5.23 Digestive System

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5.23 Digestive System
2010-12-07 00:08:04
Overview Alimentary Canal Accessory Organs Disorders Digestive System

Overview of the Digestive System, Anatomy of the Alimentary Canal and Accessory Organs, Disorders of the Digestive System
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  1. Overview of the Digestive System
    • Organs are divided into two groups:
    • Alimentary canal (gastrointestinal tract): Mouth, pharynx, and esophagus, stomach, small intestine, and large intestine

    Accessory digestive organs: Teeth and tongue, gallbladder, salivary glands, liver, and pancreas

    • *Accessory organs are connected to the alimentary canal by ducts
    • **Secretions contribute to breakdown of foodstuffs
  2. Digestive Processes
    • 1. Ingestion—the taking of food into the mouth
    • 2. Propulsion—movement of food; includes swallowing and peristalsis
    • Peristalsis—wavelike motion; major means of propulsion
    • 3. Mechanical digestion—physically prepares food for chemical digestion
    • • Chewing (mastication), churning food in stomach, segmentation
    • Segmentation is rhythmic local constrictions of intestine

    • 4. Chemical digestion—complex molecules broken down to chemical components
    • • Mouth: salivary amylase breaks down carbohydrates
    • **chemical digestion begins in the mouth
    • • Stomach
    • • Small intestine
    • carbohydrates, proteins, lipids to simple sugars, amino acids, and fatty acids and glycerol

    5. Absorption—transport of digested nutrients into blood

    6. Defecation—elimination of indigestible substances as feces
  3. Peristalsis vs Segmentation

    • Peristalsis:
    • • Major means of propulsion
    • • Adjacent segments of the alimentary canal relax and contract

    • Segmentation:
    • • Rhythmic local contractions of the intestine
    • • Mixes food with digestive juices
  4. Histology of the Alimentary Canal Wall
    Same four layers from esophagus to anus

    • Mucosa—innermost layer;
    • Consists of
    • • Epithelium: secretes mucus, absorbs nutrients
    • • Lamina propria: loose connective tissue (contains most of the MALT: mucosa associated lymphoid tissue, that defends against bacterial invasion)
    • • Muscularis mucosae: a thin layer of smooth muscle that produces local movements of the mucosa

    • Submucosa—external to the mucosa
    • Contains blood and lymphatic vessels, nerve fibers
    • *"moderately dense" intermediate between loose connective areolar connective tissue and dense irregular connective tissue; contains elastic fibers to bring back to normal shape
    • **supplies nutrients via blood to other layers

    Muscularis externa—external to the submucosa

    • Two layers
    • • Circular muscularis—inner layer; squeezes
    • • Longitudinal muscularis—outer layer; shortens
    • **responsible for peristalsis and segmentation
    • **circular layer thickens to form sphincters in some areas

    Serosa—the outermost layer (visceral peritoneum)
  5. The Peritoneal Cavity and Peritoneum
    • Peritoneum—a serous membrane
    • Visceral peritoneum—surrounds digestive organs
    • Parietal peritoneum—lines the body wall

    Peritoneal cavity—a slit-like potential space between the visceral and parietal peritoneum, filled with serous fluid
  6. Mesenteries

    • Mesentery—a double layer of peritoneum
    • • Holds organs in place
    • • Sites of fat storage
    • • Provides a route for circulatory vessels and nerves

    • Layers:
    • 1. Falciform Ligament: attaches liver to abdominal wall and diaphragm
    • 2. Lesser omentum: attaches liver to lesser curvature of the stomach
    • 3. Greater omentum: very fatty; in peritoneum, attaches stomach to posterior abdominal wall; helps limit spread of infection
    • 4. Mesentery: small intestine to posterior abdominal wall
  7. Anatomy of the Mouth
    The mouth is a mucosa lined cavity whose boundaries are the lips, cheeks, the palate, and the tongue; divided into the vestibule (between teeth and lips) and the oral cavity proper (the region internal to the teeth)

    Mouth only has two layers of tissue: an internal mucosa made of epithelium and lamina propria only, a thin submucosa in some areas, and external muscle or bone

    • Roof of the mouth is the palate
    • Front - hard palate
    • Back - soft palate; has uvula

