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

3. Overview of the Digestive System • Organization of the digestive system • Organs of digestion • 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

4. Walls of GI Tract • 4 layers of tissues: • Mucosa Deep • Submucosa • Muscularis • Serosa Superficial • Modifications of layers— layers of the GI tract have various modifications to enable it to perform various functions

5. Mouth • 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 • Oral cavity (buccal cavity) • Hard Palate • consists of four bones • Essential for clear articulation • Soft Palate • made of muscle arranged in an arch • Helps with swallowing, yawning, and ear popping • Suspended from midpoint of posterior border of the arch is the uvula • Tongue— solid mass of skeletal muscle covered by a mucous membrane • Important for mastication (chewing) and deglutition (swallowing) • Has three parts: root, tip, and body • Stratified squamous epithelium • Lingual frenulum anchors tongue to floor of mouth

8. Mouth • Salivary glands • 3 pairs of compound tubuloalveolar glands • secrete 1 liter of saliva each day • >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

9. Teeth • Organs of mastication (chewing) • 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 (gums) • Outer shell contains two additional tissues: dentin and cementum • Dentin makes up the most of the tooth shell- harder than bone! • 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. Teeth • Types of teeth • Deciduous teeth (fall off) • 20 baby teeth • Permanent teeth • 32 teeth, replace the deciduous teeth by pushing them out

11. Pharynx & Esophagus • Pharynx • Tube through which a bolus passes when moved from the mouth to the esophagus by the process of deglutition (swallowing) • Esophagus • Tube that extends from the pharynx to the stomach • First segment of digestive tube

12. Stomach • Size and position of the stomach • Size varies according to factors such as gender and amount of distention (enlarged) • 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 abdominal cavity under liver and diaphragm

13. Stomach • Divisions of the stomach • Fundus— above opening of esophagus into stomach (roller coasters) • Body— central portion of stomach • Antrum— lowest part of stomach • Pylorus— narrowing where the stomach joins the small intestine

15. 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 • If LES does not remain properly closed after intake of food, stomach acid can spill into esophagus causing damage • This is acid reflux, or GERD (gastroesophageal reflux disease) • Pyloric sphincter controls outlet of pyloric portion of stomach into duodenum

16. 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 • Helps with vitamin B12 absorption • Endocrine cells— secrete gastrin and ghrelin (grel′in)

17. Stomach • Stomach wall • Gastric muscularis • thick layer of muscle with three distinct sublayers • this allows stomach to contract strongly at many angles

18. 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 • Limited absorption • Produces gastrin and ghrelin • Helps protect body from pathogenic bacteria swallowed with food

19. Small Intestine • Size and position of the small intestine • tube approximately 2.5 cm in diameter and 6 m in length • fills 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

20. Small Intestine • Wall of the small intestine • Intestinal lining has villi • Villi— important modifications of mucosal layer • Each villus contains an arteriole, venule, and lymphatic vessel • 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

21. Large Intestine • Also called colon • Size of the large intestine • average diameter, 6 cm • length, approximately 1.5 to 1.8 m

22. Large Intestine • 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 • valve prevents material passing from large intestine back into ileum • Transverse colon passes horizontally across abdomen, above small intestine • 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

23. Large Intestine • Wall of the large intestine • Intestinal mucous glands produce lubricating mucus that coats feces as they are formed • Uneven distribution of fibers in the muscle coat

24. Vermiform Appendix • Accessory organ of digestive system • 8 to 10 cm in length • communicates with cecum • Appendicitis • If becomes blocked  increased pressure • If not treated, the appendix will break and leak infection into the body • FYI: maintains a “safe house” for the beneficial bacteria living in the human gut  may help with immune system

25. Peritoneum • Large, continuous sheet of serous membrane • Membrane covering the surfaces of organs • allows movement of each and helps prevent strangulation of the GI tract

26. 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 • 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

27. Liver • Bile ducts • Small bile ducts form right and left hepatic ducts • Right and left hepatic ducts join  common hepatic duct • Hepatic duct merges with cystic duct  common bile duct  duodenum 1, Common bile duct; 2, common hepatic duct; 3, cystic duct; 4, gallbladder; 5, left hepatic duct; 6, liver shadow with tributaries of hepatic ducts; 7, right hepatic duct.

28. Liver • Functions of the liver • Detoxification by liver cells • 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 • Metabolism of proteins, fats, and carbohydrates • Storage of substances such as iron and fat soluble vitamins • Production of important plasma proteins

29. 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

30. Pancreas • Size and location of the pancreas • grayish pink–colored gland • 12 to 15 cm long • weighs approximately 60 g • from duodenum and behind stomach to spleen • Structure of the pancreas • composed of endocrine and exocrine glandular tissue • Exocrine portion (98%) • Secretes digestive enzymes • tiny ducts unite to form main pancreatic duct, which empties into duodenum • Endocrine portion (2%) • embedded between exocrine units called pancreatic islets • made up of alpha cells and beta cells • pass secretions into capillaries

31. Pancreas • Functions of the pancreas • Secretes digestive enzymes into duodenum • Beta cells secrete insulin • Alpha cells secrete glucagon

32. 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

33. Cycle of Life: Digestive System • Young age • Mumps common in children • swelling of the salivary glands • appendicitis more common in adolescents and then decreases with advancing age

34. Cycle of Life: Digestive System • Middle age • ulcers and gallbladder disease common • Old age • decreased digestive fluids • slowing of peristalsis • reduced physical activity lead to constipation and diverticulosis