4.4: Upper Gastrointestinal Tract

Head Stand

Did you ever wonder what would happen if you tried to swallow food while standing on your head? Many people think that food travels down the gullet from the mouth by the force of gravity. If that were the case, then the food you swallowed would stay in your throat while you were standing on your head. In reality, your position doesn’t have much to do with your ability to swallow. Food will travel from your mouth to your stomach whether you are standing upright or upside down. That’s because the tube the food travels through — the esophagus — moves the food along by the muscular contractions is known as peristalsis. The esophagus is one of several organs that make up the upper gastrointestinal tract.

Figure \(\PageIndex{1}\): Headstand

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Organs of the Upper Gastrointestinal Tract

Besides the esophagus, organs of the upper gastrointestinal (GI) tract include the mouth, pharynx, and stomach. These hollow organs are all connected to form a tube through which food passes during digestion. The only role in digestion played by the pharynx and esophagus is to move food through the GI tract. The mouth and stomach, in contrast, are organs where digestion, or the breakdown of food, also occurs. In both of these organs, food is broken into smaller pieces (mechanical digestion) as well as broken down chemically (chemical digestion). It should be noted that the first part of the small intestine (duodenum) is considered in some contexts to be part of the upper GI tract, but that practice is not followed here. You can read about the small intestine (and large intestine) in the concept Lower Gastrointestinal Tract.

Mouth

The mouth is the first organ of the GI tract. Most of the oral cavity is lined with a mucous membrane. This tissue produces mucus, which helps to moisten, soften, and lubricate food. Underlying the mucous membrane is a thin layer of smooth muscle to which the mucous membrane is only loosely connected. This gives the mucous membrane considerable ability to stretch as you eat food. The roof of the mouth, called the palate, separates the oral cavity from the nasal cavity. The front part is the hard palate, consisting of a mucous membrane covering a plate of bone. The back part of the palate is softer and more pliable, consisting of mucous membrane over muscle and connective tissue. The hard surface of the front of the palate allows for the pressure needed in chewing and mixing food. The soft, pliable surface of the back of the palate can move to accommodate the passage of food while swallowing. Muscles at either side of the soft palate contract to create the swallowing action.

Several specific structures in the mouth are specialized for digestion. These include salivary glands, tongue, and teeth.

Salivary Glands

The mouth contains three pairs of major salivary glands, which are shown in Figure \(\PageIndex{2}\). These three pairs are all exocrine glands that secrete saliva into the mouth through ducts.

1. The largest of the three major pairs of salivary glands are the parotid glands, which are located on either side of the mouth in front of the ears.
2. The next largest pair is the submandibular glands, located beneath the lower jaw.
3. The third pair is the sublingual glands, located underneath the tongue.

Figure \(\PageIndex{2}\): Salivary glands in the mouth include the three major pairs of glands shown here. The sublingual gland is under the tongue. The submandibular gland is below the jaw. The parotid gland is the back of the mouth.

In addition to these three pairs of major salivary glands, there are also hundreds of minor salivary glands in the oral mucosa lining the mouth and on the tongue. Along with the major glands, most of the minor glands secrete the digestive enzyme amylase, which begins the chemical digestion of starch and glycogen (polysaccharides). However, the minor salivary glands on the tongue secrete the fat-digesting enzyme lipase, which in the mouth is called lingual lipase (to distinguish it from pancreatic lipase secreted by the pancreas).

The saliva secreted by the salivary glands mainly helps digestion, but it also plays other roles. It helps to maintain dental health by cleaning the teeth, and it contains antibodies that help protect against infection. By keeping the mouth lubricated, saliva also allows the mouth movements needed for speech.

Tongue

The tongue is a fleshy, muscular organ that is attached to the floor of the mouth by a band of ligaments that gives it great mobility. This is necessary so the tongue can manipulate food for chewing and swallowing. Movements of the tongue are also necessary for speaking. The upper surface of the tongue is covered with tiny projections called papillae, which contain taste buds. The latter are collections of chemoreceptor cells. These sensory cells sense chemicals in food and send the information to the brain via cranial nerves, thus enabling the sense of taste.

Teeth

The teeth are complex structures made of a bone-like material called dentin and covered with enamel, which is the hardest tissue in the body. Adults normally have a total of 32 teeth, with 16 in each jaw. The right and left sides of each jaw are mirror images in terms of the numbers and types of teeth they contain. Teeth have different shapes to suit them for different aspects of mastication (chewing).

Pharynx

The tube-like pharynx (Figure \(\PageIndex{3}\)) plays a dual role as an organ of both respiration and digestion. As part of the respiratory system, it conducts air between the nasal cavity and larynx. As part of the digestive system, it allows swallowed food to pass from the oral cavity to the esophagus. Anything swallowed has priority over inhaled air when passing through the pharynx. During swallowing, the backward motion of the tongue causes a flap of elastic cartilage, called the epiglottis, to close over the opening to the larynx. This prevents food or drinks from entering the larynx.

