HOW SWALLOWING WORKS
Before we look at how things can go wrong, let’s look briefly at normal swallowing. The purpose of swallowing is to bring food, liquid,
or medication safely from the mouth to the stomach.
Keep in mind that swallowing does not occur in isolation. Throughout the process, signals are sent continuously to and from the nervous system to coordinate swallowing with breathing.
Swallowing is more than a one-step gulp-and-gone. It’s a process. You can use the word “POPE” to remember its four phases:
- Preparatory Phase—Forming a Ball of Food
The Preparatory Phase begins even before food gets to your mouth. Your sense of smell—even the anticipation of food—can make your mouth water.
When you take a bite of food, a remarkable dance begins. Your jaws, teeth, cheeks, and tongue work together to make the food into more or less of a ball.
This ball of food is called a bolus after the Latin word for “lump.”
Your lips close tightly so food doesn’t escape from your mouth. Your teeth (or dentures) grind the food into smaller bits. Saliva flows from under and around your tongue. It moistens the food, gathering with it flaky food bits or nearby sugar particles.
Your tongue mixes and churns the food. It directs the bolus to one side then the other, where your teeth grind it into a dough-like paste. Your jaws move not only up-and-down but in a rotary manner that max- imizes grinding.
In addition to physically reducing the mass of solid food, chewing enhances flavor. Juices and other flavorful food elements leak out to contact your taste buds. Enzymes in saliva get to work right away. They start to break down the food while it’s still in your mouth.
Your sense of smell gets into the act once again. Aromas pass from throat to nasal cavity where smell receptors enhance the taste of the food and stimulate further saliva flow.
Coordination Required. Just as skilled dancers don’t step on each other’s toes, we (usually) do not chew on our tongue.
Why not? The tongue sends continuous signals to the brain telling it where the tongue is and what it’s doing. Your gums, cheeks, lips, and palate also send information to the brain so you can chew without worrying about chomping on your tongue.
Likewise, breathing and swallowing must work closely together for swallowing to be safe.
Ordinarily, breathing can continue during the Preparatory Phase. But when a medical or neuro- logic disorder interferes with the balance between
the two, breathing and swallowing can become dangerously out of synch.
And, if—under any circumstance—you stir things up by attempting to swallow while distracted, talking, chewing, laughing, and breathing, you’re asking for trouble. (See Chapters 4 and 5, which discuss choking and aspiration.)
2.Oral Phase—Transport From Mouth to Throat
Your tongue is now loaded with food. It’s like a bucket without a handle. The tongue squeezes the ball of food against the hard palate. This carries the bolus from the front to the back of the mouth, where the pharynx, or throat, begins.
When you open your mouth in front of a mirror, the very back part of what you see is your throat. Actually it is the oral portion of the pharynx, the oro- pharynx. Just above is the nasopharynx and below is the hypopharynx.
The exquisite coordination seen in the Preparatory Phase continues.
- Your lips remain closed to prevent food or liquid from escaping from the front of your mouth.
- Your cheeks stiffen and press against your gums and teeth, which keeps food from falling into pockets between cheek and gum.
- Your tongue presses against the bony roof of your mouth to form a muscular chute that directs food to the throat.
- These structures work together during the Oral Phase to generate air pressure that moves the bolus along.
The entire Oral Phase generally takes about one second.
3. Pharyngeal Phase
The Pharyngeal Phase includes the swallowing reflex. It is activated automatically by food, liquid (including saliva), or medication reaching the pharynx. Its entrance is marked by the fleshy pillars that frame the tonsils.
This reflex, mediated through the brainstem, coordinates the many muscles required to push the food down into the esophagus while acting as a safety net to keep it from going where it does not belong: the respiratory tract.
Six hundred or so swallows a day. That’s a big job. Without conscious direction, the swallowing reflex weaves together many elements so you can swallow safely.
Here’s what happens during the Pharyngeal Phase:
- Breathing stops briefly.
