The Digestive System


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The Digestive System

  1. 1. Chapter 24: The Digestive System
  2. 2. Digestive Tract <ul><li>Gastrointestinal (GI) tract or alimentary canal </li></ul><ul><li>Is a muscular tube </li></ul><ul><li>Extends from oral cavity to anus </li></ul>Components of the Digestive System
  3. 3. 6 Functions of the Digestive System <ul><li>Ingestion : </li></ul><ul><ul><li>occurs when materials enter digestive tract via the mouth </li></ul></ul><ul><li>Mechanical processing : </li></ul><ul><ul><li>crushing and shearing </li></ul></ul><ul><ul><li>makes materials easier to propel along digestive tract </li></ul></ul><ul><li>Digestion : </li></ul><ul><ul><li>is the chemical breakdown of food </li></ul></ul><ul><ul><li>into small organic fragments </li></ul></ul><ul><ul><li>for absorption by digestive epithelium </li></ul></ul>
  4. 4. <ul><li>Secretion : </li></ul><ul><ul><li>is the release of water, acids, enzymes, buffers, and salts </li></ul></ul><ul><ul><li>by epithelium of digestive tract </li></ul></ul><ul><ul><li>by glandular organs </li></ul></ul><ul><li>Absorption : </li></ul><ul><ul><li>movement of organic substrates, electrolytes, vitamins, and water </li></ul></ul><ul><ul><li>across digestive epithelium </li></ul></ul><ul><ul><li>into interstitial fluid of digestive tract </li></ul></ul><ul><li>Excretion : </li></ul><ul><ul><li>removal of waste products from body fluids </li></ul></ul>
  5. 5. Lining of the Digestive Tract <ul><li>Protects surrounding tissues against: </li></ul><ul><ul><li>corrosive effects of digestive acids and enzymes </li></ul></ul><ul><ul><li>mechanical stresses, such as abrasion </li></ul></ul><ul><ul><li>bacteria </li></ul></ul><ul><ul><ul><li>Is ingested with food or resides in digestive tract </li></ul></ul></ul><ul><ul><ul><li>Attacked by macrophages, and immune system cells </li></ul></ul></ul><ul><ul><ul><li>In lamina propria (underlying layer of areolar tissue) </li></ul></ul></ul>
  6. 6. Peritoneal Cavity <ul><li>Is located within the abdominopelvic cavity </li></ul><ul><li>Lined with serous membrane consisting of: </li></ul><ul><ul><li>superficial mesothelium covering a layer of areolar tissue </li></ul></ul><ul><ul><li>Divisions of the Serous Membrane </li></ul></ul><ul><ul><ul><li>Serosa, or visceral peritoneum </li></ul></ul></ul><ul><ul><ul><li>covers organs within peritoneal cavity </li></ul></ul></ul><ul><li>Parietal peritoneum : </li></ul><ul><ul><li>lines inner surfaces of body wall </li></ul></ul>
  7. 7. Peritoneal Fluid <ul><li>Is produced by serous membrane lining </li></ul><ul><li>Provides essential lubrication </li></ul><ul><li>Separates parietal and visceral surfaces </li></ul><ul><li>Allows sliding without friction or irritation </li></ul>
  8. 8. Mesenteries Figure 24–2c, d
  9. 9. Mesenteries <ul><li>Are double sheets of peritoneal membrane </li></ul><ul><li>Suspend portions of digestive tract within peritoneal cavity by sheets of serous membrane: </li></ul><ul><ul><li>that connect parietal peritoneum </li></ul></ul><ul><ul><li>with visceral peritoneum </li></ul></ul>
  10. 10. Mesenteries <ul><li>Areolar tissue between mesothelial surfaces </li></ul><ul><li>Prevent intestines from becoming entangled </li></ul>
  11. 11. Adult Ventral Mesentery <ul><li>Persists in 2 places: </li></ul><ul><ul><li>between stomach and liver ( lesser omentum ) </li></ul></ul><ul><ul><ul><li>Stabilizes position of stomach </li></ul></ul></ul><ul><ul><ul><li>Provides access route for blood vessels and other structures entering or leaving liver </li></ul></ul></ul><ul><ul><li>between liver and anterior abdominal wall ( falciform ligament ) </li></ul></ul><ul><ul><ul><li>Helps stabilize position of liver: </li></ul></ul></ul><ul><ul><ul><li>relative to diaphragm and abdominal wall </li></ul></ul></ul>
  12. 12. The Dorsal Mesentery <ul><li>Is on ventral surface of stomach </li></ul><ul><li>Enlarges to form an enormous pouch, called the greater omentum </li></ul><ul><ul><li>Extends inferiorly between: </li></ul></ul><ul><ul><ul><li>the body wall and the anterior surface of small intestine </li></ul></ul></ul><ul><ul><li>Hangs like an apron: </li></ul></ul><ul><ul><ul><li>from lateral and inferior borders of stomach </li></ul></ul></ul><ul><li>Adipose tissue in greater omentum: </li></ul><ul><ul><li>conforms to shapes of surrounding organs </li></ul></ul><ul><ul><li>pads and protects surfaces of abdomen </li></ul></ul><ul><ul><li>provides insulation to reduce heat loss </li></ul></ul><ul><ul><li>stores lipid energy reserves </li></ul></ul>
  13. 14. The Mesentery Proper <ul><li>Is a thick mesenterial sheet </li></ul><ul><li>Provides stability </li></ul><ul><li>Permits some independent movement </li></ul><ul><li>Is associated with initial portion of small intestine ( duodenum ) and pancreas </li></ul><ul><li>Fuses with posterior abdominal wall, locking structures in position </li></ul>
  14. 15. Histological Organization of the Digestive Tract <ul><li>Major layers of the digestive tract: </li></ul><ul><ul><li>mucosa </li></ul></ul><ul><ul><li>submucosa </li></ul></ul><ul><ul><li>muscularis externa </li></ul></ul><ul><ul><li>serosa </li></ul></ul>
  15. 16. The Mucosa <ul><li>Is the inner lining of digestive tract </li></ul><ul><li>Is a mucous membrane consisting of: </li></ul><ul><ul><li>epithelium, moistened by glandular secretions </li></ul></ul><ul><li>Lamina propria composed of areolar tissue </li></ul>
