Deglutition: the Biomechanics of Drinking

deglutition

Drinking requires very little thinking, but it is one of the most complex activities the human body performs. To swallow a sip of water, muscles, nerves, and skin need to choreograph a sequence of movements that allow the body to hydrate without drowning. Swallowing a liquid drives it into the pharynx, right next to the windpipe. Nobody wants fluid in the lungs, so it is important to appreciate the biomechanics of drinking. 

Eating and drinking, while similar, are not exactly the same. Certain parts of the swallowing process, technically called deglutition, are dependent on the viscosity of the food being swallowed. With liquids, there is a higher risk that the bolus, the mass of swallowed food, goes down the wrong tube. The liquids move faster from the mouth into the pharynx giving the body less time to prepare itself. Food, on the other hand, needs to be chewed and collected. To put it scientifically,  “Studies of bolus movement have shown that increased viscosity results in slower oropharyngeal transit times.” (Hanson)

The process of drinking is often conceptualized in the so-called Four Stage Model. The name is straightforward and includes the phases Oral Preparatory, Propulsive, Pharyngeal, and Esophageal. Each is fairly distinct, although the propulsive and pharyngeal can run into each other.

oral diagram
A diagram of the upper respiratory tract. from Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436.

Eating, however, does not abide by the same four stages. Swallowing solid food is better described by the Process Model. Either way, these are incredibly complex anatomical dances. The process involves over 30 nerves and muscles in the head and neck and is coordinated by both intentional impulses and nervous reflexes. (Matsuo) So, how do we drink?

To start, drinking and the parts of our anatomy that we use for drinking, evolve over our lifespans. An infant’s pallet, pharynx, and hyoid bone are all different from those of  an adult. This makes sense. Infant’s only drink milk via direct suction driven by a process called “primitive rhythmicity” (Chi-Fishman). Straws and cups, the implements of adult drinking, are foreign to our biology.

At first, we drink through a primitive instinct, but as our bodies and minds develop, we learn how to use suction and tools to help us. Recall your earliest memory of drinking from a cup. Look at a child’s transition from nipple, to sippy, to open brims. A first experience with a normal cup can be a messy learning experience. 

After we master the art of taking a sip, we can enter the realm of the Four Stage Model. During the first stage of drinking, the Oral Preparatory Stage, the liquid is held in the mouth by sealing off any exit. In the front of the mouth, this seal is created by the tip of the tongue and the roof of the mouth, the hard palate, just behind the front teeth. In the back of the mouth, this seal is formed by the back of the tongue (the tongue is very long) pushing up against the soft palate. The beverage can’t leak out of the lips, and it also can’t surprise the unsuspecting windpipe. Older adults can have a weaker seal against the soft palate, increasing the risk of choking. (Matsuo)

sword swallowing diagram
Our normal swallowing mechanisms can be subverted when we cough or laugh while drinking. We can also train ourselves to resist the reflexive muscle impulses that help us drink without choking. from Hopkins, Albert (1897) Magic; stage illusions and scientific diversions, including trick photography, London: Sampson Low, Marston and Company, p. 158

The Oral Propulsive Stage is where the action of drinking really begins. At this point, the tongue acts like someone squeezing out toothpaste. Where the tongue was holding the liquid, it now presses up against the hard palate in a wave-like manner, beginning at the front of the mouth and squeezing to the back. At the same time, the back of the tongue drops away from the soft palate, allowing the moving liquid to enter the pharynx. 

This is where eating and drinking differ. Eating does not seal off the back of the mouth entirely. This is because chewed food needs to accumulate into a bolus away from unchewed food. As we chew, the space between the tongue and soft palate moves in coordination with the jaw. This allows ready to swallow stuff to collect in the pharynx and enriches the experience by allowing air from the mouth to travel up the backdoor of our noses, the nasal cavity, to assist in smelling and tasting our food. (Matsuo) Drinking liquids does not require chewing and threatens to flood the pharynx uncontrollably. The liquid must be sealed in until swallowing occurs. 

In drinking, the Oral Propulsive Stage, which empties the mouth into the pharynx, occurs at the same time as the Pharyngeal Stage. This stage is vital in protecting our breathing tubes from solids and liquids. During this stage, the soft palate covers the nasopharynx, which is the airway leading to the nose. This prevents chocolate milk from coming out of the nose. At the same time, the larynx or voice box shifts forward and the epiglottis closes backwards. This not only allows for the bolus of food or liquid to move lower in the pharynx, but it also takes the windpipe out of harm’s way. 

breathing while swallowing
A diagram demonstrating breath and swallows. Note that when multiple swallows occur on the bottom chart, the top chart has a flat line indicating that breath is being held. From Smith, et al.

The final stage, the Esophageal Stage, is where the drama ends. In its resting state, the upper esophageal sphincter, which closes the esophagus off from the pharynx, is sealed by tonic muscle contraction. Once the sphincter opens during swallowing, the bolus of food or liquid enters the esophagus. The esophagus relaxes to make some space and then moves the bolus via peristalsis–a reflexive muscle action that pushes food along the tube. The drink hits the stomach and swallowing is complete. 

Take a breath of relief. Wait, breathe out first. Smith et al conclude in a 1989 study that swallowing is almost exclusively followed by an exhale. In fact, it typically occurs in the middle of an exhale–try to pay attention to your own breath as you eat. Swallowing will always over-power breathing. Not only does the process block our ability to inhale, the brain stem also stops the impulse to breath. (Matsuo)

In Smith’s study, they hand fed subjects donuts and water to test the interaction between deglutition and respiration. The overwhelming conclusion was that swallowing disrupts the consistency of breathing–drinking, even more so. We hold our breaths while we drink and swallow multiple times. There is no equivalent to chugging for eating. 

Drinking can be perfunctory and forgetful, that is until we cough or laugh mid sip. Milk or water then floods the sinus, chokes up the trachea, and comes screaming out in embarrassing spits. The beauty of the biomechanics must be appreciated. The head has the tough task of transporting air, food, and water to the torso through the neck. It is best that the brain knows how to keep them all separate. 

Sources Cited:

Chi-Fishman, Gloria, and Barbara C. Sonies. “Motor strategy in rapid sequential swallowing: new insights.” Journal of Speech, Language, and Hearing Research 43.6 (2000): 1481-1492.

Hanson, Ben, Mark T. O’Leary, and Christina H. Smith. “The effect of saliva on the viscosity of thickened drinks.” Dysphagia 27.1 (2012): 10-19.

Matsuo, Koichiro, and Jeffrey B. Palmer. “Anatomy and physiology of feeding and swallowing: normal and abnormal.” Physical medicine and rehabilitation clinics of North America 19.4 (2008): 691-707.

Smith, John, et al. “Coordination of eating, drinking and breathing in adults.” Chest 96.3 (1989): 578-582.

van den Engel-Hoek, Lenie, et al. “Biomechanical events of swallowing are determined more by bolus consistency than by age or gender.” Physiology & behavior 106.2 (2012): 285-290.

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