Anatomy and Physiology 2e · The Digestive System
The Mouth, Pharynx, and Esophagus
On this page 9 sections
In 30 seconds
Digestion begins in the mouth. The oral cavity — bounded by the lips and cheeks, floored by the tongue, and roofed by the hard and soft palate — receives food, and the teeth and tongue reduce it to a soft, swallowable ball, the Bolus A soft ball of chewed, moistened food Full entry →. Saliva from three pairs of salivary glands moistens the food and begins starch digestion, starting the chemical side of digestion.
From the mouth, the bolus enters the pharynx, a muscular passage shared with the respiratory system. Swallowing (Deglutition Swallowing Full entry →) is a coordinated reflex that keeps food out of the airway and moves it into the esophagus, which delivers it to the stomach. This first segment performs no absorption; its job is preparation and safe transport.
Why this matters
- Airway safety. Swallowing sits at the crossroads of the digestive and respiratory tracts. When the reflex fails — as it can after stroke, with aging, or during anesthesia — food or liquid can enter the airway (aspiration) and cause pneumonia. Health-care workers routinely assess swallowing before allowing patients to eat.
- First steps of digestion. Salivary amylase Enzyme in saliva that begins starch digestion Full entry → begins carbohydrate digestion in the mouth; dental problems can therefore affect nutrition well before food reaches the stomach.
- Extremely common conditions. Gastroesophageal reflux disease (GERD Gastroesophageal reflux disease Full entry →) involves the lower esophageal sphincter, and swallowing difficulties (dysphagia) are a frequent complaint in older adults and many medical conditions.
- Exam favorite. The three phases of swallowing and the difference between the upper and lower esophageal sphincters are classic test questions.
The college version
Core Concepts
The oral cavity and its structures
- Lips and cheeks — hold food in place during chewing; the cheeks contain the buccinator muscles.
- Palate — the hard palate (bone) forms the front roof; the soft palate (muscle) forms the back roof and ends in the uvula. The soft palate rises during swallowing to close off the nasopharynx so food does not go up the nose.
- Tongue — a mass of skeletal muscle that manipulates food, shapes the bolus, and pushes it backward to start the swallow; its surface carries taste buds.
- Teeth — specialized for mechanical digestion. Incisors cut, canines tear, premolars crush, and molars grind. Humans typically have 20 deciduous and 32 permanent teeth — commonly taught reference counts.
Saliva and the salivary glands
Three pairs of glands — parotid (near the ear), submandibular (under the jaw), and sublingual (under the tongue) — plus many small buccal glands produce saliva. Saliva is mostly water and contains:
- Salivary amylase — begins starch digestion.
- Mucus (mucin) — lubricates food and protects the lining.
- Lysozyme and immunoglobulin A (IgA) — antimicrobial defenses.
- Bicarbonate — buffers acids from food.
Saliva increases when we see, smell, chew, or even think about food (a feedforward reflex) and decreases with fear or dehydration — hence the "dry mouth" of a nervous speaker.
The pharynx: a shared passageway
The pharynx has three regions: nasopharynx (behind the nose), oropharynx (behind the mouth), and laryngopharynx (behind the larynx). Food passes through the oropharynx and laryngopharynx; the nasopharynx is normally a respiratory-only space. The Epiglottis Cartilage flap that covers the glottis during swallowing Full entry →, a flap of cartilage at the top of the larynx, tips backward during swallowing to deflect the bolus away from the airway opening (the glottis).
Swallowing: three phases
- Buccal phase (voluntary) — the tongue pushes the bolus against the hard palate and back into the oropharynx.
- Pharyngeal phase (reflex) — the soft palate seals the nasopharynx, the epiglottis covers the glottis, breathing briefly pauses, and Peristalsis Wave-like muscular contractions Full entry → of the pharyngeal muscles sweeps the bolus into the esophagus. This phase is involuntary.
- Esophageal phase (reflex) — peristaltic waves drive the bolus down the esophagus to the stomach.
The esophagus: a muscular expressway
The esophagus is a collapsible muscular tube about 25 cm long (a commonly cited approximation) connecting the pharynx to the stomach. Its wall resembles the rest of the tract, but the muscularis changes from skeletal muscle in the upper third to smooth muscle in the lower third — a transition from voluntary to involuntary control. Two sphincters guard its ends:
- Upper esophageal sphincter (UES Sphincter at the top of the esophagus Full entry →) — normally closed, opens to admit the bolus; keeps air from entering during breathing.
- Lower esophageal sphincter (LES Sphincter at the bottom of the esophagus Full entry →) — normally closed, relaxes to let food into the stomach and prevents stomach contents from refluxing upward. When the LES is weak or relaxes inappropriately, acidic stomach contents irritate the esophagus — the basis of heartburn and GERD.