  16. 17. The Digestive Epithelium <ul><li>Mucosal epithelium is simple or stratified: </li></ul><ul><ul><li>depending on location, function, and stresses </li></ul></ul><ul><li>Oral cavity, pharynx, and esophagus: </li></ul><ul><ul><li>mechanical stresses </li></ul></ul><ul><ul><li>lined by stratified squamous epithelium </li></ul></ul><ul><li>Stomach, small intestine, and most of large intestine: </li></ul><ul><ul><li>Absorption </li></ul></ul><ul><ul><li>simple columnar epithelium with goblet cells </li></ul></ul>
  17. 18. Lining of Digestive Tract <ul><li>Folding increases surface area for absorption: </li></ul><ul><ul><li>longitudinal folds, disappear as digestive tract fills </li></ul></ul><ul><ul><li>permanent transverse folds ( plicae ) </li></ul></ul>
  18. 19. <ul><li>Consists of a layer of areolar tissue that contains: </li></ul><ul><ul><li>blood vessels </li></ul></ul><ul><ul><li>sensory nerve endings </li></ul></ul><ul><ul><li>lymphatic vessels </li></ul></ul><ul><ul><li>smooth muscle cells </li></ul></ul><ul><ul><li>scattered areas of lymphoid tissue </li></ul></ul>The Lamina Propria
  19. 20. Muscularis Mucosae <ul><li>Narrow band of smooth muscle and elastic fibers in lamina propria </li></ul><ul><li>Smooth muscle cells arranged in 2 concentric layers: </li></ul>
  20. 21. The Submucosa <ul><li>Is a layer of dense irregular connective tissue </li></ul><ul><li>Surrounds muscularis mucosae </li></ul><ul><li>Has large blood vessels and lymphatic vessels </li></ul><ul><li>May contain exocrine glands: </li></ul><ul><ul><li>secrete buffers and enzymes into digestive tract </li></ul></ul>
  21. 22. Submucosal Plexus <ul><li>Also called plexus of Meissner </li></ul><ul><li>Innervates the mucosa and submucosa </li></ul>
  22. 23. Muscularis Externa Structure <ul><li>Is dominated by smooth muscle cells </li></ul><ul><li>Involved in: </li></ul><ul><ul><li>mechanical processing </li></ul></ul><ul><ul><li>movement of materials along digestive tract </li></ul></ul><ul><li>Movements coordinated by enteric nervous system (ENS) </li></ul>
  23. 24. The Serosa <ul><li>Serous membrane covering muscularis externa: </li></ul><ul><ul><li>except in oral cavity, pharynx, esophagus, and rectum </li></ul></ul>
  24. 25. The Movement of Digestive Materials <ul><li>By muscular layers of digestive tract: </li></ul><ul><ul><li>consist of visceral smooth muscle tissue </li></ul></ul>Smooth Muscle <ul><li>Along digestive tract: </li></ul><ul><ul><li>has rhythmic cycles of activity </li></ul></ul><ul><ul><li>controlled by pacesetter cells </li></ul></ul><ul><li>Cells undergo spontaneous depolarization: </li></ul><ul><ul><li>triggering wave of contraction through entire muscular sheet </li></ul></ul>
  25. 26. Pacesetter Cells <ul><li>Located in muscularis mucosae and muscularis externa: </li></ul><ul><ul><li>surrounding lumen of digestive tract </li></ul></ul>
  26. 27. Peristalsis <ul><li>Consists of waves of muscular contractions </li></ul><ul><li>Moves a bolus along the length of the digestive tract </li></ul>Figure 24–4 <ul><li>Bolus is a small, oval mass of digestive contents </li></ul>
  27. 28. Peristaltic Motion <ul><li>Circular muscles contract behind bolus: </li></ul><ul><ul><li>while circular muscles ahead of bolus relax </li></ul></ul><ul><li>Longitudinal muscles ahead of bolus contract: </li></ul><ul><ul><li>shortening adjacent segments </li></ul></ul><ul><li>Wave of contraction in circular muscles: </li></ul><ul><ul><li>forces bolus forward </li></ul></ul>
  28. 29. Segmentation <ul><li>Cycles of contraction: </li></ul><ul><ul><li>Churn and fragment bolus </li></ul></ul><ul><ul><li>mix contents with intestinal secretions </li></ul></ul><ul><li>Does not follow a set pattern: </li></ul><ul><ul><li>does not push materials in any 1 direction </li></ul></ul>
  29. 30. Control of Digestive Function <ul><li>Neural mechanisms </li></ul><ul><li>Hormonal mechanisms </li></ul><ul><li>Local mechanisms </li></ul>
  30. 31. Neural Mechanisms <ul><li>Control: </li></ul><ul><ul><li>movement of materials along digestive tract </li></ul></ul><ul><ul><li>secretory functions </li></ul></ul><ul><li>Motor neurons: </li></ul><ul><ul><li>control smooth muscle contraction and glandular secretion </li></ul></ul><ul><ul><li>located in myenteric plexus </li></ul></ul>
  31. 32. Digestive Hormones Mechanisms <ul><li>At least 18 hormones that affect: </li></ul><ul><ul><li>most aspects of digestive function </li></ul></ul><ul><ul><li>activities of other systems </li></ul></ul><ul><li>Are peptides </li></ul><ul><li>Are produced by enteroendocrine cells in digestive tract </li></ul><ul><li>Reach target organs after distribution in bloodstream </li></ul>
  32. 33. Local Mechanisms <ul><li>Prostaglandins, histamine, and other chemicals </li></ul><ul><li>Released into interstitial fluid </li></ul><ul><li>Affect adjacent cells within small segment of digestive tract </li></ul><ul><li>Coordinating response to changing conditions: </li></ul><ul><ul><li>e.g., variations in local pH, chemical, or physical stimuli </li></ul></ul><ul><li>Affect only a portion of tract </li></ul>
  33. 34. The Oral Cavity Figure 24–6
  34. 35. 4 Functions of the Oral Cavity <ul><li>Lubrication : </li></ul><ul><ul><li>mixing with mucus and salivary gland secretions </li></ul></ul><ul><li>Limited digestion : </li></ul><ul><ul><li>of carbohydrates and lipids </li></ul></ul><ul><li>Sensory analysis: </li></ul><ul><ul><li>of material before swallowing </li></ul></ul><ul><li>Mechanical processing: </li></ul><ul><ul><li>through actions of teeth, tongue, and palatal surfaces </li></ul></ul>
  35. 36. Oral Mucosa <ul><li>Lining of oral cavity </li></ul><ul><li>Has stratified squamous epithelium </li></ul>Layer of Keratinized Cells <ul><li>Covers only regions exposed to severe abrasion </li></ul>The Epithelial Lining <ul><li>Of cheeks, lips, and inferior surface of tongue: </li></ul><ul><ul><li>is relatively thin, nonkeratinized, and delicate </li></ul></ul>
