12.4: From the Mouth to the Stomach
- Page ID
- 121667
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)The mouth, pharynx, esophagus, and stomach work together to begin digestion by mechanically and chemically processing food, safely directing it toward the stomach, and mixing it with secretions that transform it into chyme for the small intestine.
- Identify the major structures of the mouth, salivary glands, teeth, pharynx, and esophagus, and explain how they begin digestion and move a bolus safely from mouth to stomach.
- Label the main regions and curvatures of the stomach, and summarize how stomach structure, secretory cells, and protective mechanisms support mechanical and chemical digestion.
Mouth, Pharynx, and Esophagus
Let's start with a quick overview of the anatomy and functions of the three main organs of the upper alimentary canal — the mouth, pharynx, and esophagus — as well as two associated accessory organs — the salivary glands, and teeth.
The Mouth (Oral Cavity)
The cheeks, lips, tongue, and palate frame the mouth, also called the oral cavity. The lips mark the entrance to the mouth and are highly vascular, which gives them their reddish color. A small fold of tissue, the labial frenulum, connects each lip to the gums. The cheeks form the sidewalls of the oral cavity and contain the buccinator muscles, which help keep food in your mouth while you chew and speak.
The space between the lips/cheeks and the gums/teeth is called the oral vestibule. Farther inside is the oral cavity proper, which extends back to the opening to the throat (oropharynx).
The palate forms the roof of the mouth and separates the oral and nasal cavities. The front portion, the hard palate, is made of bone and provides a firm surface for the tongue during chewing. The back portion, the soft palate, is made of skeletal muscle and can move during swallowing, yawning, or speaking. Hanging from the soft palate is the uvula, a small projection that helps prevent food and liquids from entering the nasal cavity during swallowing.
On either side of the uvula are two muscular arches. Between them lie the palatine tonsils, part of the immune system that helps protect the pharynx. Additional lymphoid tissues, the lingual tonsils, are located at the base of the tongue.
The Salivary Glands
Scattered throughout the lining of the mouth and tongue are many small salivary glands that constantly release tiny amounts of saliva, even when you sleep. Their job is to keep the mouth moist and comfortable. When you begin eating, saliva production increases so food can be moistened, shaped into a bolus, and chemically broken down.
In addition to these hundreds of small glands, three pairs of major salivary glands supply most of the saliva that enters the mouth:
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Parotid glands – located near the ears and about the size of a prune. They send saliva into the mouth through the parotid duct near the second upper molar.
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Submandibular glands – found along the floor of the mouth and about the size of a walnut. Their ducts open just under the tongue.
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Sublingual glands – sitting beneath the tongue and about the size of an almond. They release saliva through several small ducts in the floor of the mouth.
Together, all salivary glands produce about 1 to 1.5 liters of saliva each day.
Saliva
Saliva is mostly water, but it contains important substances that help with digestion, protection, and lubrication. One key enzyme, salivary amylase, begins the chemical breakdown of carbohydrates in the mouth and continues to work until the food reaches the acidic environment of the stomach. Saliva also contains mucus that helps lubricate food for swallowing, as well as antimicrobial molecules such as lysozyme and immunoglobulin A, which help protect the mouth from pathogens.
Each major salivary gland produces slightly different saliva. The parotid glands produce a thin, watery saliva rich in amylase. The submandibular glands make a mixture of watery and mucous saliva. The sublingual glands primarily secrete thicker, mucus-rich saliva with smaller amounts of amylase. This combination ensures that the mouth stays moist and that chewing, swallowing, and early digestion occur smoothly.
Infections of the nasal passages and pharynx can involve any of the salivary glands, but the parotid glands are the classic site of infection with the mumps virus, a paramyxovirus. Mumps typically causes painful enlargement and inflammation of one or both parotid glands, producing the familiar swelling between the ears and jaw. Symptoms often include fever, headache, and sore throat, and swallowing acidic drinks such as orange juice can be especially uncomfortable. CDC1(opens in new window)
In some adolescents and adults, the virus spreads beyond the salivary glands. After puberty, mumps can cause inflammation of the testes (mumps orchitis) in a substantial fraction of infected males, especially if they are unvaccinated; the condition is usually unilateral and painful, can lead to testicular shrinkage, but only rarely results in permanent sterility. Mumps can also inflame the ovaries or breast tissue, the pancreas, the coverings of the brain and spinal cord (meningitis), or the brain itself, and can occasionally cause hearing loss. CDC2(opens in new window)
The good news is that routine use of the measles–mumps–rubella (MMR) vaccine has reduced U.S. mumps cases by more than 99 percent compared with the pre-vaccine era, when well over 100,000 cases occurred each year. Today, most years see only a few hundred to a few thousand reported cases, often clustered in outbreaks on college campuses or in other close-contact settings, and vaccinated people who do get mumps usually have milder disease and fewer complications.
