24.1 Anatomy and normal microbiota of the digestive system  (Page 17/6)

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

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.

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.

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.