  36. 37. Mucosa Inferior to the Tongue <ul><li>Is thin and vascular enough to rapidly absorb lipid-soluble drugs </li></ul>The Mucosae of the Cheeks <ul><li>Are supported by pads of fat and the buccinator muscles </li></ul>Labia <ul><li>Also called lips </li></ul><ul><li>Anteriorly, the mucosa of each cheek is continuous with that of the lips </li></ul>
  37. 38. Vestibule <ul><li>Space between the cheeks (or lips) and the teeth </li></ul>Gingivae (Gums) <ul><li>Ridges of oral mucosa </li></ul><ul><li>Surround base of each tooth on alveolar processes of maxillary bones and mandible </li></ul>The Oral Cavity <ul><li>Roof formed by hard and soft palates </li></ul><ul><li>Tongue dominates the floor </li></ul>
  38. 39. Hard Palate <ul><li>Is formed by: </li></ul><ul><ul><li>palatine processes of maxillary bones </li></ul></ul><ul><ul><li>horizontal plates of palatine bones </li></ul></ul>Raphe <ul><li>Prominent central ridge </li></ul><ul><li>Extends along midline of hard palate </li></ul>Mucosae <ul><li>Mucosa lateral and anterior to raphe: </li></ul><ul><ul><li>is thick, with complex ridges </li></ul></ul><ul><li>Thin, delicate mucosa: </li></ul><ul><ul><li>covers posterior margin of hard palate </li></ul></ul><ul><ul><li>extends onto soft palate </li></ul></ul>
  39. 40. Uvula <ul><li>A dangling process </li></ul><ul><li>Helps prevent food from entering pharynx prematurely </li></ul><ul><li>Is supported by posterior margin of soft palate </li></ul>Soft Palate <ul><li>Lies posterior to hard palate </li></ul>
  40. 42. Tonsil <ul><li>Lies between palatoglossal and palatopharyngeal arches, on each side </li></ul>
  41. 43. The Tongue <ul><li>Manipulates materials inside mouth </li></ul><ul><li>May bring foods into oral cavity </li></ul>4 Functions of the Tongue <ul><li>Mechanical processing: </li></ul><ul><ul><li>compression, abrasion, and distortion </li></ul></ul><ul><li>Manipulation: </li></ul><ul><ul><li>assists in chewing </li></ul></ul><ul><ul><li>prepares material for swallowing </li></ul></ul><ul><li>Sensory analysis: </li></ul><ul><ul><li>touch, temperature, and taste receptors </li></ul></ul><ul><li>Secretion: </li></ul><ul><ul><li>mucins </li></ul></ul><ul><ul><li>enzyme lingual lipase </li></ul></ul>
  42. 44. Structure of the Tongue <ul><li>Anterior body ( oral portion ) </li></ul><ul><li>Posterior root ( pharyngeal portion ) </li></ul>Lingual Papillae <ul><li>Fine projections on superior surface ( dorsum ) of tongue </li></ul><ul><li>Covered in thick epithelium </li></ul><ul><li>Assists in moving materials </li></ul><ul><li>In the oropharynx </li></ul><ul><li>A V-shaped line </li></ul><ul><li>Rough boundary between body and root of tongue </li></ul>Circumvallate Papillae
  43. 45. Lingual Frenulum <ul><li>Is a thin fold of mucous membrane along inferior midline </li></ul><ul><li>Prevents extreme movements of the tongue </li></ul><ul><li>Connects: </li></ul><ul><ul><li>body of tongue to floor of oral cavity </li></ul></ul>Sublingual Glands <ul><li>Small glands extend into underlying lamina propria </li></ul><ul><li>Secretions flush tongue’s epithelium </li></ul><ul><li>Contain water, mucins, and enzyme lingual lipase </li></ul>
  44. 46. The Salivary Glands <ul><li>3 pairs secrete into oral cavity </li></ul><ul><li>Each pair has distinctive cellular organization: </li></ul><ul><ul><li>and produces saliva with different properties </li></ul></ul>
  45. 47. Parotid Salivary Glands <ul><li>Inferior to zygomatic arch </li></ul><ul><li>Produce serous secretion: </li></ul><ul><ul><li>enzyme salivary amylase (breaks down starches) </li></ul></ul><ul><li>Drained by parotid duct ( Stensen’s duct ): </li></ul><ul><ul><li>which empties into vestibule at second molar </li></ul></ul>Sublingual Salivary Glands <ul><li>Covered by mucous membrane of floor of mouth </li></ul><ul><li>Produce mucous secretion: </li></ul><ul><ul><li>buffer and lubricant </li></ul></ul><ul><li>Sublingual ducts ( Rivinus’ ducts ): </li></ul><ul><ul><li>either side of lingual frenulum </li></ul></ul>
  46. 48. Submandibular Salivary Glands <ul><li>In floor of mouth </li></ul><ul><li>Within mandibular groove </li></ul><ul><li>Secrete buffers, glycoproteins ( mucins ), and salivary amylase </li></ul><ul><li>Submandibular ducts ( Wharton’s ducts ): </li></ul><ul><ul><li>open immediately posterior to teeth </li></ul></ul><ul><ul><li>either side of lingual frenulum </li></ul></ul>
  47. 49. Salivary Glands <ul><li>Produce 1.0–1.5 liters of saliva each day: </li></ul><ul><ul><li>70% by submandibular glands </li></ul></ul><ul><ul><li>25% by parotids </li></ul></ul><ul><ul><li>5% by sublingual glands </li></ul></ul>Saliva <ul><li>99.4% water </li></ul><ul><li>0.6% includes: </li></ul><ul><ul><li>electrolytes (Na + , Cl — , and HCO 3 — ) </li></ul></ul><ul><ul><li>buffers </li></ul></ul><ul><ul><li>glycoproteins (mucins) </li></ul></ul><ul><ul><li>antibodies </li></ul></ul><ul><ul><li>enzymes </li></ul></ul><ul><ul><li>waste products </li></ul></ul>
  48. 50. 4 Functions of Saliva <ul><li>Lubricating the mouth </li></ul><ul><li>Moistening and lubricating materials in the mouth </li></ul><ul><li>Dissolving chemicals that: </li></ul><ul><ul><li>stimulate taste buds </li></ul></ul><ul><ul><li>provide sensory information </li></ul></ul><ul><ul><li>4. Initiate digestion of: </li></ul></ul><ul><ul><li>complex carbohydrates by enzyme salivary amylase (ptyalin or alpha-amylase) </li></ul></ul><ul><ul><li>lipids by enzyme lingual lipase </li></ul></ul>
  49. 51. Teeth Figure 24–8
  50. 52. The Teeth <ul><li>Tongue movements pass food across occlusal surfaces of teeth </li></ul><ul><li>Chew ( masticate ) food </li></ul>Tooth Structure <ul><li>Dentin : </li></ul><ul><ul><li>a mineralized matrix similar to that of bone </li></ul></ul><ul><ul><li>does not contain cells </li></ul></ul><ul><li>Pulp cavity : </li></ul><ul><ul><li>receives blood vessels and nerves through the root canal </li></ul></ul>