The Teeth
You develop two sets of teeth in your lifetime. The first 20 baby (deciduous) teeth begin to appear around 6 months of age. Between ages 6 and 12, they are replaced by 32 permanent teeth. From front to back, the teeth include incisors for cutting, canines for tearing, premolars for mashing, and molars for crushing food. The third molars are the wisdom teeth; they often erupt in early adulthood and may stay trapped in the bone or gum tissue without being able to fully erupt into the mouth. This is why they are commonly removed.
Each tooth sits in a socket in the upper or lower jaw and is held in place by the gums and a strong periodontal ligament. A tooth has two main regions: the crown above the gum line and the root anchored in the jaw. Inside is the pulp cavity, which contains blood vessels and nerves. Surrounding the pulp is dentin, a hard tissue covered by cementum in the root and by enamel in the crown. Enamel is the hardest substance in the body, but it can still be damaged by acids.
Tooth decay (cavities) occurs when bacteria feeding on sugars produce acids that dissolve enamel and irritate deeper tissues. Good brushing, flossing, and limiting sugary foods help keep enamel intact and protect the rest of the tooth.
The Pharynx
The wall of the oropharynx is similar to the lining of the mouth, with a protective stratified squamous epithelium and mucus-secreting glands. During swallowing, skeletal muscles in the pharynx lift and widen the space to receive the bolus of food, then relax as the constrictor muscles squeeze it into the esophagus to begin peristalsis. At the same time, the soft palate and uvula rise to seal off the nasopharynx, and the larynx moves upward so the epiglottis can fold down over the glottis to protect the airway. If a small amount of food slips into the trachea, the cough reflex usually expels it and sends it back toward the pharynx.
The Esophagus
The esophagus is a muscular tube that carries food from the pharynx to the stomach. It is about 25 cm (10 in) long, lies posterior to the trachea, and stays collapsed except during swallowing. As shown in the figure, the esophagus travels straight through the mediastinum of the thorax and enters the abdomen by passing through the esophageal hiatus of the diaphragm.
The upper esophageal sphincter (UES) controls the entry of food from the pharynx. The upper two-thirds of the esophagus contain both skeletal and smooth muscle fibers, while the lower third is entirely smooth muscle. Waves of peristalsis begin near the top of the esophagus and push the bolus downward. Mucus secreted by the esophageal lining lubricates both the tube and the food as it travels.
At the stomach entrance, the lower esophageal sphincter LES) relaxes so the bolus can enter the stomach, then contracts to prevent acidic stomach contents from refluxing upward. The surrounding diaphragm also helps keep this sphincter closed between swallows. When the sphincter does not close completely, gastric contents can move into the esophagus and cause heartburn or GERD (gastroesophageal reflux disease).
The esophageal mucosa is lined with non-keratinized stratified squamous epithelium, which protects the surface from abrasion. The lamina propria contains mucus-secreting glands that help lubricate the lumen.
The muscularis layer changes along the length of the esophagus: the upper third is skeletal muscle, the middle third is a mix of skeletal and smooth muscle, and the lower third is entirely smooth muscle. The outermost layer is the adventitia, a connective tissue covering that lacks the visceral peritoneum found on the stomach and intestines.
| Action | Outcome |
|---|---|
| Upper esophageal sphincter relaxation | Bolus moves from the laryngopharynx into the esophagus |
| Peristalsis | Propels the bolus through the esophagus |
| Lower esophageal sphincter relaxation | Bolus enters the stomach and reflux is prevented |
| Mucus secretion | Lubricates the bolus and eases passage |
Deglutition (Swallowing)
Deglutition, or swallowing, is the process that moves a bolus of food from the mouth to the stomach. It normally takes only a few seconds and depends on smooth cooperation between the tongue, pharynx, esophagus, and plenty of lubricating saliva. Swallowing begins voluntarily but soon becomes an automatic reflex. The three phases are:
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Voluntary (oral) phase: The tongue pushes the bolus to the back of the mouth to start swallowing.
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Pharyngeal phase: Breathing briefly pauses while muscles guide the bolus safely past the airway and into the esophagus.
