Chapter 21 The Digestive System

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ChipzThatLeo
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270470
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Chapter 21 The Digestive System
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2014-04-14 01:16:02
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Chapter 21 Digestive System
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The Digestive System
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  1. What are functions of GI tract?
    • • Digest food and protect self from digestion
    • • Mass balance: match fluid input with fluid output
    • • Constant surveillance prevents infection
    •    – Environment is external to body
    •    – Gut‐associated lymphoid tissue(GALT) responds to infectious agents
  2. Four processes of GI tract:
    1. Digestion: breakdown of food

    2. Absorption: transport of substances and water from lumen of GI tract to extracellular fluid

    3. Motility: movement of material in the GI tract

    • 4. Secretion: movement into GI lumen
    •    – transport of waterand substances
    •    – release of hormonesand paracrinemolecules
  3. Anatomy of the Digestive System
    • • Gross structures of the digestive system:
    • 1. Mouth (oral cavity)
    • 2. Esophagus
    • 3. Stomach
    • 4. Small intestine
    • 5. Large intestine

    • Esophagus passes through thoracic cavity

    • Stomach, intestines, and accessory organs are inabdominal cavity

    • • Accessory glands and organs of GI tract:
    • 1. Salivary glands (in oral cavity)
    • 2. Liver
    • 3. Pancreas
    • 4. Gall bladder
  4. • The GI wall has four layers
    • 1. Mucosa faces lumen
    • 2. Submucosa
    • 3. Muscularis externa has two layers
    • 4. Serosa is outer covering
  5. Mucosa layer
    • • Inner lining of GI tract consisting of:
    • 1. Epithelium
    • – Cell layer
    • – Fold out into villi
    • – Fold in into crypts
    • 2. Lamina propria connective tissue with capillaries and lacteals
    • 3. Muscularis mucosae(muscle)
  6. What does the mucosa layer do?
    • 1. Epithelial layer absorbs substances from lumen of gut
    • 2. Substances exported to capillaries in lamina propria
    • 3. Stem cells in crypt replace worn out epithelial cells of villi
  7. Submucosa
    • Connective tissuelayer:1. Arteries and veins2. Submucosal plexus– enteric nervoussystem I– is one of twomajor nervenetworks3. Lymph vessels
  8. muscularis externa
    • • Outer muscle layer
    • – Circular smooth muscle
    • – Longitudinal smooth muscle

    • • Myenteric plexus
    • – enteric nervous system II
    • – is second of two major nerve networks

    1. Motor efferent neurons of myenteric plexus innervate smooth muscle

    2. Contraction of circular smooth muscle constricts tube

    3. Contraction of longitudinal smooth muscle shortens tube
  9. Motility
    • • Progressive contractions behind a bolus of food
    • – Smooth muscle
    • – Circular muscles contract and relax
    • • Propels food in one direction
    • • Important in esophagus; less instomach and intestines

    • • Gastric motility (intestine and stomach)
    • • Segmental contractions:
    • – Coordinate circular and longitudinal muscle contractions
    • – Circular muscle contraction traps food in segment
    • – Interior contractions break up food in segment
    • • Gastric motility churns food; peristalsis moves food toward anus

    • • gastric motility is controlled by myenteric plexus
    • • Tonic stimulation maintain contraction
    • • Phasic stimulation generate slow wave potentials
    •    – Pacemaker
    •    – Control cycle of contraction and relaxation of smooth muscle
  10. Secretion
    • • Secretion: any process which delivers substances from the interstitial fluid to the GI lumen
    • • 7L/day of fluid enters digestive tract from body
    • • Most secretions are water and ions (Na+, K+, Cl‐, HCO3 ‐, H+)
    • • Specialized cells provide function to segments of GI tract
    • 1. Salivary glands secrete hyposmotic fluid to wet food
    • 2. Parietal cells of stomach secrete acid and pepsin to denature food
    • 3. Pancreas and small intestine near stomach (duodenum) secrete bicarbonate to neutralize acid
    • 4. Liver secretes bile to break up fats
    • 5. Goblet cells along GI tract secrete mucus to protect and lubricate

    • • Parietal cell secretion is one example
    • – Stomach epithelium
    • – Secretes HCl
    • – Primary active transport: H+‐K+ transporter pumps H+ into lumen
    • – Cl‐ follows H+ by facilitated diffusion
  11. How is digestion distinguished from absorption
    • Digestion progressively breaks food down to component parts
    • • Absorption moves components of food from GI lumen into mucosa epithelium
    • • Microvilli of epithelium
    • – Brush border cells
    • – on apical surface
    • – increase area for absorption
    • • Absorbed substances transported to capillaries and lacteal
  12. How are carbohydrates absorbed?
    • • Carbohydrates digested to monosaccharides in lumen of Intestine
    • • Transported through cells into blood
    • • All sugars absorbed by enterocyte are transported to blood
  13. How are proteins absorbed?
    • • Proteins are digested by peptidases in stomach and intestine
    • • Products absorbed:
    •    1. Amino acid are cotransported with Na+
    •    2. Di and tri peptides are cotransported with H+
    •    3. Small peptides move by trancytosis
    • • All are delivered to blood
  14. How are fats absorbed?
    • • Fats are digested by lipases in intestinal lumen
    • • Products of digestion diffuse into enterocyte
    • • Triglycerides and cholesterol form into chylomicrons (4)
    • • Exocytosis releases chylomicrons intointerstitial fluid (5)
    • • Lacteal transports chylomicron in lymph
  15. How is NaCl and water reabsorbed?
    • • Na+ and Cl‐ are reabsorbed from lumen of intestine
    • • Na,K‐ATPase pumps Na+ into interstitial fluid
    • • Cl‐ follows Na+
    • • Na+ and Cl‐ enterfrom lumen by facilitated transport
    • • osmosis moves H2O by paracellular pathway
  16. What is the cephalic phase?
    • • Long reflexes that begin in the brain and create feed forward responses
    • • Medulla initiates reflex in response tofood stimuli
    • 1. Salivary glands stimulated
    • 2. Vagus nerve stimulates secretions and motility of gut
  17. How are secretions and motility regulated?
    1. Long reflexes are integrated in the CNS (Cephalic Reflexes)

    2. Short reflexes are integrated in the Enteric Nervous System (ENS)

    3. Peptide‐derived reflexes
  18. What is the gastric phase?
    • • Food entering stomach stimulates short reflex of secretion and motility
    • 1. Acid secretion
    • 2. Pepsin secretion
    • 3. Churning and motility
    • • Hormones, paracrine, and neurocrine signals also regulate
    • • Product of stomach is chyme: pasty denatured mix of carbohydrates,protein, and fat
  19. What are functions of small intestine?
    • • Functions include
    • 1. Most absorption takes place
    • 2. Most digestion takesplace
    • • >80% of fluid entering small intestine is absorbed
    • • Water and soluble nutrient enter hepatic portal circulation
    • • Liver filters xenobiotics and drugs

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