  51. 53. Root Canal <ul><li>A narrow tunnel located at root of tooth </li></ul><ul><li>Blood vessels and nerves enter root canal through apical foramen to supply pulp cavity </li></ul>
  52. 54. The Root <ul><li>Of each tooth sits in a bony socket ( alveolus ) </li></ul><ul><li>A layer of cementum covers dentin of the root: </li></ul><ul><ul><li>providing protection and anchoring periodontal ligament </li></ul></ul>The Neck <ul><li>Of the tooth marks boundary between root and crown </li></ul>
  53. 55. Gingival Sulcus <ul><li>A shallow groove surrounding the neck of each tooth </li></ul>The Crown <ul><li>Exposed portion of tooth </li></ul><ul><li>Projects beyond soft tissue of gingiva </li></ul><ul><li>Dentin covered by layer of enamel </li></ul>
  54. 56. Dental Arches <ul><li>Contain 4 types of teeth: </li></ul><ul><ul><li>incisors </li></ul></ul><ul><ul><li>cuspids ( canines ) </li></ul></ul><ul><ul><li>bicuspids ( premolars ) </li></ul></ul><ul><ul><li>molars </li></ul></ul>
  55. 57. Incisors <ul><li>Blade-shaped teeth </li></ul><ul><li>Located at front of mouth </li></ul><ul><li>Used for clipping or cutting </li></ul><ul><li>Have a single root </li></ul>
  56. 58. Cuspids (Canines) <ul><li>Conical </li></ul><ul><li>Sharp ridgeline </li></ul><ul><li>Pointed tip </li></ul><ul><li>Used for tearing or slashing </li></ul><ul><li>Have a single root </li></ul>
  57. 59. Bicuspids (Premolars) <ul><li>Flattened crowns </li></ul><ul><li>Prominent ridges </li></ul><ul><li>Used to crush, mash, and grind </li></ul><ul><li>Have 1 or 2 roots </li></ul>
  58. 60. Molars <ul><li>Very large, flat crowns </li></ul><ul><li>With prominent ridges </li></ul><ul><li>Used for crushing and grinding </li></ul><ul><li>Have 3 or more roots </li></ul>
  59. 61. Primary and Secondary Dentitions Figure 24–9
  60. 62. Dental Succession <ul><li>During embryonic development, 2 sets of teeth form: </li></ul><ul><ul><li>primary dentition, or deciduous teeth </li></ul></ul><ul><ul><li>secondary dentition, or permanent dentition </li></ul></ul>Deciduous Teeth <ul><li>Also called primary teeth, milk teeth, or baby teeth </li></ul><ul><li>20 temporary teeth of primary dentition </li></ul><ul><li>5 on each side of upper and lower jaws: </li></ul><ul><ul><li>2 incisors </li></ul></ul><ul><ul><li>1 cuspid </li></ul></ul><ul><ul><li>2 deciduous molars </li></ul></ul>
  61. 63. Permanent Dentition <ul><li>Also called secondary dentition </li></ul><ul><li>Replaces deciduous teeth </li></ul><ul><li>32 permanent teeth </li></ul><ul><li>8 on each side, upper and lower: </li></ul><ul><ul><li>2 incisors </li></ul></ul><ul><ul><li>1 cuspid </li></ul></ul><ul><ul><li>5 molars </li></ul></ul>
  62. 64. Mastication <ul><li>Also called chewing </li></ul><ul><li>Food is forced from oral cavity to vestibule and back: </li></ul><ul><ul><li>crossing and recrossing occlusal surfaces </li></ul></ul>Muscles of Mastication <ul><li>Close the jaws </li></ul><ul><li>Slide or rock lower jaw from side to side </li></ul><ul><li>Chewing involves mandibular: </li></ul><ul><ul><li>elevation and depression </li></ul></ul><ul><ul><li>protraction and retraction </li></ul></ul><ul><ul><li>medial and lateral movement </li></ul></ul>
  63. 65. What are the anatomy and functions of the pharynx? <ul><li>A common passageway for solid food, liquids, and air </li></ul>The Pharynx Regions of the Pharynx <ul><li>Nasopharynx </li></ul><ul><li>Oropharynx </li></ul><ul><li>Laryngopharynx </li></ul><ul><li>Food passes through oropharynx and laryngopharynx to esophagus </li></ul>
  64. 66. Anatomy and functions of the esophagus
  65. 67. The Esophagus <ul><li>A hollow muscular tube </li></ul><ul><li>About 25 cm long and 2 cm wide </li></ul><ul><li>Conveys solid food and liquids to the stomach </li></ul><ul><li>Begins posterior to cricoid cartilage </li></ul><ul><li>Is innervated by fibers from the esophageal plexus </li></ul>
  66. 68. 5 Characteristics of the Esophageal Wall <ul><li>Mucosa contains: </li></ul><ul><ul><li>nonkeratinized and stratified squamous epithelium </li></ul></ul><ul><li>Mucosa and submucosa: </li></ul><ul><ul><li>form large folds that extend the length of the esophagus </li></ul></ul><ul><li>Muscularis mucosae : </li></ul><ul><ul><li>consists of irregular layer of smooth muscle </li></ul></ul><ul><li>Submucosa contains esophageal glands : </li></ul><ul><ul><li>which produce mucous secretion </li></ul></ul><ul><ul><li>reduces friction between bolus and esophageal lining </li></ul></ul><ul><li>Muscularis externa : </li></ul><ul><ul><li>has usual inner circular and outer longitudinal layers </li></ul></ul>
  67. 69. The Swallowing Process Figure 24–11
  68. 70. Swallowing <ul><li>Also called deglutition </li></ul><ul><li>Can be initiated voluntarily </li></ul><ul><li>Proceeds automatically </li></ul><ul><li>Is divided in 3 phases: </li></ul><ul><ul><li>Buccal </li></ul></ul><ul><ul><ul><li>Compression of bolus against hard palate </li></ul></ul></ul><ul><ul><ul><li>Retraction of tongue forces bolus into oropharynx: </li></ul></ul></ul><ul><ul><ul><li>assists elevation of soft palate </li></ul></ul></ul><ul><ul><ul><li>seals off nasopharynx </li></ul></ul></ul><ul><ul><li>Pharyngeal </li></ul></ul><ul><ul><ul><li>Bolus contacts: </li></ul></ul></ul><ul><ul><ul><li>palatoglossal arches </li></ul></ul></ul><ul><ul><ul><li>palatopharyngeal arches </li></ul></ul></ul><ul><ul><ul><li>posterior pharyngeal wall </li></ul></ul></ul><ul><ul><li>Esophogeal </li></ul></ul><ul><ul><ul><li>Contraction of pharyngeal muscles forces bolus through entrance to esophagus </li></ul></ul></ul>
