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Chapter 22 – The Digestive System . 2 main components. Alimentary canal Continuous passageway from mouth to anus Accessory organs/structures Assists with the physical and/or chemical breakdown of food Teeth, salivary glands, pancreas, liver . Digestive processes. Ingestion

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2 main components
2 main components
  • Alimentary canal
    • Continuous passageway from mouth to anus
  • Accessory organs/structures
    • Assists with the physical and/or chemical breakdown of food
    • Teeth, salivary glands, pancreas, liver
digestive processes
Digestive processes
  • Ingestion
    • Bringing food into body’s interior
  • Digestion
    • Mechanical/physical
      • Breaking down into smaller components
        • Chewing/mastication
        • Stomach churning
        • Segmentation
          • Back and forth movement in small intestine
    • Chemical/enzymatic
      • Breaking of chemical bonds in nutrients into their smaller building blocks
  • Absorption
    • Nutrients pass through intestinal wall into bloodstream
  • Elimination
    • Removal of waste/undigested material
    • Defecation
histology of alimentary canal
Histology of alimentary canal
  • 4 tunics
    • Mucosa – deepest
      • Secretes mucus, enzymes
      • Contains lymphoid cells to fight infection
    • Submucosa
      • Blood and lymphatic vessels, nerve fibers
    • Muscularisexterna – smooth muscle
      • Deep – circular layer
      • Superficial – longitudinal layer
    • Serosa – most superficial
      • Visceral peritoneum
      • Reduces friction from organ movement
peristalsis
Peristalsis
  • Waves of smooth muscle contraction that propels the contents through the alimentary canal/GI tract
mouth
Mouth
  • Roof
    • Anterior hard palate
      • Maxillary and palatine bones
    • Posterior soft palate
      • Underlying muscle only; no bone
      • Uvula
        • Elicits gag response
        • Blocks entrance into nasopharynx when swallowing
  • Tongue
    • Moves food around during mastication to form bolus, and to push bolus to back of throat
      • Bolus – chewed food mixed with saliva
      • Deglutition = swallowing
    • Covered with taste buds
      • Also scattered in walls or oral cavity and oropharynx
salivary glands
Salivary glands
  • Produce saliva
    • Moistens food
    • Contains lysozyme to kill bacteria
    • Dissolves chemicals for sense of taste
    • Contains enzyme amylase
      • Begins chemical digestion of carbohydrates
  • 3
    • Parotid
    • Sublingual
    • Submandibular
tooth structure
Tooth structure
  • Crown – exposed region; root – below gumline; neck – separates crown and root
  • Enamel covers crown
    • Calcium salts
  • Cementum – calcified connective tissue
    • Attachment site of periodontal ligament
  • Dentin – bulk of tooth
    • Similar to bone composition
    • Central pulp cavity
      • Extends through root canal
      • Vessels, nerve endings, connective tissue
teeth type
Teeth #/type
  • Baby/

deciduous

teeth

    • 20 total
    • 2-1-0-2

2-1-0-2

  • Adult teeth
    • 32 total
    • 2-1-2-3

2-1-2-3

pharynx and esophagus
Pharynx and Esophagus

Stratified squamous epithelium

Esophageal/stomach junction

  • Pharynx
    • Oropharynx and laryngopharynx
  • Esophagus
    • Joins with stomach and cardiac orifice
      • Cardiac or gastroesophageal sphincter
        • Diaphragm helps keep it closed
stomach
Stomach
  • Muscularisexterna has a third layer – oblique (deep to circular layer)
  • Rugae – inner lining folds
    • Increases surface area
    • Allows stomach to expand
  • Regions
    • Cardia/cardiac
    • Fundus
    • Body
      • Lesser curvature – medial
      • Greater curvature – lateral
    • Pylorus
      • Plyoric sphincter at junction with small intestine
  • Food and gastric juice mix to form thick acid chyme
    • Exits from stomach in spurts
    • Higher fat content stays in stomach longer
stomach histology
Stomach histology
  • Mucous cells
  • Parietal cells
    • Secrete hydrochloric acid
  • Chief cells
    • Secrete pepsinogen
      • In presence of HCl, converts to active pepsin
        • Chemical digests protein
small intestine
Small intestine
  • Site where most of chemical digestion occurs, and absorption of nutrients
  • 3 regions
    • Duodenum
      • Smallest section; site of most digestion
      • Entrance for hepatic and pancreatic secretions
    • Jejunum
    • Ileum
      • Contains Peyer’s patches
        • Large aggregates of lymphoid material
      • Ileocecal valve – junction with large intestine
small intestine modifications for absorption
Small intestine modifications for absorption
  • Microvilli – plasma membrane extensions of cell
  • Villi – projections of mucosa
    • Both villi and microvilli increase surface area
  • Pilcaecirculares
    • Allows chyme to swirl through lumen
      • Slows movement and increases contact with walls for absorption
pancreas
Pancreas
  • Tail – next to spleen; body – surrounded by duodenum
  • Pancreatic secretions into duodenum
    • Bicarbonate ions
      • Neutralized acidity of chyme
    • Enzymes for digestion
      • Proteases, amylase, lipase
liver
Liver
  • Main digestive function is to filter and process nutrient-rich blood from hepatic portal vein
  • Produces bile salts
    • Emulsifies fat
      • Mechanical breakdown of lipids into smaller droplets
        • Provides a greater surface area for enzymes to work on
gallbladder
Gallbladder
  • Concentrates and stores bile
  • Bile exits through cystic duct to bile duct, then into duodenum
  • Excess cholesterol can crystallize to form gallstones
histology
Histology

Pancreas

Liver

large intestine
Large intestine
  • Absorbs water and compacts waste
  • Contains bacteria
    • Produces B12 and K vitamins, which are then absorbed
    • Breakdown of molecules cause gaseous byproducts – flatus
  • Longitudinal layer of muscularis is reduced to three bands – teniae coli
    • Causes walls to ‘pucker’ into pouches called haustra
  • Epiploic appendages
large intestine sections
Large intestine sections
  • Cecum “blind pouch”
    • Ileocecal valve
    • Vermiform appendix – mass of lymphoid tissue
  • Ascending colon – right side of body
    • Turns at hepatic flexure
  • Transverse colon
    • Turns at splenic flexure
  • Descending colon – left side of body
  • Sigmoid colon – “S” shape curves posteriorly
large intestine sections cont
Large intestine sections cont
  • Rectum
  • Anus
    • Internal sphincter
      • Smooth muscle
    • External sphincter
      • Skeletal muscle
chemical digestion
Chemical digestion
  • Large macromolecules (polymers) are broken down into monomer building blocks
    • Hydrolysis – addition of water molecule breaks one bond
  • Carbohydrates
    • Enzymatic breakdown begins in mouth with amylase
    • Monomer = monosaccharide
      • Glucose, fructose, galactose
    • Dissaccharide = 2 monosaccharides
      • Sucrose, lactose
    • Polysaccharide = complex carbohydrate
      • Glycogen, starch, cellulose (indigestible)
chemical digestion cont
Chemical digestion cont
  • Proteins
    • Enzymatic breakdown begins in stomach by pepsin
    • monomer = amino acids
  • Lipids
    • Enzymatic breakdown begins in duodenum by lipase in adults
      • In children, begins in stomach
    • Monomer = fatty acid (and glycerol)
  • Nucleic acids
    • Enzymatic breakdown begins in duodenum
    • Does not contribute to cellular fuel/creation of ATP
    • Monomer = nucleotide