Chapter 25 Anatomy of the Digestive System

1. Chapter 25Anatomy of the Digestive System

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

4. Overview of the Digestive System • Organization of the digestive system • 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 • 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

5. Mouth • Structure of the oral cavity (buccal cavity) • 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 • 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

6. Mouth • Structure of the oral cavity (buccal cavity) (cont.) • 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 • 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

7. 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 • Parotid glands—largest of the paired salivary glands; produce watery saliva containing enzymes • Submandibular glands—compound glands that contain enzyme and mucus-producing elements • Sublingual glands—smallest of the salivary glands; produce a mucous type of saliva

8. Mouth • Teeth—organs of mastication • 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

10. Mouth • Teeth (cont.) • Types of teeth • Deciduous teeth—20 baby teeth, which appear early in life • Permanent teeth—32 teeth, which replace the deciduous teeth

12. Pharynx • Tube through which a bolus passes when moved from the mouth to the esophagus by the process of deglutition

14. Esophagus • Tube that extends from the pharynx to the stomach • First segment of digestive tube

16. Stomach • Size and position of the stomach • 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 • Stomach location: upper part of abdominal cavity under liver and diaphragm • Divisions of the stomach • Fundus—enlarged portion to the left and above opening of esophagus into stomach • Body—central portion of stomach • Pylorus—lower part of stomach

18. Stomach • Curves of the stomach • Lesser curvature—upper right curve of stomach • 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 • Lower esophageal sphincter (LES) or cardiac sphincter controls opening of esophagus into stomach • Pyloric sphincter controls outlet of pyloric portion of stomach into duodenum

19. Stomach • Stomach wall • 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 B12 absorption • Endocrine cells—secrete gastrin and ghrelin • 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

20. Stomach • Functions of the stomach • Reservoir for food until it is partially digested and moved further along GI tract • Secretes gastric juice to aid in digestion of food • Breaks food into small particles and mixes them with gastric juice • Secretes intrinsic factor • Limited absorption • Produces gastrin and ghrelin • Helps protect body from pathogenic bacteria swallowed with food

21. 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 • Duodenum—uppermost division; approximately 25 cm long, shaped roughly like the letter C • Jejunum—approximately 2.5 m long • Ileum—approximately 3.5 m long

23. Small Intestine • Wall of the small intestine • Intestinal lining has plicae with villi • 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

25. 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 • Colon • 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)

27. Large Intestine • Wall of the large intestine (Figure 25-19) • Intestinal mucous glands produce lubricating mucus that coats feces as they are formed • Uneven distribution of fibers in the muscle coat

28. Vermiform Appendix • Accessory organ of digestive system; 8 to 10 cm in length; communicates with cecum

29. 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

30. 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 • Left lobe—forms about one sixth of liver • Right lobe—forms about five sixths of liver; divides into right lobe proper, caudate lobe, and quadrate lobe • Hepatic lobules—anatomical units of liver; small branch of hepatic vein extends through the center of each lobule

32. Liver • Bile ducts • Small bile ducts form right and left hepatic ducts • Right and left hepatic ducts immediately join to form one hepatic duct • Hepatic duct merges with cystic duct to form common bile duct, which opens into duodenum

33. Liver • Functions of the liver • Detoxification by liver cells—ingested toxic substances and toxic substances formed in intestines may be changed to nontoxic substances • 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 • Liver metabolism carries out numerous important steps in the metabolizing of proteins, fats, and carbohydrates • Storage of substances such as iron and some vitamins • Production of important plasma proteins

34. 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: • Storage of bile • Concentration of bile fivefold to tenfold • Ejection of the concentrated bile into duodenum

35. 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 • Exocrine portion makes up majority of pancreas; has a compound acinar arrangement; tiny ducts unite to form main pancreatic duct, which empties into duodenum • 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

36. Pancreas • Functions of the pancreas • Acinar units secrete digestive enzymes • Beta cells secrete insulin • Alpha cells secrete glucagon

37. Cycle of Life: Digestive System • Changes in digestive function and structure are age-related • Result in diseases or pathological conditions • May occur in any segment of intestinal tract • 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

38. 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