The Digestive System

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The Digestive System
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Two Divisions

• Alimentary Canal (or gastrointestinal (GI) tract)
  
• Where food passes from mouth to anus
• Organs mouth, pharynx, esophagus, stomach,
  small intestine, large intestine
• Accessory Digestive Organs
• Contribute to digestion and absorption
• Teeth, tongue, salivary glands, liver,
  gallbladder, pancreas

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Layers of the GI Tract

• Same four layers from esophagus to anus

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Layers of the GI Tract

• Mucosa innermost lining of tract
• Secretion mucus, enzymes, hormones
• Protection disease
• Absorption nutrients, ions, water, vitamins
• Plica folds that increase surface area

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Layers of the GI Tract

• Submucosa dense connective tissue
• Binding mucosa to muscularis
• Contains blood vessels, lymph vessels, lymph
  nodes, nerves

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Layers of the GI Tract

• Muscularis externa smooth muscle
• Contains circular and longitudinal fibers
• Serosa - outermost layer
• AKA visceral peritoneum

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Functions of the Digestive System

• Ingestion eating
• Propulsion moving food through GI
• Deglutition (swallowing) voluntary
• Peristalsis wave-like contractions that move
  food involuntary
• Mass movements large
  contractions in large
  intestine

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Functions of the Digestive System

• Mechanical digestion food breakdown
• chewing mixing by tongue teeth (mastication)
• churning in the stomach
• Segmentation rhythmic contractions of small
  intestine that mixes food

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Functions of the Digestive System

• Chemical digestion food breaks down into
  chemical building blocks
• Uses enzymes
• Mouth through small intestine
• Hydrolysis reactions
• Carbohydrates? monosaccharides
• Protein? amino acids
• DNA? nucleotides

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Functions of the Digestive System

• Absorption - movement of digested end products
  from lumen of GI into blood or lymph
• Defecation elimination of wastes

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The Mouth, Pharynx, Esophagus

• Salivary glands produce salivary amylase to start
  break down of complex carbohydrates.
• Mastication why?
• Bolus formed in palate
• Deglutition
• Epiglottis protects trachea
  as food passes through
  pharynx
• Peristalsis moves food
  through esophagus to
  stomach

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The Stomach

• Cardioesophageal sphincter controls
  entrance
• Rugae expand to
  accommodate food
• Enteroendocrine cells
  produce the hormone
  gastrin to stimulate release of gastric
  juice
• Parietal cells release hydrochloric acid to
  activate enzymes and kill bacteria
• Mucous neck cells secrete mucus to protect the
  stomach wall from acid

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The Stomach

• Chief cells produce pepsinogen which is activated
  by acid to form pepsin (digests protein).
• Processed food (chyme) enters the small intestine
  through the pyloric sphincter

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The Small Intestine

• Three Parts
• Duodenum 5 of length, but most activity is
  here, C-shaped
• Jejunum almost 40, coiled
• Ileum almost 60, coiled
• Suspended from abdominal wall by mesenteries

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Duodenum

• Trypsin produced by pancreas
  furthers protein digestion
• Pancreatic amylase continues
  carbohydrate breakdown
• Brush border enzymes of small
  intestine finish carb and protein
  digestion
• Bile (produced by liver stored in gall bladder)
  emulsifies fats for absorption
• Pancreatic enzymes bile production is
  controlled by hormones secretin and CCK, produced
  by small intestine when chyme is present

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Jejunum Ileum

• Villi microvilli (brush border) increase
  surface area of small intestine
• Fat is absorbed by diffusion
• Other molecules are absorbed through active
  transport

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Large Intestine

• Remaining water is absorbed
• Residue is further digested by resident bacteria,
  who produce vitamins (K some B) and gas (more
  with carb-rich food)
• Goblet cells produce mucus to ease passage of
  solid waste

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Rectum Anus

• Mass movements push feces past internal anal
  sphincter (involuntary)
• Relaxation of external anal sphincter is
  voluntary and results in defecation

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Leptin Ghrelin