Digestive System

1. Unit 12 Digestive System

2. Parts of the digestive system • Two main groups – alimentary canal and accessory digestive organs • Alimentary canal: • Aka gastrointestinal (GI) tract • Continuous, coiled, hollow, muscular tube that winds through the ventral body cavity • Open at both ends • Consists of mouth, pharynx, esophagus, stomach, small intestine, and large intestine

3. Mouth • Oral cavity  mucous membrane-lined cavity • Anterior opening protected by lips, lateral walls formed by cheeks • Hard palate forms anterior roof, soft palate forms posterior roof • Uvula = small, fleshy projection of soft palate at back of throat • Vestibule = space between lips and cheeks externally, space between teeth and gums internally • Oral cavity proper = space contained by teeth

4. Mouth • Food enters digestive tract • Contains tongue  secured to floor of mouth by lingual frenulum • Sometimes children are born with an extremely short lingual frenulum = “tongue-tied” • Food enters the mouth, mixes with saliva, and gets masticated (chewed) • Tongue initiates swallowing, and food moves on to pharynx

5. Mouth

6. Pharynx • Divided into nasopharynx (part of respiratory passageway), oropharynx (posterior to oral cavity), and laryngopharynx (continuous with esophagus) • Contains two skeletal muscle layers • Inner layer runs longitudinally (up and down) • Outer layer runs around in a circular pattern • Alternating contractions of the two muscle layers help the food move down into the esophagus = peristalsis

7. Esophagus • “Gullet” • Runs from pharynx to stomach through diaphragm • Four basic muscle layers: • 1. Mucosa – innermost layer, mainly surface epithelium • 2. Submucosa – just beneath mucosa, soft connective tissue with blood vessels, nerve endings, lymph nodes, and lymphatic vessels • 3. Muscularisexterna – next layer of muscle, typically made of an inner circular layer and an outer longitudinal layer of smooth muscle cells • 4. Serosa – Outermost layer, single layer of flat, fluid-producing cells (visceral peritoneum) which is continuous with the slippery parietal peritoneum lining the abdominal cavity

8. Esophagus • Alimentary canal wall contains two nerve plexuses: • Submucosal nerve plexus • Myenteric nerve plexus • Part of autonomic nervous system • Regulate mobility and secretory activity of GI tract organs

9. Stomach • C-shaped • Left side of abdominal cavity, hidden by liver and diaphragm • Cardiac region = area near heart • Fundus = expanded part of stomach lateral to cardiac region • Body = midportion • Pylorus = funnel-shaped terminal part of stomach • Rugae = large folds of mucosa that form when an empty stomach collapses inward on itself • Greater curvature = convex lateral surface • Lesser curvature = concave medial surface

10. Stomach • Lesser omentum = double layer of peritoneum, runs from liver to lesser curvature • Greater omentum = extension of peritoneum, drapes down over abdominal organs like a curtain or apron, attaches to posterior body wall • Contains gastric juice, which helps break down food • After food has been broken down, it resembles heavy cream and is called chyme • Food enters from the esophagus through the cardioesophageal sphincter  in cardiac region of stomach • Food exits to the small intestine through the pyloric sphincter, or valve  in pyloric section of stomach

11. Stomach • Intrinsic factor = substance produced by some stomach cells that helps small intestine absorb vitamin B12 • Chief cells = produce pepsinogens protein digesting enzymes • Parietal cells = produce hydrochloric acid • Mucous neck cells = produce a sticky alkaline mucus which protects the stomach wall from its acidic content

12. Stomach

13. Small intestine • Major digestive organ – nutrients get absorbed into bloodstream • Extends from pyloric sphincter to ileocecal valve • Usually ~21 feet long • Three sections: • Duodenum – curves around head of pancreas, ~10 inches long • Jejunum - ~8 feet long, middle section • Ileum – ~12 feet long • Chemical digestion of food really kicks in

14. Small intestine • Can only handle a small amount of food at a time • Pyloric sphincter acts as gatekeeper, controls how much food from the stomach enters the small intestine • Pancreas makes enzymes which enter small intestine through pancreatic ducts  help break down food • Liver makes bile which enters through the bile duct  also helps break down food

15. Small intestine • Three structures that increase absorptive surface: • Microvilli – tiny projections of the plasma membrane of the mucosa cells, gives a “fuzzy” appearance • Villi – fingerlike projections of the mucosa that give it a velvety appearance and feel • Circular folds – deep folds of both mucosa and submucosa layers • These structures decrease in number towards the end of the small intestine, while Peyer’s patches can be found more frequently  undigested food at that point has huge numbers of bacteria, which should not enter the bloodstream

