Chapter 26 Outline General Structure and Functions of the Digestive System Oral Cavity Pharynx General Arrangement of Abdominal GI Organs Esophagus The Swallowing Process Stomach Small Intestine Large Intestine Accessory Digestive Organs Aging and the Digestive System Development of the Digestive System
Introduction The digestive system includes organs that: ingest the food transport the ingested material digest the material into smaller usable components absorb the necessary digested nutrients into the bloodstream expel waste products from the body
Introduction The digestive system is composed of two separate categories of organs: 1.Digestive organs –collectively make up the gastrointestinal (GI) tract, also called the digestive tract or the alimentary canal 2.Accessory digestive organs
Digestive System Figure 26.1
GI Tract Organs The GI tract organs are as follows: oral cavity pharynx esophagus stomach small intestine large intestine
Accessory Digestive Organs The accessory digestive organs are not part of the long GI tube, but often develop as outgrowths of that tube. They are as follows: ……….. teeth ……….. tongue ……….. salivary glands ………. liver ……….. gall bladder ……….. pancreas
Digestive System Functions Ingestion Digestion Propulsion Secretion Absorption Elimination of wastes
Peristalsis and Segmentation Propulsion of food along the GI tube involves two types of movement: –peristalsis –segmentation Peristalsis is the ripple-like wave of muscular contraction that forces material to move further along the GI tract. Segmentation is the churning and mixing of material helping to disperse the material and mix it and combine it with digestive organ secretions.
Peristalsis and Segmentation Figure 26.2
Oral Cavity Contains the following structural features: cheeks, lips, and palate tongue salivary glands teeth
Cheeks, Lips, and Palate Cheeks form the lateral wall of the oral cavity and are comprised mainly of the buccinator muscles. The cheeks end anteriorly as the lips. The gingivae (gums) cover the alveolar processes of the teeth. The internal surface of the upper and lower lips are attached to the gingivae by a thin, midline mucosa fold called the labial frenulum.
Cheeks, Lips, and Palate The palate forms the roof of the oral cavity. The anterior two-thirds of the palate is called the hard palate because it is comprised of bone. The posterior one-third of the palate is soft and muscular and is called the soft palate. Extending from the soft palate posteriorly is the uvula, which elevates during swallowing and closes off the posterior entrance to the nasopharynx.
Cheeks, Lips, and Palate Figure 26.3
Cheeks, Lips, and Palate The fauces represent the opening from the oral cavity to the oropharynx. The fauces are bounded laterally by paired muscular folds: –palatoglossal arch –palatopharyngeal arch The palatine tonsils (see Chapter 24) are housed between the two arches.
Tongue The tongue manipulates and mixes ingested materials during chewing and helps compress the partially digested materials into a bolus. A bolus is a globular mass of ingested materials that can be more easily swallowed. The inferior surface of the tongue attaches to the floor of the oral cavity by a thin, midline mucous membrane called the lingual frenulum.
