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Understanding the Digestive System Functions

Explore the intricate workings of the digestive system, from the alimentary canal structure to the roles of organs like the liver, pancreas, and intestines. Discover the processes of digestion, absorption, and metabolism in detail.

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Understanding the Digestive System Functions

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  1. Chapter 15The Digestive SystemJeanelle F. Jimenez RN, BSN, CCRN

  2. DIGESTIVE SYSTEM (FIGURE 15-1) • Irregular tube called alimentary canal or gastrointestinal (GI) tract • Food must first be digested, then absorbed, and later metabolized

  3. WALL OF THE DIGESTIVE TRACT (FIGURE 15-2) • Digestive tract described as tube that extends from mouth to anus • Wall of the digestive tube is formed by four layers of tissue: • Mucosa—mucous epithelium • Submucosa—connective tissue • Muscularis—two layers of smooth muscle • Serosa—serous membrane that covers the outside of abdominal organs; it attaches the digestive tract to the wall of the abdominopelvic cavity by forming folds called mesenteries

  4. MOUTH • Roof—formed by hard palate (parts of maxillary and palatine bones) and soft palate, an arch-shaped muscle separating mouth from pharynx; uvula, a downward projection of soft palate (Figure 15-4) • Floor—formed by tongue and its muscles; papillae, small elevations on mucosa of tongue; taste buds, found in many papillae; lingual frenulum, fold of mucous membrane that helps anchor tongue to floor of mouth (Figure 15-4) • Typical tooth (Figure 15-5) • Three main parts—crown, neck, and root • Enamel, which covers the crown, is hardest tissue in body

  5. MOUTH • Types of teeth—incisors, cuspids, bicuspids, and tricuspids • Twenty teeth in temporary set; average age for cutting first tooth about 6 months; set complete at about 2 years of age • Thirty-two teeth in permanent set; 6 years about average age for starting to cut first permanent tooth; set complete usually between ages of 17 and 24 years (Figure 15-6)

  6. SALIVARY GLANDS (FIGURE 15-7) • Parotid glands—largest salivary glands • Submandibular glands—open into mouth on either side of frenulum • Sublingual glands—open into floor of mouth

  7. PHARYNX • Subdivided into three anatomical components: • Nasopharynx • Oropharynx • Laryngopharynx

  8. ESOPHAGUS • Connects pharynx to stomach • Dynamic passageway for food

  9. STOMACH (Figure 15-8) • Size—expands after large meal; about size of large sausage when empty • Food enters stomach through gastroesophageal (cardiac) sphincter • Pyloric sphincter muscle closes opening between pylorus (lower part of stomach) and duodenum • Wall—many smooth muscle fibers; contractions produce churning movements (peristalsis) • Lining—mucous membrane; many microscopic glands that secrete gastric juice and hydrochloric acid into stomach; mucous membrane lies in folds (rugae) when stomach is empty

  10. SMALL INTESTINE (FIGURE 15-9) • Size—about 7 meters (20 feet) long but only 2 cm or so in diameter • Divisions • Duodenum • Jejunum • Ileum

  11. SMALL INTESTINE • Wall—contains smooth muscle fibers that contract to produce peristalsis • Lining—mucous membrane; many microscopic glands (intestinal glands) secrete intestinal juice; villi (microscopic finger-shaped projections from surface of mucosa into intestinal cavity) contain blood and lymph capillaries

  12. LIVER AND GALLBLADDER • Size and location—liver is largest gland; fills upper right section of abdominal cavity and extends over into left side • Liver secretes bile • Ducts (Figure 15-10) • Hepatic—drains bile from liver • Cystic—duct by which bile enters and leaves gallbladder • Common bile—formed by union of hepatic and cystic ducts; drains bile from hepatic or cystic ducts into duodenum • Gallbladder • Location—undersurface of the liver • Function—concentrates and stores bile produced in the liver

  13. PANCREAS • Exocrine gland that lies behind stomach • Functions • Pancreatic cells secrete pancreatic juice (most important digestive juice) into pancreatic ducts; main duct empties into duodenum • Pancreatic islets (of Langerhans)—cells not connected with pancreatic ducts; secrete hormones glucagon and insulin into the blood

  14. LARGE INTESTINE (FIGURE 15-12) • Divisions • Cecum • Colon—ascending, transverse, descending, and sigmoid • Rectum • Food enters through ileocecal valve; external opening called anus • Wall—contains smooth muscle fibers that contract to produce churning, peristalsis, and defecation • Lining—mucous membrane

  15. APPENDIX • Blind tube off cecum • No important digestive functions in humans

  16. PERITONEUM (FIGURE 15-14) • Definitions—peritoneum, serous membrane lining abdominal cavity and covering abdominal organs; parietal layer of peritoneum lines abdominal cavity; visceral layer of peritoneum covers abdominal organs; peritoneal space lies between parietal and visceral layers • Extensions—largest are the mesentery and greater omentum • Mesentery is extension of parietal peritoneum, which attaches most of small intestine to posterior abdominal wall • Greater omentum, or “lace apron,” hangs down from lower edge of stomach and transverse colon over intestines • X-ray studies of the GI tract—radiopaque contrast medium used to help visualize structures in study images

  17. DIGESTION (TABLE 15-2) • Definition—transforms foods into substances that can be absorbed and used by cells • Mechanical digestion—chewing (mastication), swallowing (deglutition), and peristalsis break food into tiny particles, mix them well with digestive juices, and move them along the digestive tract • Chemical digestion—breaks up large food molecules into compounds that have smaller molecules; brought about by digestive enzymes (Figure 15-15) • Enzymes and chemical digestion • Enzymes are specialized protein molecules that act as catalysts • Breakdown process called hydrolysis

  18. DIGESTION • Carbohydrate digestion—mainly in small intestine • Pancreatic amylase—breaks polysaccharides down to disaccharides • Intestinal juice enzymes • Maltase—changes maltose to glucose • Sucrase—changes sucrose to glucose • Lactase—changes lactose to glucose

  19. DIGESTION • Protein digestion—starts in stomach; completed in small intestine • Gastric juice enzyme pepsin partially digests proteins • Pancreatic enzyme, trypsin, continues digestion of proteins • Intestinal enzymes, peptidases, complete digestion of partially digested proteins and convert them to amino acids • Fat digestion • Bile contains no enzymes but emulsifies fats (breaks fat droplets into very small droplets) • Pancreatic lipase changes emulsified fats to fatty acids and glycerol in small intestine

  20. ABSORPTION • Definition—process by which digested food moves from intestine into blood or lymph • Foods and most water minerals and vitamins are absorbed from small intestine; some water and vitamin K also absorbed from large intestine • Surface area absorption • Structural adaptations increase absorptive surface area • Fractal geometry—study of fragmented geometric irregular shapes such as those in lining of intestine

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