  69. 71. Stomach 4 Functions of the Stomach <ul><li>Storage of ingested food </li></ul><ul><li>Mechanical breakdown of ingested food </li></ul><ul><li>Disruption of chemical bonds in food material: </li></ul><ul><ul><li>by acids and enzymes </li></ul></ul><ul><li>4. Production of intrinsic factor : </li></ul><ul><ul><li>glycoprotein required for absorption of vitamin B 12 in small intestine </li></ul></ul>
  70. 72. The Stomach Figure 24–12a
  71. 73. Anatomy of the Stomach <ul><li>The stomach is shaped like an expanded J: </li></ul><ul><ul><li>short lesser curvature forms medial surface </li></ul></ul><ul><ul><li>long greater curvature forms lateral surface </li></ul></ul><ul><li>Anterior and posterior surfaces are smoothly rounded </li></ul><ul><li>Shape and size vary: </li></ul><ul><ul><li>from individual to individual </li></ul></ul><ul><ul><li>from 1 meal to the next </li></ul></ul><ul><li>Stomach typically extends between levels of vertebrae T 7 and L 3 </li></ul>
  72. 74. 4 Regions of the Stomach <ul><li>Cardia </li></ul><ul><li>Fundus </li></ul><ul><li>Body </li></ul><ul><li>Pylorus </li></ul>
  73. 75. Smooth Muscle <ul><li>Muscularis mucosae and muscularis externa : </li></ul><ul><ul><li>contain extra layers of smooth muscle cells </li></ul></ul><ul><ul><li>in addition to circular and longitudinal layers </li></ul></ul>
  74. 76. The Stomach Lining <ul><li>Are shallow depressions that open onto the gastric surface </li></ul><ul><li>Mucous cells: </li></ul><ul><ul><li>at base, or neck, of each gastric pit </li></ul></ul><ul><ul><li>actively divide, replacing superficial cells </li></ul></ul>Figure 24–13 Gastric Pits
  75. 77. Gastric Glands <ul><li>In fundus and body of stomach: </li></ul><ul><ul><li>extend deep into underlying lamina propria </li></ul></ul><ul><li>Each gastric pit communicates with several gastric glands </li></ul>2 Types of Secretory Cells <ul><li>In gastric glands: both secrete HCl </li></ul><ul><ul><li>parietal cells </li></ul></ul><ul><ul><li>chief cells </li></ul></ul>
  76. 78. Small intestine and Associated Glandular Organs <ul><li>Stomach : </li></ul><ul><ul><li>gastric juices </li></ul></ul><ul><ul><li>stomach acids </li></ul></ul><ul><ul><li>pepsin </li></ul></ul><ul><li>Pancreas : </li></ul><ul><ul><li>digestive enzymes </li></ul></ul><ul><ul><li>buffers </li></ul></ul><ul><li>Liver : </li></ul><ul><ul><li>Bile </li></ul></ul><ul><ul><ul><li>Produced in liver </li></ul></ul></ul><ul><ul><li>Contains buffers and bile salts </li></ul></ul><ul><ul><li>Stored in gallbladder </li></ul></ul><ul><ul><li>Discharge into small intestine </li></ul></ul>
  77. 79. Segments of the Intestine Figure 24–16
  78. 80. The Small Intestine <ul><li>Plays key role in digestion and absorption of nutrients </li></ul><ul><li>90% of nutrient absorption occurs in the small intestine </li></ul>The Duodenum <ul><li>The segment of small intestine closest to stomach </li></ul><ul><li>25 cm (10 in.) long </li></ul><ul><li>“ Mixing bowl” that receives: </li></ul><ul><ul><li>chyme from stomach (Mixture of secretions and food in the stomach) </li></ul></ul><ul><ul><li>digestive secretions from pancreas and liver </li></ul></ul>
  79. 81. The Jejunum <ul><li>Is the middle segment of small intestine </li></ul><ul><li>2.5 meters (8.2 ft) long </li></ul><ul><li>Is the location of most: </li></ul><ul><ul><li>chemical digestion </li></ul></ul><ul><ul><li>nutrient absorption </li></ul></ul>The Ileum <ul><li>The final segment of small intestine </li></ul><ul><li>3.5 meters (11.48 ft) long </li></ul>
  80. 82. Plicae <ul><li>Also called plicae circulares </li></ul><ul><li>Transverse folds in intestinal lining </li></ul><ul><li>Are permanent features: </li></ul><ul><ul><li>Do not disappear when small intestine fills </li></ul></ul>
  81. 83. Intestinal Villi <ul><li>A series of fingerlike projections: </li></ul><ul><ul><li>in mucosa of small intestine </li></ul></ul><ul><li>Covered by simple columnar epithelium: </li></ul><ul><ul><li>covered with microvilli </li></ul></ul>
  82. 84. Intestinal Glands <ul><li>Goblet cells between columnar epithelial cells </li></ul><ul><li>Eject mucins onto intestinal surfaces </li></ul>
  83. 85. The Duodenum <ul><li>Has few plicae </li></ul><ul><li>Small villi </li></ul>Functions of the Duodenum <ul><li>To receive chyme from stomach </li></ul><ul><li>To neutralize acids before they can damage the absorptive surfaces of the small intestine </li></ul>
  84. 86. Intestinal Secretions <ul><li>Watery intestinal juice </li></ul><ul><li>1.8 liters per day enter intestinal lumen </li></ul><ul><li>Moistens chyme </li></ul><ul><li>Assists in buffering acids </li></ul><ul><li>Keeps digestive enzymes and products of digestion in solution </li></ul>Intestinal Movements <ul><li>Chyme arrives in duodenum </li></ul><ul><li>Weak peristaltic contractions move it slowly toward jejunum </li></ul>
  85. 87. The Pancreas <ul><li>A compound tubuloalveolar gland </li></ul>Figure 24–18
  86. 88. The Pancreas <ul><li>Lies posterior to stomach: </li></ul><ul><ul><li>from duodenum toward spleen </li></ul></ul><ul><li>Is bound to posterior wall of abdominal cavity </li></ul><ul><li>Is wrapped in thin, connective-tissue capsule </li></ul>
  87. 89. Regions of the Pancreas <ul><li>Head : </li></ul><ul><ul><li>broad </li></ul></ul><ul><ul><li>in loop of duodenum </li></ul></ul><ul><li>Body : </li></ul><ul><ul><li>slender </li></ul></ul><ul><ul><li>extends toward spleen </li></ul></ul><ul><li>Tail : </li></ul><ul><ul><li>short and rounded </li></ul></ul>
  88. 90. Duct of Wirsung <ul><li>Large pancreatic duct </li></ul><ul><li>Delivers digestive enzymes and buffers to duodenum </li></ul>