16. Small intestine

17. Large intestine • Larger in diameter than small intestine, but shorter in length • Dries out indigestible food and eliminates it as feces • Subdivided into cecum, appendix, colon, rectum, and anal canal • Appendix can cause problems  usually twisted = ideal location for bacteria to accumulate  appendicitis • Ascending colon  transverse colon  descending colon  sigmoid colon  rectum  anus • Anal canal ends at anus, which opens to exterior

18. Accessory digestive organs • Not part of gastrointestinal tract, but still part of digestion • Can play major or minor roles in digestion

19. Salivary Glands • Three pairs: • Parotid glands – large glands that lie anterior to the ears, when they are inflamed  mumps • Submandibular glands – posterior floor of mouth • Sublingual glands – anterior floor of mouth • The glands produce saliva, a mixture of mucus and serous fluids • Helps moisten and bind food together into a bolus (a clump or mass), which is easier to chew and swall0w • Salivary amylase = enzyme that starts the process of starch digestion

20. Teeth • Help masticate (chew) food • Two sets: • Deciduous teeth (baby teeth) = start to erupt around 6 months, full set by 2 years, lost during childhood • Permanent teeth = enlarge and develop during childhood, roots of deciduous teeth are reabsorbed  tooth is “lost” • Most permanent teeth are present by the end of adolescence, but 3rd molars (wisdom teeth) don’t erupt until ~17-25

21. Teeth • Incisors = cutting • Canines = tearing • Premolars (bicuspids) and molars = grinding • Two major regions: • Crown = exposed part of tooth above gingiva (gum), covered by enamel • Root = below gum, connected to crown by “neck” region of tooth, outer surface covered by cementum, contains dentin (bonelike material) inside, surrounds a central pulp cavity, which contains blood vessels, nerves, connective tissue, etc.

22. Teeth

23. Pancreas • Soft, pink, triangular gland that extends across the abdomen from the spleen to the duodenum • Produces enzymes that are secreted into duodenum in an alkaline fluid  helps neutralize the acidic chyme from the stomach

24. Liver • Largest gland in body • Under diaphragm, more on right side of body • 4 lobes • Produces bile  a greenish-yellow or brownish fluid that helps digest lipids • Leaves liver and enters duodenum through common hepatic duct

25. Gallbladder • Small, thin-walled green sac located beneath liver • Bile is stored here while it is not aiding in digestion, water is removed  bile becomes more concentrated • If bile is stored too long, it can crystallize and become a gallstone

26. Gastrointestinal Processes and Controls • 1. Ingestion – active, voluntary process of putting food in the mouth • 2. Propulsion – moving food from one organ to the next • Peristalsis – involuntary movement of food due to waves of contraction and relaxation of smooth muscle • Segmentation – movement of food back and forth (usually in small intestine), which mixes it with digestive juices

27. Gastrointestinal Processes and Controls • 3. Food breakdown: mechanical digestion – Chewing in the mouth, churning food in the stomach, segmentation in the small intestine • 4. Food breakdown: chemical digestion – breaking food molecules into their molecular building blocks, i.e. simple sugars, amino acids, fatty acids, etc.

28. Gastrointestinal Processes and Controls • 5. Absorption – Transport of digested end products from the GI tract to the blood and/or lymph • 6. Defecation – elimination of indigestible substances from the body via the anus as feces

29. Activities occurring in the mouth, pharynx, and esophagus • Food ingestion and breakdown – mechanical (chewing) and chemical digestion (salivary amylase) • Food Propulsion – Swallowing and Peristalsis • Deglutition(swallowing) – a complicated process that involves the coordinated activity of several structures (tongue, soft palate, pharynx, and esophagus) • Buccal phase – voluntary, occurs in mouth, bolus is forced into pharynx by tongue • Pharyngeal-esophageal phase – involuntary, transports food from esophagus to tongue

30. Activities of the stomach • Food breakdown – gastric juice is secreted, regulated by neural and hormonal factors, breaks food down into chyme • Food propulsion – peristalsis occurs in lower half of stomach, moves chyme through pyloric sphincter into the small intestine • Emesis – vomiting, a reverse peristalsis in the stomach accompanied by contractions of the abdominal muscles and diaphragm

31. Activities of the small intestine • Food breakdown and absorption – food is chemically digested by enzymes, pancreatic juice, and bile. When chyme enters small intestine, it stimulates the release of two hormones: secretin and cholecystokinin (CCK), which control the release of bile and pancreatic juice. • Food propulsion – peristalsis moves food through the small intestine, segmentation helps digest and break down food

