Chapter 25 Anatomy of the Digestive System Mosby

Overview of the Digestive System Role of the digestive system Ø Prepares food for absorption and use by all the cells of the body Ø Food material not absorbed becomes feces that is eliminated Ø Digestion depends on both endocrine and exocrine secretions and the controlled movement of ingested food materials through the gastrointestinal (GI) tract Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 2

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Overview of the Digestive System Organization of the digestive system Ø Organs of digestion • Main organs of the digestive system form the GI tract that extends through the abdominopelvic cavity • Ingested food material passing through the lumen of the GI tract is outside the internal environment of the body Ø Wall of the GI tract • Layers—GI tract is made of four layers of tissues: mucosa, submucosa, muscularis, and serosa • Modifications of layers—layers of the GI tract have various modifications to enable it to perform various functions Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 4

Mouth Structure of the oral cavity (buccal cavity) Ø Lips—covered externally by skin and internally by mucous membrane; junction between skin and mucous membrane is highly sensitive; when lips are closed, line of contact is oral fissure Ø Cheeks—lateral boundaries of oral cavity, continuous with lips and lined by mucous membrane; formed in large part by buccinator muscle covered by adipose tissue; contain mucus-secreting glands Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 5

Mouth Structure of the oral cavity (buccal cavity) (cont. ) Ø Hard and soft palates • Hard palate consists of portions of four bones: two maxillae and two palatines • Soft palate forms partition between the mouth and the nasopharynx and is made of muscle arranged in an arch • Suspended from midpoint of posterior border of the arch is the uvula Ø Tongue—solid mass of skeletal muscle covered by a mucous membrane; extremely maneuverable (Figure 25 -4) • • Important for mastication and deglutition Has three parts: root, tip, and body Papillae located on dorsal surface of tongue Lingual frenulum anchors tongue to floor of mouth Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 6

Mouth Salivary glands—three pairs of compound tubuloalveolar glands secrete approximately 1 liter of saliva each day; buccal glands contribute less than 5% of total salivary volume but provide for hygiene and comfort of oral tissues Ø Parotid glands—largest of the paired salivary glands; produce watery saliva containing enzymes Ø Submandibular glands—compound glands that contain enzyme and mucus-producing elements Ø Sublingual glands—smallest of the salivary glands; produce a mucous type of saliva Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 7

Mouth Teeth—organs of mastication Ø Typical tooth • Crown—exposed portion of a tooth, covered by enamel; ideally suited to withstand abrasion during mastication • Neck—narrow portion that joins the crown to the root; surrounded by the gingivae • Root fits into socket of alveolar process and is suspended by fibrous periodontal membrane • Outer shell contains two additional tissues: dentin and cementum Dentin makes up the greatest portion of the tooth shell; at crown, covered by enamel, and at neck and root, covered by cementum Pulp cavity—located in dentin, contains connective tissue, blood, and lymphatic vessels and sensory nerves Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 8

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Mouth Teeth (cont. ) Ø Types of teeth • Deciduous teeth— 20 baby teeth, which appear early in life • Permanent teeth— 32 teeth, which replace the deciduous teeth Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 10

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Pharynx Tube through which a bolus passes when moved from the mouth to the

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Esophagus Tube that extends from the pharynx to the stomach First segment of digestive

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Stomach Size and position of the stomach Ø Size varies according to factors such as gender and amount of distention • When no food is in stomach, it is about the size of a large sausage • In adults, capacity ranges from 1. 0 to 1. 5 liters Ø Stomach location: upper part of abdominal cavity under liver and diaphragm Divisions of the stomach Ø Fundus—enlarged portion to the left and above opening of esophagus into stomach Ø Body—central portion of stomach Ø Pylorus—lower part of stomach Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 16

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Stomach Curves of the stomach Ø Lesser curvature—upper right curve of stomach Ø Greater curvature—lower left curve of stomach Sphincter muscles—circular fibers arranged so that there is an opening in the center when relaxed and no opening when contracted Ø Lower esophageal sphincter (LES) or cardiac sphincter controls opening of esophagus into stomach Ø Pyloric sphincter controls outlet of pyloric portion of stomach into duodenum Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 18

Stomach wall Ø Gastric mucosa • Epithelial lining has rugae marked by gastric pits Gastric glands—found below level of the pits; secrete most of gastric juice • Chief cells—secretory cells found in gastric glands; secrete the enzymes of gastric juice • Parietal cells—secretory cells found in gastric glands; secrete hydrochloric acid; thought to produce intrinsic factor needed for vitamin B 12 absorption • Endocrine cells—secrete gastrin and ghrelin Ø Gastric muscularis—thick layer of muscle with three distinct sublayers of smooth muscle tissue arranged in a crisscrossing pattern; this pattern allows stomach to contract strongly at many angles Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 19

Stomach Functions of the stomach Reservoir food until it is partially digested and moved further along GI tract Ø Secretes gastric juice to aid in digestion of food Ø Breaks food into small particles and mixes them with gastric juice Ø Secretes intrinsic factor Ø Limited absorption Ø Produces gastrin and ghrelin Ø Helps protect body from pathogenic bacteria swallowed with food Ø Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 20