Figure \(\PageIndex{3}\): The tongue moves backward during swallowing to cause the epiglottis to cover the opening to the larynx so food passes from the pharynx to the esophagus and not into the larynx.

Esophagus

The esophagus, which is shown in Figure \(\PageIndex{4}\), is a muscular tube through which food is pushed from the pharynx to the stomach. The esophagus passes through an opening in the diaphragm (the large breathing muscle that separates the abdomen from the thorax) before reaching the stomach. In adults, the esophagus averages about 25 cm (10 in.) in length, depending on a person’s height. The inner lining of the esophagus consists of a mucous membrane, which provides a smooth, slippery surface for the passage of food. The cells of this membrane are constantly being replaced as they are worn away from the frequent passage of food over them.

When food is not being swallowed, the esophagus is closed at both ends by upper and lower esophageal sphincters. Sphincters are rings of muscle that can contract to close off openings between structures. The upper esophageal sphincter is triggered to relax and open by the act of swallowing, allowing a bolus of food to enter the esophagus from the pharynx. Then the esophageal sphincter closes again to prevent food from moving back into the pharynx.

Once in the esophagus, the food bolus travels down to the stomach, pushed along by the rhythmic contraction and relaxation of muscles (peristalsis). The lower esophageal sphincter is located at the junction between the esophagus and the stomach. This sphincter opens when the bolus reaches it, allowing the food to enter the stomach. The sphincter normally remains closed at other times to prevent the contents of the stomach from entering the esophagus. Failure of this sphincter to remain completely closed can lead to heartburn. If it happens chronically, it can lead to gastroesophageal reflux disease (GERD), in which the mucous membrane of the esophagus may become damaged by the highly acidic contents of the stomach.

Figure \(\PageIndex{4}\): The esophagus is located behind the trachea and it connects the pharynx to the stomach. The stomach is connected at the bottom to the small intestine.

 

Stomach

The stomach (Figure \(\PageIndex{4}\) is a J-shaped organ that is joined to the esophagus at its upper end and to the first part of the small intestine (duodenum) at its lower end. When the stomach is empty of food, it normally has a volume of about 75 mL. However, it can expand to hold up to about a liter of food. Waves of muscle contractions (peristalsis) passing through the muscular walls of the stomach cause the food inside to be mixed and churned. The wall of the stomach has an extra layer of muscle tissue not found in other organs of the GI tract that helps it squeeze and mix the food. These movements of the stomach wall contribute greatly to mechanical digestion by breaking the food into much smaller pieces. The churning also helps to mix the food with stomach secretions that aid in its chemical digestion.

Secretions of the stomach include gastric acid, which consists mainly of hydrochloric acid. This makes the stomach contents highly acidic, which is necessary so that the enzyme pepsin — also secreted by the stomach — can begin the digestion of protein. Mucus is secreted by the lining of the stomach to provide a slimy protective coating against the otherwise damaging effects of gastric acid. The fat-digesting enzyme lipase is secreted in small amounts in the stomach, but very little fat digestion occurs there.

By the time food has been in the stomach for about an hour, it has become the thick, semi-liquid chyme. When the small intestine is ready to receive chyme, a sphincter between the stomach and duodenum, called the pyloric sphincter, opens to allow the chyme to enter the small intestine for further digestion and absorption.

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Review

1. List organs of the upper gastrointestinal tract.
2. Identify structures in the mouth that are specialized for digestion.
3. Describe digestion in the mouth.
4. What general role do the pharynx and esophagus play in the digestion of food?
5. How does food travel through the esophagus?
6. Describe digestion in the stomach.
7. In which of the following structures does chemical digestion occur?

   A. Mouth

   B. Esophagus

   C. Stomach

   D. A and C

   E. A, B, and C

8. In which of the following structures does mechanical digestion occur?

   A. Mouth

   B. Pharynx

   C. Stomach

   D. A and C

   E. A, B, and C

9. Describe the differences between how air and food normally move past the pharynx.
10. Name two structures in the mouth that contribute to mechanical digestion.
11. What structure normally keeps stomach contents from backing up into the esophagus?
12. True or False. Peristalsis occurs in the esophagus, but not in the stomach.
13. True or False. The sense of taste is due to the detection of chemicals by the tongue.
14. Where is most of your food located thirty minutes after you eat a meal? Explain your answer.
15. What are the two roles of mucus in the upper GI tract?

Attributions

1. Yoga by Anant_762 via Pixabay license
2. Salivary glands by Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. CC BY 3.0 via Wikimedia Commons
3. Head and neck by Prof. Squirrel, Public Domain via Wikimedia Commons
4. Esophagus and stomach by NIAID, CC BY 2.0 via Wikimedia Commons
5. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0