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Esophageal phase: Peristaltic waves move the bolus through the esophagus and relax the lower esophageal sphincter so it can enter the stomach.
Watch This Video:
Watch this animation to see how swallowing is a complex process that involves the nervous system to coordinate the actions of upper respiratory and digestive activities. During which stage of swallowing is there a risk of food entering respiratory pathways and how is this risk blocked?
Stomach
Situated just below the esophagus, the stomach forms a muscular, expandable chamber that links the esophagus to the duodenum. Although anchored at both ends, it is highly dynamic in between, shifting shape and rhythmically contracting to aid mechanical digestion. When empty, it is roughly the size of a fist, yet it can expand to hold up to 4 liters of food and fluid — more than 75 times its resting volume — and then return to its original size. Stomach size does not predict how much a person typically eats; instead, it simply stretches more when larger meals, such as holiday dinners, are consumed.
The stomach does not operate as an open tube; instead, it is bounded by two muscular gates that regulate what enters and leaves this organ. A sphincter is a muscular ring that can tighten like a drawstring to control the passage of materials from one region of the digestive tract to another. The stomach has two of these muscular gates. At the top — as you learned further up — is the lower esophageal sphincter helping to prevent acidic stomach contents from flowing backward into the esophagus. At the bottom, the pyloric sphincter regulates the release of small amounts of chyme from the stomach into the duodenum.
Chemical digestion begins in the mouth with amylase breaking down starch, but it accelerates in the stomach. However, despite common belief, the stomach is not where most digestion occurs; that job belongs to the small intestine. Instead, the stomach functions primarily as a temporary storage and mixing chamber. Because you can eat much faster than your small intestine can process nutrients, the stomach holds food, mixes it with digestive secretions, and releases small amounts of chyme into the duodenum at a controlled pace.
Anatomy of the Stomach
There are four main regions in the stomach: the cardia, fundus, body, and pylorus. It is unique with three layers of the muscularis externa: outer longitudinal layer, middle circular layer, and inner oblique layer. The cardia (or cardiac region) is the point where the esophagus connects to the stomach and through which food passes into the stomach. Located inferior to the diaphragm, above and to the left of the cardia, is the dome-shaped fundus. Below the fundus is the body, the main part of the stomach. The funnel-shaped pylorus connects the stomach to the duodenum. The wider end of the funnel, the pyloric antrum, connects to the body of the stomach. The narrower end is called the pyloric canal, which connects to the duodenum. The smooth muscle pyloric sphincter is located at this latter point of connection and controls stomach emptying. In the absence of food, the stomach deflates inward, and its mucosa and submucosa fall into large folds called rugae.
Histology
The wall of the stomach is built from the same four layers that line most of the alimentary canal, but its mucosa and muscularis include special adaptations that support the stomach’s unique roles in digestion.
Mucosa: Gastric Pits and Gastric Glands
The surface of the mucosa is not smooth. Countless indentations give it the appearance of a well-used pincushion. The shallow openings on the surface are the gastric pits. Each pit serves as the opening to deeper exocrine tubules called gastric glands.
What Do Gastric Pit Cells Produce?
The cells on the surface of the stomach and the ones lining the gastric pits are called surface mucous cells. Their main job is to secrete a thick, alkaline (bicarbonate-rich) mucus. This coating of alkaline mucus shields the stomach lining, preventing the acid and enzymes in gastric juice from damaging the stomach wall.
What Do Gastric Gland Cells Produce?
Beneath the gastric pits lie the gastric glands. These deeper tubules contain several types of secretory cells that collectively produce the components of gastric juice. Their secretions include hydrochloric acid, digestive enzymes, and mucus, all of which work together to support efficient digestion in the stomach.
Mucous neck cells
Located in the upper “neck” region of the glands, these cells secrete a thinner, slightly acidic mucus. This mucus helps lubricate the gland and mixes with the secretions moving upward through the pit.
Parietal cells
Found mainly in the middle of the gastric glands, the parietal cells produce hydrochloric acid (HCl), which creates the highly acidic environment of the stomach (pH 1.5 to 3.5). This acidity activates pepsin, helps break down proteins, and kills many microbes in food. Parietal cells also release intrinsic factor, a glycoprotein required for vitamin B12 absorption in the small intestine.
Chief cells
Located deeper in the glands, chief cells secrete pepsinogen, the inactive precursor of pepsin, and a small amount of gastric lipase. Pepsinogen is converted to pepsin by HCl and begins the digestion of proteins, while gastric lipase contributes modestly to fat digestion.
