are so close together. But then came the need to preserve respiratory function while still allowing for nutrition and hydration through the oral cavity.  You now understand how this is achieved in our bodies 

Now take a look at the diagram again. Hold your hand over your voice box, and then swallow. Feel the motion of the structures brought on by the involuntary action of about 25 different pairs of muscles directed by the brain’s swallow center, enabling you to be able to survive in the world.  And it all takes place in less than a second !! 

It’s been said that a picture is worth a thousand words and that beauty is in the mind of the beholder.  Well, I’d like to leave you with a link that shows a video of swallowing.

As you watch it, consider what you’ve just learned about what’s really going on every time you swallow, about 1,000 times a day. That’s 1,000 chances per day for aspiration causing disease, dysfunction and death to take place, and the body handles it beautifully. 

 www.linkstudio.info/images/portfolio/medani/Swallow.swf

Next time we’ll be looking at gastrointestinal function…see you then.

Dr.G.
drhglicksman@yahoo.com

Howard Glicksman M.D. graduated from the University of Toronto in 1978. He practiced primary care medicine for almost 25 yrs in Oakville, Ontario and Spring Hill, Florida. He now practices palliative medicine for a Hospice organization in his community. He has a special interest in how the ethos of our culture has been influenced by modern science’s understanding and promotion of what it means to be a human being. Comments and questions about this column or any of the previous ones are welcome at drhglicksman@yahoo.com

Copyright 2007 Dr. Howard Glicksman. All rights reserved. International copyright secured.
File Date: 15.01.07