    The labial frenulum connects lips to gum

    The palate forms the roof of the mouth

    The lingual frenulum secures the tongue to the floor of the mouth

    Ducts secrete saliva
  8. Anatomy of Teeth
    Teeth lie in sockets called alveoli

    • Deciduous teeth—20 teeth
    • • First appear at 6 months of age

    • Permanent teeth—32 teeth
    • • Most erupt by the end of adolescence

    • Dental formula—shorthand
    • • Way to indicate number and position of teeth
    • 2I, 1C, 2P, 3M = 1/2 mouth (x2 = 32 teeth total)
    • 2I, 1C, 2P, 3M

    • Deciduous dental formula:
    • 2I, 1C, 2M x2 = 20 teeth
    • 2I, 1C, 2M

  9. Tooth Structure

    Longitudinal section of tooth in alveolus
    • Crown: superior portion
    • - Clinical crown: visible above gingiva (gums); covered by enamel (calcium salt similar to bone with some differences)
    • Root: portion of tooth embedded in the alveolar portion of the jaw

    **Clinical crown : enamel :: root : cementum

    • Neck: where the crown and root meet at the gumline
    • Cementum: covers the outermost surface of the root
    • Periodontal ligament: attaches the tooth to the bone
    • Dentin: composes the bulk of the tooth; produced by odontoblasts
    • Pulp cavity: central portion of the tooth
    • Root canal: pulp cavity in the distal portions of the root; innervated
  10. Salivary Glands
    Small glands throughout tongue, palate, lips, cheeks.

    • Produce saliva ( water, ions, mucus, and enzymes), that moistens the mouth, dissolves food chemicals so they can be tasted, wets food, binds the food together into a bolus, and enzymes begin carbohydrate digestion

    Contains: bicarbonate buffer to neutralizes acids, bactericidal enzymes, antiviral substances, antibodies, and a cyanide compound that helps kill microorganisms

    • Three salivary glands: (compound tuboalveolar glands)
    • 1. Parotid glands: largest, lies near ears between masseter muscle and skin

    2. Submandibular glands: lies along the medial surface of the mandibular body

    3. Sublingual glands: lies in the floor of the mouth, under the tongue

    • The cells of the salivary glands are serous cells (saliva) and mucous cells (mucus)
  11. The Pharynx
    Oropharynx and laryngopharynx

    • • Passages for both air and food
    • • Lined with stratified squamous epithelium
  12. The Esophagus
    Muscular tube that connects the pharynx to the stomach

    • Cardiac sphincter: closes lumen to prevent stomach acid from entering the esophagus
    • *GERD: weakness of the sphincter; leads to heartburn and regurgitation

    • Microscopic anatomy
    • - Epithelium composed of stratefied squamous epithelium (adapted for constant abrasion), then changes to simple columnar (secretion)
    • - Mucous glands: secretes lubricating mucus
    • - muscularis externa: skeletal muscle first third of length, then smooth muscle
    • - adventitia: most external layer (takes place of serosa)
  13. The Stomach
    • Site where food is churned into chyme
    • • Secretion of pepsin begins protein digestion
    • • Functions under acidic conditions
    • • Food remains in stomach approximately 4 hours
    • *water, electrolytes, and some drugs (aspirin and alcohol) are absorbed through the stomach

    • Regions of the stomach:
    • •Cardiac region: ring shaped zone encircling the area near the esophagus
    • •Fundus: stomach's dome
    • •Body: large midportion
    • •Pyloric region: funnel-shaped end composed of antrum and canal

    Pyloric sphincter – between stomach and duodenum.

    Greater curvature – convex left surface

    Lesser curvature – concave right surface

    • The internal surface of the stomach has numerous longitudinal folds called rugae, which flattens as the stomach fills.
  14. Microscopic Anatomy of the Stomach
    • Muscularis has three layers
    • • Circular and longitudinal layers and oblique layer ( helps vigorous churning)

    • Epithelium is simple columnar epithelium (adapted for secretion)