  89. 91. Common Bile Duct <ul><li>From the liver and gallbladder </li></ul><ul><li>Meets pancreatic duct near duodenum </li></ul>Duodenal Ampulla <ul><li>Chamber that receives secretions from: </li></ul><ul><ul><li>common bile duct </li></ul></ul><ul><ul><li>pancreatic duct </li></ul></ul><ul><li>Located halfway along length of duodenum </li></ul>
  90. 92. Lobules of the Pancreas <ul><li>Are separated by connective tissue partitions (septa) </li></ul><ul><li>Contain blood vessels and tributaries of pancreatic ducts </li></ul><ul><li>In each lobule: </li></ul><ul><ul><li>ducts branch repeatedly </li></ul></ul><ul><ul><li>end in blind pockets- pancreatic acini - Blind pockets </li></ul></ul><ul><ul><ul><li>Are lined with simple cuboidal epithelium </li></ul></ul></ul><ul><ul><ul><li>Contain scattered pancreatic islets- </li></ul></ul></ul><ul><ul><ul><ul><li>Endocrine tissues of pancreas </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Scattered (1% of pancreatic cells) </li></ul></ul></ul></ul>
  91. 93. Functions of the Pancreas <ul><li>Endocrine cells: </li></ul><ul><ul><li>of pancreatic islets </li></ul></ul><ul><ul><li>secrete insulin and glucagon into bloodstream </li></ul></ul><ul><li>Exocrine cells: </li></ul><ul><ul><li>acinar cells </li></ul></ul><ul><ul><li>epithelial cells of duct system </li></ul></ul>Pancreatic Secretions <ul><li>1000 ml (1 qt) pancreatic juice per day </li></ul><ul><li>Controlled by hormones from duodenum </li></ul><ul><li>Contain pancreatic enzymes </li></ul>
  92. 94. Pancreatic Enzymes <ul><li>Pancreatic alpha-amylase : </li></ul><ul><ul><li>a carbohydrase </li></ul></ul><ul><ul><li>breaks down starches </li></ul></ul><ul><ul><li>similar to salivary amylase </li></ul></ul><ul><li>Pancreatic lipase : </li></ul><ul><ul><li>breaks down complex lipids </li></ul></ul><ul><ul><li>releases products ( e.g., fatty acids) that are easily absorbed </li></ul></ul><ul><li>Nucleases : </li></ul><ul><ul><ul><ul><ul><li>break down nucleic acids </li></ul></ul></ul></ul></ul><ul><li>Proteolytic enzymes : </li></ul><ul><ul><li>break certain proteins apart </li></ul></ul><ul><ul><li>proteases break large protein complexes </li></ul></ul><ul><ul><li>peptidases break small peptides into amino acids </li></ul></ul>
  93. 95. Proteolytic Enzymes <ul><li>70% of all pancreatic enzyme production </li></ul><ul><li>Secreted as inactive proenzymes </li></ul><ul><li>Activated after reaching small intestine </li></ul>
  94. 96. The Liver <ul><li>Is the largest visceral organ (1.5 kg) </li></ul><ul><li>Lies in right hypochondriac and epigastric regions </li></ul><ul><li>Extends to left hypochondriac and umbilical regions </li></ul><ul><li>Performs essential metabolic and synthetic functions </li></ul>
  95. 97. Anatomy of the Liver <ul><li>Is wrapped in tough fibrous capsule </li></ul><ul><li>Is covered by visceral peritoneum </li></ul><ul><li>Is divided into lobes </li></ul>
  96. 98. The Falciform Ligament <ul><li>On anterior surface </li></ul><ul><li>Divides left lobe and right lobe </li></ul>
  97. 99. The Posterior Surface <ul><li>Depression at inferior vena cava marks division between: </li></ul><ul><ul><li>right lobe </li></ul></ul><ul><ul><li>small caudate lobe </li></ul></ul>
  98. 100. The Quadrate Lobe <ul><li>Inferior to caudate lobe </li></ul><ul><li>Between left lobe and gallbladder </li></ul>
  99. 101. The Bile Duct System <ul><li>Liver secretes bile fluid: </li></ul><ul><ul><li>into a network of narrow channels ( bile canaliculi ) </li></ul></ul><ul><ul><li>between opposing membranes of adjacent liver cells </li></ul></ul>Bile Canaliculi <ul><li>Extend outward, away from central vein </li></ul><ul><li>Connect with fine bile ductules : </li></ul><ul><ul><li>which carry bile to bile ducts in nearest portal area </li></ul></ul>
  100. 102. The Right and Left Hepatic Ducts <ul><li>Collect bile from all bile ducts of liver lobes </li></ul><ul><li>Unite to form common hepatic duct which leaves the liver </li></ul>Bile Flow <ul><li>From common hepatic duct to either: </li></ul><ul><ul><li>the common bile duct , which empties into duodenal ampulla </li></ul></ul><ul><ul><li>the cystic duct , which leads to gallbladder </li></ul></ul>
  101. 103. The Common Bile Duct <ul><li>Is formed by union of: </li></ul><ul><ul><li>cystic duct </li></ul></ul><ul><ul><li>common hepatic duct </li></ul></ul><ul><li>Passes within the lesser omentum toward stomach </li></ul><ul><li>Penetrates wall of duodenum </li></ul><ul><li>Meets pancreatic duct at duodenal ampulla </li></ul>
  102. 104. 3 Functions of the Liver <ul><li>Metabolic regulation </li></ul><ul><li>Hematological regulation </li></ul><ul><li>Bile production </li></ul>Metabolic Regulation <ul><li>The liver regulates: </li></ul><ul><ul><li>composition of circulating blood </li></ul></ul><ul><ul><li>nutrient metabolism </li></ul></ul><ul><ul><li>waste product removal </li></ul></ul><ul><ul><li>nutrient storage </li></ul></ul><ul><ul><li>drug inactivation </li></ul></ul>
  103. 105. Composition of Circulating Blood <ul><li>All blood leaving absorptive surfaces of digestive tract: </li></ul><ul><ul><li>enters hepatic portal system </li></ul></ul><ul><ul><li>flows into the liver </li></ul></ul><ul><li>Liver cells extract nutrients or toxins from blood: </li></ul><ul><ul><li>before it reaches systemic circulation through hepatic veins </li></ul></ul><ul><li>Liver removes and stores excess nutrients: </li></ul><ul><ul><li>corrects nutrient deficiencies by mobilizing stored reserves or performing synthetic activities </li></ul></ul>
  104. 106. Metabolic Activities of the Liver <ul><li>Carbohydrate metabolism </li></ul><ul><li>Lipid metabolism </li></ul><ul><li>Amino acid metabolism </li></ul><ul><li>Waste product removal </li></ul><ul><li>Vitamin storage </li></ul><ul><li>Mineral storage </li></ul><ul><li>Drug inactivation </li></ul>
