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Learning objectives

  • Describe the major anatomical features of the human digestive system
  • Describe the normal microbiota of various regions in the human digestive system
  • Explain how microorganisms overcome the defenses of the digestive tract to cause infection or intoxication
  • Describe general signs and symptoms associated with infections of the digestive system

Part 1

After a morning of playing outside, four-year-old Carli ran inside for lunch. After taking a bite of her fried egg, she pushed it away and whined, “It’s too slimy, Mommy. I don’t want any more.” But her mother, in no mood for games, curtly replied that if she wanted to go back outside she had better finish her lunch. Reluctantly, Carli complied, trying hard not to gag as she choked down the runny egg.

That night, Carli woke up feeling nauseated. She cried for her parents and then began to vomit. Her parents tried to comfort her, but she continued to vomit all night and began to have diarrhea and run a fever. By the morning, her parents were very worried. They rushed her to the emergency room.

  • What could have caused Carli’s signs and symptoms?

Jump to the next Clinical Focus box.

The human digestive system , or the gastrointestinal (GI) tract, begins with the mouth and ends with the anus. The parts of the mouth include the teeth, the gums, the tongue, the oral vestibule (the space between the gums, lips, and teeth), and the oral cavity proper (the space behind the teeth and gums). Other parts of the GI tract are the pharynx, esophagus, stomach, small intestine, large intestine, rectum, and anus ( [link] ). Accessory digestive organs include the salivary glands, liver, gallbladder, spleen, and pancreas.

The digestive system contains normal microbiota, including archaea, bacteria, fungi, protists, and even viruses. Because this microbiota is important for normal functioning of the digestive system, alterations to the microbiota by antibiotics or diet can be harmful. Additionally, the introduction of pathogens to the GI tract can cause infections and diseases. In this section, we will review the microbiota found in a healthy digestive tract and the general signs and symptoms associated with oral and GI infections.

Diagram of the digestive system. The system begins with the mouth and tongue. There are salivary glands in this region: the sublingual gland is under the tongue, the submandibular gland is below the jaw and the parotid gland is in the very back of the mouth. The mouth leads to the pharynx (a tube) that leads to the esophagus, that leads to the stomach, that leads to the small intestines. The small intestine is divided into 3 regions: first is the duodenum, next is the jejunim and finally the ileum. This leads to the large intestines which is divided into  regions: first the cecum, then the ascending colon, then the transverse colon, then the descending colon, then the sigmoid colon, and finally the rectum, anal canal and anus.  The appendix is a small projection off the cecum. Also part of the digestive system is the large liver (above and to the right of the stomach), the gallbladder (a small sac under the liver), the pancrease (a structure below and behind the stomach) and the spleen (a structure below and to the left of the stomach).
The digestive system, or the gastrointestinal tract, includes all of the organs associated with the digestion of food.

Anatomy and normal microbiota of the oral cavity

Food enters the digestive tract through the mouth, where mechanical digestion (by chewing) and chemical digestion (by enzymes in saliva ) begin. Within the mouth are the tongue , teeth , and salivary glands , including the parotid, sublingual, and submandibular glands ( [link] ). The salivary glands produce saliva, which lubricates food and contains digestive enzymes.

a) Structures of the head and neck: lips, jaw, nasal cavity (large space behind the nose), oral cavity (space in the mouth), tongue, uvula (structure in at the back of the mouth), pharyx (tube at the back of the mouth), esophagus (the pharyx is the top part of this tube which is now called the esophagus in the throat), and the larynx (this is also continuous with the pharynx but leads to the respiratory system). B) Components of the mouth region: teeth, sublingual gland (Below the tongue), submandibular gland (at the back and to the bottom of the mouth), and the parotid gland (a large gland at the very back of the mouth).
(a) When food enters the mouth, digestion begins. (b) Salivary glands are accessory digestive organs. (credit: modification of work by National Cancer Institute)

The structure of a tooth ( [link] ) begins with the visible outer surface, called the crown , which has to be extremely hard to withstand the force of biting and chewing. The crown is covered with enamel , which is the hardest material in the body. Underneath the crown, a layer of relatively hard dentin extends into the root of the tooth around the innermost pulp cavity, which includes the pulp chamber at the top of the tooth and pulp canal, or root canal , located in the root. The pulp that fills the pulp cavity is rich in blood vessels, lymphatic vessels, connective tissue, and nerves. The root of the tooth and some of the crown are covered with cementum , which works with the periodontal ligament to anchor the tooth in place in the jaw bone. The soft tissues surrounding the teeth and bones are called gums , or gingiva . The gingival space or gingival crevice is located between the gums and teeth.

Questions & Answers

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Source:  OpenStax, Microbiology. OpenStax CNX. Nov 01, 2016 Download for free at http://cnx.org/content/col12087/1.4
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