The esophagus secretes mucus but no digestive enzymes and absorbs nothing; its entire job is transport.
Common Confusions
| Do not confuse | With | Difference |
|---|---|---|
| Epiglottis | Glottis | The glottis is the opening of the airway; the epiglottis is the flap that covers it during swallowing |
| Pharynx | Larynx | Pharynx is the shared food/air passage (throat); larynx is the voice box below it |
| Esophagus | Trachea | Esophagus carries food (behind the trachea); trachea carries air |
| UES | LES | UES is at the top (guards air entry); LES is at the bottom (prevents reflux) |
| Bolus | Chyme | Bolus is the mouth-stage ball; chyme is the semi-liquid mixture in the stomach |
| Salivary amylase | Pancreatic amylase | Salivary amylase works in the mouth; pancreatic amylase continues starch digestion in the small intestine |
| Nasopharynx | Oropharynx | Nasopharynx is respiratory-only; food passes through the oropharynx and laryngopharynx |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Eating is like sending a package down a slide. Your teeth chop the food into small bits, spit makes it slippery, and your tongue packs it into a soft ball. Then a trapdoor at the back of your throat closes over your windpipe — like a lid — so the package goes down the food slide instead of into your lungs. Muscles squeeze the slide, pushing the ball down to your stomach.
Worked example
Follow a sip of water and you will see the safety system in action:
- You close your lips, and the water enters the mouth; your tongue cups it against the hard palate.
- In the buccal phase, the tongue pushes the water backward into the oropharynx.
- The moment water touches the sensitive back of the throat, the pharyngeal reflex fires: the soft palate rises to seal the nasopharynx, the epiglottis folds over the glottis, and your breathing pauses for a fraction of a second while peristalsis sweeps the water past the larynx.
- The water passes through the relaxed upper esophageal sphincter into the esophagus, where peristaltic waves — helped by gravity — carry it down.
- The lower esophageal sphincter relaxes, the water enters the stomach, and the LES closes again.
Now imagine the same reflex after a stroke: if the epiglottis does not seal in time, liquid slips toward the glottis, triggering coughing or, worse, silent aspiration. That is why swallowing assessments are standard before stroke survivors eat or drink.
Key takeaways
- Oral cavity structures: lips/cheeks, hard and soft palate, tongue, teeth; soft palate and epiglottis protect the airway and nasal passages.
- Saliva: water + amylase (starch), mucin (lubrication), lysozyme/IgA (defense), bicarbonate (buffer); three main gland pairs: parotid, submandibular, sublingual.
- Teeth types by function: incisors cut, canines tear, premolars crush, molars grind; 20 deciduous, 32 permanent (commonly taught counts).
- Swallowing phases: buccal (voluntary), pharyngeal (reflex), esophageal (reflex) — the pharyngeal phase protects the airway.
- Esophagus: no digestion or absorption; UES and LES guard the ends; LES dysfunction underlies GERD.
- Pharynx regions: nasopharynx, oropharynx, laryngopharynx — food uses the lower two.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
Name the three pairs of salivary glands and one key component of saliva.
Show answer
Parotid, submandibular, and sublingual glands. Saliva contains water plus salivary amylase, mucin, lysozyme/IgA, and bicarbonate (any one component is an acceptable answer).
List the three phases of swallowing and state which one is voluntary.
Show answer
Buccal (voluntary), pharyngeal (reflex), esophageal (reflex). Only the buccal phase is voluntary.
What structures protect the nasopharynx and the larynx during swallowing?
Show answer
The soft palate (and uvula) seals the nasopharynx; the epiglottis covers the glottis.
Which sphincter keeps stomach contents from refluxing into the esophagus, and what happens when it fails?
Show answer
The lower esophageal sphincter (LES). When it is weak or relaxes inappropriately, acidic stomach contents reflux into the esophagus, causing heartburn and, when persistent, GERD.
Where in the esophagus is skeletal muscle found, and where is smooth muscle found?
Show answer
Skeletal muscle in the upper third, smooth muscle in the lower third (with a mixed middle zone).
What are the four tooth types, and what does each do?
Show answer
Incisors cut, canines tear, premolars crush, molars grind.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Bolus
- A soft ball of chewed, moistened food
- Mastication
- Chewing
- Salivary amylase
- Enzyme in saliva that begins starch digestion
- Deglutition
- Swallowing
- Epiglottis
- Cartilage flap that covers the glottis during swallowing
- UES
- Sphincter at the top of the esophagus
- LES
- Sphincter at the bottom of the esophagus
- Peristalsis
- Wave-like muscular contractions
- GERD
- Gastroesophageal reflux disease
Sources & references
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