32. Activities of large intestine • Food breakdown and absorption – bacteria within intestine metabolize the few remaining nutrients, producing methane and hydrogen sulfide  contributes to odor of feces • Propulsion of residue and defecation – peristalsis is slower, mass movements are long, slow, powerful contractile waves that move over large areas of the colon and force contents towards rectum  occur 3-4 times per day, usually during or just after eating

33. Nutrition and Metabolism • Food is used as metabolic fuel  become ATP (adenosine triphosphate), the chemical cells use for energy • Energy value of food is measured in kilocalories (kcal) or “large calories” (C) • Nutrient = substance in food that is used by the body to promote natural growth, maintenance, and repair • Major nutrients = carbohydrates, lipids, and proteins • Also needed: vitamins, minerals, and water • Most foods contain a combination of nutrients

34. Dietary sources of major nutrients • Carbohydrates – most come from plants (sugars and starches), some from meat (glycogen), and milk (lactose) • Lipids – neutral fats, from animal products such as meat, eggs, and dairy (saturated fats) and plant products such as seeds, nuts, and vegetable oils (unsaturated fats) • Proteins – highest quality from animal products (eggs, milk, and meat), incomplete proteins from plant products (legumes, nuts, and cereals) • Vitamins – from a variety of foods, no one food contains all the vitamins we need to survive • Minerals – inorganic substances, mainly from vegetables, legumes, milk, and meats

35. Metabolism • All chemical reactions that are necessary to maintain life • Catabolism = substances are broken down into simpler substances • Anabolism = larger molecules are built from smaller ones

36. Carbohydrate, Fat, and Protein Metabolism in Body Cells • Glucose (blood sugar) is the major fuel for making ATP in the body • Aerobic respiration = oxygen is used, three main pathways: • Glycolysis = occurs in cytoplasm, produces 2 ATP • Krebs cycle = occurs in mitochondria, produces 2 ATP • Electron transport chain = occurs in mitochondria, produces 34 ATP • Anaerobic respiration = no oxygen is used, glycolysis only

37. Carbohydrate, Fat, and Protein Metabolism in Body Cells • Liver cells use fats (lipids) to make ATP for their own use • Fats also form myelin sheaths around neurons • Fats are the most concentrated form of energy in the body, broken down using acid like acetoacetic acid and acetone • Ketosis (acidosis) = blood becomes acidic because fat is the only source of energy being used, breath has a “fruity” odor

38. Carbohydrate, Fat, and Protein Metabolism in Body Cells • Proteins form the bulk of cell structure and most functional molecules  carefully conserved • Cells actively take up amino acids in order to have enough materials to build proteins • 20 amino acids total, 8 cannot be made by cells but must come from diet  essential amino acids

39. The Central Role of the Liver in Metabolism • Manufactures bile, detoxifies drugs and alcohol, degrades hormones, processes nutrients, and makes cholesterol, blood proteins, and lipoproteins • We have a surplus of liver tissue, and the liver can regenerate quickly and easily if it is damaged or partially removed • Hepatic portal circulation brings nutrient rich blood straight from digestive organs

40. The Central Role of the Liver in Metabolism • Glycogenesis = glycogen formation, glucose is removed from blood and turned into glycogen • Glycogenolysis = glycogen splitting, glycogen molecules are split into glucose molecules and released into blood • Gluconeogenesis = formation of new sugar, liver makes glucose out of noncarbohydrates, such as fats or proteins

41. The Central Role of the Liver in Metabolism • Cholesterol production – only 15% of cholesterol comes from diet, the other 85% is made by the liver • Cholesterol serves as the building blocks of steroid hormones and vitamin D • Low-density lipoproteins (LDLs) = transport cholesterol and other lipids to body cells, increase chance of depositing fatty substances in arteries  atherosclerosis • High-density lipoproteins (HDLs) = transport cholesterol from body cells to liver, where it will break down

42. Body Energy Balance • Ideally, energy intake = total energy output (heat + work + energy storage) • Energy intake = energy liberated during food oxidation (glycolysis, Krebs cycle, and ETC) • Energy output = energy lost as heat (60%), energy to do work, and energy stored as fat or glycogen • Basal metabolic rate (BMR) = amount of heat produced by the body per unit of time while at rest • Total metabolic rate (TMR) = total amount of kilocalories the body must consume to fuel all ongoing activities

43. Developmental Aspects of the Digestive System • Embryo folds to form a hollow cylinder  internal cavity becomes alimentary canal by 5 weeks • Digestive glands (salivary glands, liver, and pancreas) bud out of tube shortly after • Rooting reflex and sucking reflex help infant find food • Metabolism decreases with age • Stomach cancer and colon cancer are more common in the elderly • Taste and smell decline, periodontal disease can develop