  105. 107. The Liver and Hematological Regulation (1 of 2) <ul><li>Largest blood reservoir in body </li></ul><ul><li>Receives 25% of cardiac output </li></ul><ul><li>Performs 6 hematological regulation functions </li></ul>
  106. 108. The Liver and Hematological Regulation (2 of 2) <ul><li>Phagocytosis and antigen presentation </li></ul><ul><li>Synthesis of plasma proteins </li></ul><ul><li>Removal of circulating hormones </li></ul><ul><li>Removal of antibodies </li></ul><ul><li>Removal or storage of toxins </li></ul><ul><li>Synthesis and secretion of bile </li></ul>
  107. 109. Lipid Digestion and Absorption <ul><li>Dietary lipids are not water soluble </li></ul><ul><li>Mechanical processing in stomach creates large drops containing lipids </li></ul><ul><li>Pancreatic lipase is not lipid soluble: </li></ul><ul><ul><li>interacts only at surface of lipid droplet </li></ul></ul>
  108. 110. Functions of Bile <ul><li>Bile salts break droplets apart ( emulsification ): </li></ul><ul><ul><li>increases surface area exposed to enzymatic attack </li></ul></ul><ul><ul><li>creates tiny emulsion droplets coated with bile salts </li></ul></ul>
  109. 111. The Gallbladder <ul><li>Is a pear-shaped, muscular sac </li></ul><ul><li>Stores and concentrates bile prior to excretion into small intestine </li></ul><ul><li>Is located in the fossa on the posterior surface of the liver’s right lobe </li></ul>
  110. 112. 3 Regions of the Gallbladder <ul><li>Fundus </li></ul><ul><li>Body </li></ul><ul><li>Neck </li></ul>
  111. 113. The Cystic Duct <ul><li>Extends from gallbladder </li></ul><ul><li>Union with common hepatic duct forms common bile duct </li></ul>
  112. 114. The Common Bile Duct <ul><li>Meets pancreatic duct at duodenum: </li></ul><ul><ul><li>before emptying into duodenal ampulla </li></ul></ul>
  113. 115. The Duodenal Ampulla <ul><li>Receives: </li></ul><ul><ul><li>buffers and enzymes from pancreas </li></ul></ul><ul><ul><li>bile from the liver and gallbladder </li></ul></ul><ul><li>Opens into duodenum at duodenal papilla </li></ul>
  114. 116. The Gallbladder <ul><li>Stores bile </li></ul><ul><li>Releases bile into duodenum: </li></ul><ul><ul><li>only under stimulation of hormone cholecystokinin ( CCK ) </li></ul></ul>
  115. 117. The Gallbladder & Bile Modification <ul><li>Full gallbladder contains 40–70 ml bile </li></ul><ul><li>Bile composition gradually changes in gallbladder: </li></ul><ul><ul><li>water is absorbed </li></ul></ul><ul><ul><li>bile salts and solutes become concentrated </li></ul></ul>Gallstones <ul><li>Are crystals of insoluble minerals and salts </li></ul><ul><li>Form if bile is too concentrated </li></ul><ul><li>Small stones may be flushed through bile duct and excreted </li></ul>
  116. 118. Activities of Major Digestive Tract Hormones Figure 24–22
  117. 119. Intestinal Hormones <ul><li>Intestinal tract secretes peptide hormones with multiple effects: </li></ul><ul><ul><li>in several regions of digestive tract </li></ul></ul><ul><ul><li>in accessory glandular organs </li></ul></ul>
  118. 120. Secretin <ul><li>Is released when chyme arrives in duodenum </li></ul><ul><li>Increases secretion of bile and buffers by liver and pancreas </li></ul>Cholecystokinin (CCK) <ul><li>Is secreted in duodenum: </li></ul><ul><ul><li>when chyme contains lipids and partially digested proteins </li></ul></ul><ul><li>Accelerates pancreatic production and secretion of digestive enzymes </li></ul>
  119. 121. Cholecystokinin (CCK) <ul><li>Relaxes hepatopancreatic sphincter and gallbladder: </li></ul><ul><ul><li>ejecting bile and pancreatic juice into duodenum </li></ul></ul>Gastric Inhibitory Peptide (GIP) <ul><li>Is secreted when fats and carbohydrates enter small intestine </li></ul>Vasoactive Intestinal Peptide (VIP) <ul><li>Stimulates secretion of intestinal glands </li></ul><ul><li>Dilates regional capillaries </li></ul><ul><li>Inhibits acid production in stomach </li></ul>
  120. 122. Gastrin <ul><li>Is secreted by G cells in duodenum: </li></ul><ul><ul><li>when exposed to incompletely digested proteins </li></ul></ul><ul><li>Promotes increased stomach motility </li></ul><ul><li>Stimulates acids and enzyme production </li></ul>
  121. 123. Intestinal Absorption <ul><li>It takes about 5 hours for materials to pass: </li></ul><ul><ul><li>from duodenum </li></ul></ul><ul><ul><li>to end of ileum </li></ul></ul><ul><li>Movements of the mucosa increases absorptive effectiveness: </li></ul><ul><ul><li>stir and mix intestinal contents </li></ul></ul><ul><ul><li>constantly change environment around epithelial cells </li></ul></ul>
  122. 124. The Large Intestine <ul><li>Is horseshoe-shaped </li></ul><ul><li>Extends from end of ileum to anus </li></ul><ul><li>Lies inferior to stomach and liver </li></ul><ul><li>Frames the small intestine </li></ul><ul><li>Also called large bowel </li></ul><ul><li>Is about 1.5 meters long and 7.5 cm wide </li></ul>Figure 24–23
  123. 125. Functions of the Large Intestine <ul><li>Reabsorption of water </li></ul><ul><li>Compaction of intestinal contents into feces </li></ul><ul><li>Absorption of important vitamins produced by bacteria </li></ul><ul><li>Storage of fecal material prior to defecation </li></ul>
  124. 126. 3 Parts of the Large Intestine <ul><li>Cecum : </li></ul><ul><ul><li>the pouchlike first portion </li></ul></ul><ul><li>Colon : </li></ul><ul><ul><li>the largest portion </li></ul></ul><ul><li>Rectum : </li></ul><ul><ul><li>the last 15 cm of digestive tract </li></ul></ul>
  125. 127. The Ileum <ul><li>Attaches to the medial surface of cecum </li></ul><ul><li>Opens into the cecum at the ileocecal valve </li></ul>