Enteroendocrine cells (including G cells)
Scattered among the glandular cells, these cells release hormones into the surrounding tissue fluid rather than into the gland lumen. In the pyloric region, G cells produce gastrin, a hormone that stimulates the parietal and chief cells and increases stomach motility.
Cells Involved in the Digestive Processes in the Stomach
| Type of Cell | Secrete |
|---|---|
| Neck | Mucus |
| Chief | Pepsinogen and gastric lipase |
| Parietal | Hydrochloric acid |
| G | Gastrin |
Muscularis Externa: A Third Layer in the Stomach
Most regions of the digestive tract contain two layers of smooth muscle in the muscularis externa, an inner circular layer and an outer longitudinal layer. The stomach is an exception. It includes a third, inner oblique layer of smooth muscle.
Why Does the Stomach Have an Oblique Layer?
The added oblique layer strengthens the stomach’s contractions. Working together, the three layers create powerful mixing motions that blend food with gastric juice and mechanically break it into very small particles. This vigorous mixing helps transform the food into chyme, the semi-fluid material that moves into the duodenum.
For a quick visual review, this short Khan Academy video offers a helpful overview of stomach physiology.
Digestive Functions of the Stomach
The stomach participates in virtually all the digestive activities with the exception of ingestion and defecation. Although almost all digestions and absorption takes place in the small intestine, the stomach does absorb some nonpolar substances, such as alcohol and aspirin.
Mechanical Digestion in the Stomach
Within a few moments after food enters your stomach, rhythmic mixing waves begin about every 20 seconds. These waves represent a special type of peristalsis that blends and softens food with gastric juices, forming a thick, creamy mixture called chyme. Early waves are gentle, but they become stronger as they move from the body of the stomach toward the pylorus. By the time your sushi approaches the pyloric sphincter, it no longer resembles the sushi you originally ate.
The pylorus, which holds about 30 mL (1 fluid ounce) of chyme, acts like a selective gatekeeper. Only liquids and very small particles pass through the mostly closed pyloric sphincter. During gastric emptying, each wave pushes roughly 3 mL of chyme into the duodenum. Sending more than this at once would overwhelm the small intestine. The chyme that does not pass through is pushed back into the stomach to continue mixing until the next wave repeats the cycle. Such mixing waves occur roughly every 20 seconds.
Gastric emptying is controlled by both the stomach and the duodenum. As chyme enters the duodenum, receptors there trigger signals that slow gastric secretion and delay further emptying. This coordination ensures the duodenum receives chyme only at a rate it can safely process.
Chemical Digestion in the Stomach
Hydrochloric Acid (HCl) from Parietal Cells
Parietal cells release hydrochloric acid, creating the stomach’s extremely acidic environment. This acidity helps denature dietary proteins, unfolding them so enzymes can reach their chemical bonds. It also converts the inactive enzyme precursor pepsinogen into its active form, pepsin, which begins the chemical breakdown of proteins.
Pepsinogen and Gastric Lipase from Chief Cells
Chief cells secrete pepsinogen, an inactive enzyme (= zymogen) that becomes the active enzyme pepsin when exposed to HCl. Pepsin is the stomach’s primary protease — protein digesting enzyme — and begins cleaving large proteins into smaller peptides. Chief cells also release gastric lipase, which initiates the digestion of triglycerides. Gastric lipase is a relatively weak fat-digesting enzyme compared to those of the small intestine, but it contributes to the early breakdown of dietary fats.
Its numerous digestive functions notwithstanding, there is only one stomach function necessary to life: the production of intrinsic factor. The intestinal absorption of vitamin B12, which is necessary for both the production of mature red blood cells and normal neurological functioning, cannot occur without intrinsic factor. People who undergo total gastrectomy (stomach removal) — for life-threatening stomach cancer, for example — can survive with minimal digestive dysfunction if they receive vitamin B12 injections.
The contents of the stomach are completely emptied into the duodenum within 2 to 4 hours after you eat a meal. Different types of food take different amounts of time to process. Foods heavy in carbohydrates empty fastest, followed by high-protein foods. Meals with a high triglyceride content remain in the stomach the longest. Since enzymes in the small intestine digest fats slowly, food can stay in the stomach for 6 hours or longer when the duodenum is processing fatty chyme. However, note that this is still a fraction of the 24 to 72 hours that full digestion typically takes from start to finish.