Small Intestine Size and position of the small intestine—tube approximately 2. 5 cm in diameter and 6 m in length; coiled loops fill most of abdominal cavity Divisions of the small intestine Ø Duodenum—uppermost division; approximately 25 cm long, shaped roughly like the letter C Ø Jejunum—approximately 2. 5 m long Ø Ileum—approximately 3. 5 m long Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 21

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Small Intestine Wall of the small intestine Ø Intestinal lining has plicae with villi Ø Villi—important modifications of mucosal layer • Each villus contains an arteriole, venule, and lacteal • Covered by a brush border made up of 1, 700 ultrafine microvilli per cell • Villi and microvilli increase surface area of small intestine hundreds of times Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 23

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Large Intestine Size of the large intestine—average diameter, 6 cm; length, approximately 1. 5 to 1. 8 m Divisions of the large intestine Cecum—first 5 to 8 cm of large intestine, blind pouch located in lower right quadrant of abdomen Ø Colon Ø • Ascending colon—vertical position on right side of abdomen; • • ileocecal valve prevents material passing from large intestine into ileum Transverse colon passes horizontally across abdomen, above small intestine; extends from hepatic flexure to splenic flexure Descending colon—vertical position on left side of abdomen Sigmoid colon joins descending colon to rectum Rectum—last 7 or 8 inches of intestinal tube; terminal inch is anal canal with opening called the anus (Figure 25 -17) Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 25

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Large Intestine Wall of the large intestine (Figure 25 -19) Ø Intestinal mucous glands produce lubricating mucus that coats feces as they are formed Ø Uneven distribution of fibers in the muscle coat Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 27

Vermiform Appendix Accessory organ of digestive system; 8 to 10 cm in length; communicates

Peritoneum Large, continuous sheet of serous membrane Made up of parietal and visceral layers Mesentery—projection of parietal peritoneum; allows free movement of each coil of the intestine and helps prevent strangulation of the long tube Transverse mesocolon—extension of peritoneum that supports transverse colon Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 29

Liver Location and size of the liver—largest gland in body, weighs approximately 1. 5 kg; lies under diaphragm; occupies most of right hypochondrium and part of epigastrium Liver lobes and lobules—two lobes separated by falciform ligament Ø Left lobe—forms about one sixth of liver Ø Right lobe—forms about five sixths of liver; divides into right lobe proper, caudate lobe, and quadrate lobe Ø Hepatic lobules—anatomical units of liver; small branch of hepatic vein extends through the center of each lobule Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 30

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Liver Bile ducts Ø Small bile ducts form right and left hepatic ducts Ø Right and left hepatic ducts immediately join to form one hepatic duct Ø Hepatic duct merges with cystic duct to form common bile duct, which opens into duodenum Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 32

Liver Functions of the liver Ø Detoxification by liver cells—ingested toxic substances and toxic substances formed in intestines may be changed to nontoxic substances Ø Bile secretion by liver—bile salts are formed in liver from cholesterol and are the most essential part of bile; liver cells secrete approximately 1 pint of bile per day Ø Liver metabolism carries out numerous important steps in the metabolizing of proteins, fats, and carbohydrates Ø Storage of substances such as iron and some vitamins Ø Production of important plasma proteins Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 33

Gallbladder Size and location of the gallbladder—pear-shaped sac from 7 to 10 cm long and 3 cm wide at its broadest point; holds 30 to 50 ml of bile; lies on undersurface of liver Structure of gallbladder—serous, muscular, and mucous layers compose the gallbladder wall; mucosal lining has rugae Functions of gallbladder: Ø Storage of bile Ø Concentration of bile fivefold to tenfold Ø Ejection of the concentrated bile into duodenum Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 34

Pancreas Size and location of the pancreas—grayish pink–colored gland; 12 to 15 cm long; weighs approximately 60 g; runs from duodenum and behind stomach to spleen Structure of the pancreas—composed of endocrine and exocrine glandular tissue Ø Exocrine portion makes up majority of pancreas; has a compound acinar arrangement; tiny ducts unite to form main pancreatic duct, which empties into duodenum Ø Endocrine portion—embedded between exocrine units; called pancreatic islets; constitute only 2% of total mass of pancreas; made up of alpha cells and beta cells; pass secretions into capillaries Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 35

Pancreas Functions of the pancreas Ø Acinar units secrete digestive enzymes Ø Beta cells secrete insulin Ø Alpha cells secrete glucagon Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 36

Cycle of Life: Digestive System Changes in digestive function and structure age-related Ø Result in diseases or pathological conditions Ø May occur in any segment of intestinal tract Ø Changes involve accessory organs: teeth, salivary glands, liver, gallbladder, and pancreas Infants—immature intestinal mucosa; intact proteins can pass through epithelial cells lining the tract and trigger allergic response Lactose intolerance affects infants who lack the enzyme lactase Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 37

Cycle of Life: Digestive System Young age—mumps common in children; appendicitis more common in adolescents and then decreases with advancing age Middle age—ulcers and gallbladder disease common Old age—decreased digestive fluids, slowing of peristalsis, and reduced physical activity lead to constipation and diverticulosis Mosby items and derived items © 2007, 2003 by Mosby, Inc. Slide 38