  126. 128. The Cecum <ul><li>Is an expanded pouch </li></ul><ul><li>Receives material arriving from the ileum </li></ul><ul><li>Stores materials and begins compaction </li></ul>
  127. 129. The Appendix <ul><li>Also called vermiform appendix </li></ul><ul><li>Is a slender, hollow appendage (about 9 cm long) </li></ul><ul><li>Is dominated by lymphoid nodules (a lymphoid organ) </li></ul><ul><li>Is attached to posteromedial surface of cecum: </li></ul><ul><ul><li>mesoappendix connects appendix to ileum and cecum </li></ul></ul>
  128. 130. The Colon <ul><li>Has a larger diameter and thinner wall than small intestine </li></ul><ul><li>The wall of the colon: </li></ul><ul><ul><li>forms a series of pouches ( haustra ) </li></ul></ul><ul><ul><li>Haustra permit expansion and elongation of colon </li></ul></ul>
  129. 131. Colon Muscles <ul><li>3 longitudinal bands of smooth muscle ( taeniae coli ): </li></ul><ul><ul><li>run along outer surfaces of colon </li></ul></ul><ul><ul><li>deep to the serosa </li></ul></ul><ul><ul><li>similar to outer layer of muscularis externa </li></ul></ul><ul><li>Muscle tone in taeniae coli creates the haustra </li></ul>
  130. 132. Serous Membrane of the Colon <ul><li>Contains numerous teardrop-shaped sacs of fat: </li></ul><ul><ul><li>fatty appendices or epiploic appendages </li></ul></ul>
  131. 133. 4 Colon Regions <ul><li>Ascending colon </li></ul><ul><li>Transverse colon </li></ul><ul><li>Descending colon </li></ul><ul><li>Sigmoid colon </li></ul>
  132. 134. The Rectum <ul><li>Forms last 15 cm of digestive tract </li></ul><ul><li>Is an expandable organ for temporary storage of feces </li></ul><ul><li>Movement of fecal material into rectum triggers urge to defecate </li></ul>The Anal Canal <ul><li>Is the last portion of the rectum </li></ul><ul><li>Contains small longitudinal folds called anal columns </li></ul>The Anus <ul><li>Also called anal orifice </li></ul><ul><li>Is exit of the anal canal </li></ul>
  133. 135. Anal Sphincters <ul><li>Internal anal sphincter : </li></ul><ul><ul><li>circular muscle layer of muscularis externa </li></ul></ul><ul><ul><li>has smooth muscle cells, not under voluntary control </li></ul></ul><ul><li>External anal sphincter : </li></ul><ul><ul><li>encircles distal portion of anal canal </li></ul></ul><ul><ul><li>a ring of skeletal muscle fibers, under voluntary control </li></ul></ul>
  134. 136. Characteristics of the Colon <ul><li>Lack of villi </li></ul><ul><li>Abundance of goblet cells </li></ul><ul><li>Presence distinctive intestinal glands </li></ul>Glands of the Large Intestine <ul><li>Are deeper than glands of small intestine </li></ul><ul><li>Are dominated by goblet cells </li></ul>Mucosa of the Large Intestine <ul><li>Does not produce enzymes </li></ul><ul><li>Provides lubrication for fecal material </li></ul>
  135. 137. Absorption in the Large Intestine <ul><li>Reabsorption of water </li></ul><ul><li>Reabsorption of bile salts : </li></ul><ul><ul><li>in the cecum </li></ul></ul><ul><ul><li>transported in blood to liver </li></ul></ul><ul><li>Absorption of vitamins produced by bacteria </li></ul><ul><li>Absorption of organic wastes </li></ul>
  136. 138. 3 Vitamins Produced in the Large Intestine <ul><li>Vitamin K: </li></ul><ul><ul><li>a fat-soluble vitamin </li></ul></ul><ul><ul><li>required by liver for synthesizing 4 clotting factors, including prothrombin </li></ul></ul><ul><li>Biotin : </li></ul><ul><ul><li>a water-soluble vitamin </li></ul></ul><ul><ul><li>important in glucose metabolism </li></ul></ul><ul><li>Pantothenic acid : </li></ul><ul><ul><li>a water-soluble vitamin </li></ul></ul><ul><ul><li>required in manufacture of steroid hormones and some neurotransmitters </li></ul></ul>
  137. 139. Organic Wastes <ul><li>Bacteria convert bilirubin to urobilinogens and stercobilinogens : </li></ul><ul><li>Bacteria break down peptides in feces and generate: </li></ul><ul><ul><li>ammonia: </li></ul></ul><ul><ul><ul><li>as soluble ammonium ions </li></ul></ul></ul><ul><ul><li>indole and skatole : </li></ul></ul><ul><ul><ul><li>nitrogen compounds responsible for odor of feces </li></ul></ul></ul><ul><ul><li>hydrogen sulfide : </li></ul></ul><ul><ul><ul><li>gas that produces “rotten egg” odor </li></ul></ul></ul><ul><li>Bacteria feed on indigestible carbohydrates (complex polysaccharides): </li></ul><ul><ul><li>produce flatus , or intestinal gas, in large intestine </li></ul></ul>
  138. 140. Chemical Events in Digestion Figure 24–26
  139. 141. Processing Nutrients <ul><li>The digestive system: </li></ul><ul><ul><li>breaks down physical structure of food </li></ul></ul><ul><ul><li>disassembles component molecules </li></ul></ul><ul><li>Molecules released into bloodstream are: </li></ul><ul><ul><li>absorbed by cells </li></ul></ul><ul><li>Broken down to provide energy for ATP synthesis: </li></ul><ul><ul><li>used to synthesize carbohydrates, proteins, and lipids </li></ul></ul>
  140. 142. Digestive Enzymes <ul><li>Are secreted by: </li></ul><ul><ul><li>salivary glands </li></ul></ul><ul><ul><li>tongue </li></ul></ul><ul><ul><li>stomach </li></ul></ul><ul><ul><li>pancreas </li></ul></ul><ul><li>Break molecular bonds in large organic molecules: </li></ul><ul><ul><li>carbohydrates, proteins, lipids, and nucleic acids </li></ul></ul><ul><ul><li>in a process called hydrolysis </li></ul></ul>
  141. 143. <ul><li>Are divided into classes by targets: </li></ul><ul><ul><li>carbohydrases : </li></ul></ul><ul><ul><ul><li>break bonds between simple sugars </li></ul></ul></ul><ul><ul><li>proteases : </li></ul></ul><ul><ul><ul><li>break bonds between amino acids </li></ul></ul></ul><ul><ul><li>lipases : </li></ul></ul><ul><ul><ul><li>separate fatty acids from glycerides </li></ul></ul></ul>