Tongue Figure 26.3
Salivary Glands Salivary glands produce and secrete saliva into the oral cavity. Saliva serves the following functions: –moistens ingested materials to become a slick bolus –moistens, cleanses, and lubricates the structures of the oral cavity –chemical digestion of ingested materials –antibacterial action –dissolves materials so that taste receptors on the tongue can be stimulated
Salivary Glands Three pairs of salivary glands are located external to the oral cavity: parotid glands submandibular glands sublingual glands
Salivary Glands Figure 26.4
Parotid Salivary Glands Largest of the three salivary glands Located anterior and inferior to the ear. Lateral side of face, under ears Secrete 25–30% of total saliva Parotid duct runs parallel to the zygomatic arch and pierces the buccinator muscle just opposite the second upper molar secrete amylaseAlso secrete amylase
Parotid Salivary Glands Figure 26.4
Submandibular Salivary Glands Reside inferior/lateral to the body of the mandible Produce the majority of the saliva (60–70%) A submandibular duct transports saliva from each gland through a papilla in the floor of the mouth on the lateral sides of the lingual frenulum
Submandibular Salivary Glands
Sublingual Salivary Glands Inferior/anterior to the tongue Each gland extends multiple tiny sublingual ducts that open onto the inferior surface of the oral cavity just posterior to the submandibular duct papilla Contribute only 3–5% of total saliva
Sublingual Salivary Glands Figure 26.4
Salivary Gland Secretion Two types of secretory cells are found in salivary glands: Mucous cells—secrete mucin, which forms mucus upon hydration Serous cells—secrete a watery fluid containing ions, lysozyme, and salivary amylase
Salivary Gland Secretion Figure 26.4
Salivary Gland Secretion
Teeth The teeth are collectively known as the dentition. A tooth has an exposed crown, a constricted neck, and one or more roots that fit into dental alveoli. Dentin forms the primary mass of the tooth. It is harder than bone. Each root is covered with cementum. The external surface of the dentin is covered with a layer of enamel that forms the crown of the tooth.
Teeth The center of the tooth is a pulp cavity that contains connective tissue called pulp. A root canal opens into the connective tissue through an opening called the apical foramen. Blood vessels and nerves pass through this opening and are housed in the pulp.
Teeth Figure 26.5
Surfaces of the Teeth Teeth have several surfaces: mesial surface distal surface buccal surface labial surface lingual surface occlusal surface
Teeth Two sets of teeth develop and erupt in a normal lifetime: deciduous teeth—erupt between 6– 30 months, 20 in number, and are often called milk teeth permanent teeth—replace the deciduous teeth and are 32 in number
Permanent Teeth Incisors—most anteriorly placed, shaped like chisels, and have a single root Canines—posterolateral to the incisors, pointed tips for puncturing and tearing Premolars—posterolateral to canines, have flat crowns with prominent ridges called cusps for crushing and grinding Molars—thickest and most posterior teeth, also adapted for crushing and grinding of ingested materials
Oral Cavity Structures
Pharynx Shared by the respiratory and digestive systems Three skeletal muscle pairs of pharyngeal constrictors (superior, middle, and inferior) form the wall of the pharynx and participate in swallowing CN X innervates most pharyngeal muscles Branches of external carotid arteries supply the pharynx Internal jugular veins drain the pharynx
Peritoneum Abdominopelvic cavity is covered with moist serous membranes: parietal peritoneum—lines the inside surface of the body wall visceral peritoneum—covers the surface of internal organs within the cavity
Peritoneum Organs that are completely surrounded by visceral peritoneum are called intraperitoneal organs. They include the stomach and most of the small intestines. Organs that lie in direct contact with the posterior abdominal and pelvic walls and are only covered on their anterolateral surfaces with visceral peritoneum are called retroperitoneal organs. Examples are the pancreas, ascending and descending colon of the large intestines, and the rectum.
Peritoneum Figure 26.7
Mesenteries Folds of peritoneum that support and stabilize intraperitoneal GI tract organs Blood vessels, lymphatic vessels, and nerves are sandwiched between the two folds and supply the digestive organs
Mesenteries Figure 26.7
Mesenteries The greater omentum extends inferiorly like an apron from the greater curvature of the stomach and covers most of the abdominal organs. The lesser omentum connects the lesser curvature of the stomach and the proximal end of the duodenum to the liver. The mesentery proper suspends most of the small intestines from the posterior abdominal wall. The mesocolon is a peritoneal fold that attaches parts of the large intestine to the posterior abdominal wall.
Mesenteries Figure 26.8
Mesenteries Figure 26.8
The Wall of the Abdominal GI Tract The wall of the GI tract from the esophagus to the large intestine is composed of four concentric layers called tunics. From deep (in contact with ingested materials) to superficial (the external covering) they are: mucosa submucosa muscularis adventitia or serosa