- The tongue and soft palate seal off the roof of the mouth so food doesn’t escape through the nose.
- The hinge-like epiglottis closes over the entrance to the larynx (voice box), which helps keep food, liquid, or pills from entering the airway.
- The larynx moves forward and upward, opening the upper esophagus and further preventing food from getting to the lungs.
- Within the larynx, the vocal cords come together to keep food or liquid from entering the trachea (windpipe).
- Muscles of the pharynx and tongue contract to move the bolus to and through the upper part of the esophagus.
- The upper esophageal sphincter (UES), a ring- shaped muscle at the uppermost esophagus, relaxes to allow food to pass from throat to esophagus.
Because many different foods and liquids of different amounts and textures pass from your mouth to your stomach, it will come as no surprise that your swallowing apparatus is constantly adjusting to these ever-changing demands for safe swallowing.
The entire Pharyngeal Phase normally takes a second or less. Breathing resumes after the swallow.
A word or two about gagging and vomiting. While the swallowing reflex invites food in, gagging and
Esophageal Sphincters. Food enters the esophagus through the upper esophageal sphincter (UES) and enters the stomach through the lower esophageal sphincter (LES).
vomiting reflexively send material out. A brisk gag does not mean a person has a normal swallow. And, as we will discuss in a later chapter, gagging does not mean choking.
4. Esophageal Phase—Getting to the Stomach
Once the bolus gets past the upper esophageal sphincter (UES), this muscular structure contracts tightly to guard against backflow.
The bolus works its way through the esophagus, moved along by orderly waves of contraction (called peristalsis) to reach the lower esophageal sphincter (LES). Like its upper counterpart, the lower esophageal sphincter is contracted (closed) at rest. So it must relax to let food pass into the stom- ach. The entire trip through the esophagus normally takes from eight to twenty seconds.
Putting It All Together
Let’s go for a swallowing walk-through. Tear off a piece of warm, crusty, freshly-baked bread. Hold it in front of you and look at it. Your nose picks up the pleasant smell and saliva flows as you prepare for food to enter your mouth.
The bread passes your lips, and you bite off a hunk. Your tongue greets it and mixes it with saliva, forming a ball of food as it gathers up bits of flaky crust so you don’t inhale them. It moves the food to the side where your teeth begin to chew.
Your jaws move up and down, side to side. This rotary action grinds the food until it becomes pasty. You continue to breathe while you chew. Your tongue gathers the bolus into a central valley in your mouth, then presses it against the bony palate. Your lips continue their tight seal and your cheeks stiffen further, pressing against your teeth and gums.
The food is brought to the back of your mouth where muscles of the throat and palate block escape through your nose. The bolus enters the pharynx. Now comes the moment of truth—the swallow itself.
Like a track athlete getting ready for a jump, you pause, gather energy, and hold your breath. Your tongue and throat muscles forcefully contract. Your Adam’s apple rises as your larynx elevates and squeezes tightly shut, closing off the airway. You swallow. The food leaves your throat and begins its downward descent. You release air through your nose as you resume breathing.
Your tongue searches out remnants of food that may have been left behind between gum and cheek or stuck to the roof of your mouth. You swallow one or several times more to finish things off, perhaps clearing your throat between swallows. The final swallow is little more than saliva.
You can turn this thought exercise into a real-life experience by getting a piece of bread or chunk of apple and observing yourself as you chew and swallow.
See if you can follow the various phases. Notice especially how many roles the tongue plays. Making food into a ball. Positioning it for chewing. Bringing it to the back of the mouth. Pushing against the hard palate and down. Cleaning up afterwards. Did you notice how breathing stops during the swallow itself?
We think you’ll agree—there’s a lot going on here. Swallowing is a complicated process, something we generally take for granted when all is well.
But all is not always well. In the next chapter, keeping the phases of swallowing in mind, we look at how swallowing can break down.