    • Mucosa dotted with gastric pits

    Gastric glands—deep to gastric pits; secretes enzymes

  15. Microscopic Anatomy of Stomach: Mucosa
    • Mucosa of the stomach has gastric pits, which have gastric glands

    • Gastric glands of the Fundus and the Body
    • Mucus neck cells: secrete a special type of mucus
    • Parietal (oxyntic) cells: secrete HCl and gastric intrinsic factor (impt for vitamin B12 absorption)
    • Chief (zymogenic) cells: secrete pepsinogen
    • **pepsinogen is activated to pepsin when it encounters acid in the gastric glands

    pepsinogen --HCl---> pepsin (impt for digestion of protein)

    • **digestion of carbohydrates begins in the mouth
    • **digestion of proteins begins in the stomach
  16. Small intestine
    • Longest portion of the alimentary canal;
    • site of most enzymatic digestion and absorption
    • Three subdivisions: duodenum, jejunum, ileum

    • Modifications for nutrient absorption: increases surface area, forces chyme to move and spiral around, spend more time inside small intestine
    • Circular folds (plicae circulares): transverse ridges of mucosa and submucosa
    • Villi: finger-like projections of the mucosa; covered with simple columnar epithelium
    • Microvilli; further increase surface area for absorption

    • Absorptive cells: Uptake digested nutrients; all along microvilli
    • Goblet cells: secrete mucus that lubricates chyme
    • - CCK (choleocystokinin): hormone produced by duodenum; signals the release of bile/ pancreatic enzymes
    • Enteroendocrine cells: secrete hormones
    • Intestinal crypts: epithelial cells secrete intestinal juice
  17. The Small Intestine: Duodenum
    Receives digestive enzymes and bile

    • Main pancreatic duct (passageway for pancreatic enzymes from pancreas to duodenum) and common bile duct (passageway for bile from liver to duodenum) enter the duodenum
    • *sphincters control entry of bile and pancreatic juices
    • **junction where the two ducts meet is called the hepatopancreatic ampulla
  18. The Large Intestine
    • Digested residue contains few nutrients; small amount of digestion by bacteria
    • Main functions: absorbs water and electrolytes
    • Mass peristaltic movements force feces toward rectum

    • Subdivided into:
    • Cecum (blind pouch where Lg. intestine begins; near ileum junction lies the ileocecal valve)
    • Vermiform appendix (has no function or purpose; possibly repopulates the lg intestine with good bacteria)
    • Colon (ascending (right colic or hepatic flexure), transverse (left colic or splenic flexure), descending, sigmoid (s-shaped))
    • Rectum (descends along the inferior half of the sacrum)
    • Anal canal (the last subdivision of the large intestine; wall is lined with two sphincter muscles: internal anal sphincter (involuntary smooth muscle) and external anal sphincter (voluntary skeletal muscle)
    • Special features:
    • Teniae coli: thickening of longitudinal muscularis
    • Haustra: puckering created by teniae coli
    • Epiploic appendages: fat-filled pouches of visceral peritoneum
  19. Microscopic Anatomy of the Large Intestine
    Villi are absent

    Contains numerous goblet cells and some absorptive cells

    Lined with simple columnar tissue
  20. The Liver
    • Largest gland in the body; performs over 500 functions
    • Digestive function: bile production
    • Performs many metabolic functions

    • Has 4 Lobes: right, left, caudate, quadrate
  21. Liver hepatocyte functions
    • Functions of hepatocytes
    • - Rough ER manufactures blood proteins
    • - Smooth ER produces bile salts, detoxifies poisons
    • - Peroxisomes detoxify poisons (alcohol)
    • - Glycosomes store sugar
    • - Great capacity for regeneration
  22. Microscopic anatomy of the Liver
    • Liver contains over a million lobules
    • Hepatocyte: functional cells of the liver
    • *at every corner of a lobule, there is a portal triad
    • Portal triad composed of
    • - bile duct tributary
    • - branch of hepatic portal vein
    • - branch of hepatic artery
    • Kupffer cells: destroy bacteria

  23. Gallbladder and Pancreas
    • Gallbladder:
    • - Stores and concentrates bile
    • - Expels bile into the duodenum through cystic duct (bile emulsifies fats)
    • - Cholecystokinin (CCK) released from entereoendocrine cells in response to fatty chyme

    • Pancreas:
    • Exocrine function - acinar cells make, store, and secrete pancreatic enzymes that are activated in the duodenum
    • Endocrine function - islet cells produce insulin and glucagon (GLUT-1), which regulate blood sugar levels