1'Overview of the Gastrointestinal System

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The Gastrointestinal System

• 1.Overview of the Gastrointestinal System
• A.Anatomical features of the gastrointestinal
  system
• The digestive tract consists of
• Mouth
• Pharynx
• Esophagus
• Stomach
• Small intestines (duodenum, jejunum and ileum)
• large intestines (cecum with appendix, ascending
  colon, transverse colon, descending colon,
  sigmoid colon, rectum, anus)

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• Accessory structures
• Salivary glands
• Pancreas
• Liver
• Gallbladder
• The wall of the digestive tract is composed of
  four coats
• Mucosa
• epithelial lining
• lamina propria
• muscularis mucosa
• Submucosa (a layer of connective tissue with
  lymph and blood vessels, exocrine glands, and a
  plexus of nerve cells)

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• Muscularis externa
• circular muscle layers
• longitudinal muscle layers
• a network of nerve cells
• Serosa (a layer of connective tissue)
• The salivary glands and pancreas contain clusters
  of exocrine secretary cells called the acini,
  which lead into a system of ducts that converge
  and empty into the mouth or duodenum
• Gastrointestinal circulation
• splanchnic circulation (circulation of the
  stomach, small and large intestines, pancreas,
  and liver)
• portal circulation (which brings blood from the
  stomach, intestines, and pancreas to the liver)

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• B. The processes of the GI system
• Motility
• Mixing movements
• Propulsive movements (rate depends on the
  function served by the organ)
• Secretion
• Inorganic ions (H, HCO3-)
• Organic components
• digestive enzymes (salivary glands, stomach, and
  pancreas)
• bile salts
• pigments
• cholesterol
• water and solutes

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• Digestion (Large organic molecules are broken
  down into smaller molecules that can be absorbed)
• Absorption (Various transport systems are used to
  move digestive products from the lumen to the
  blood and lymph)
• C. Regulation of GI functions by neural and
  hormonal mechanisms
• Motility and secretion are regulated by neural
  and hormonal mechanisms, while digestion and
  absorption are not
• Many of the stimuli that initiate the regulatory
  events are present in the lumen of the digestive
  system
• Chemoreceptors and mechanoreceptors respond to
  the stimuli

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• Neural control of GI processes
• Short reflexes mediated by nerves lying within
  the wall of the digestive tract (the enteric
  nervous systme little brain)
• excitatory
• inhibitory
• Long reflexes mediated by nerves outside of the
  tract
• CNS
• peripheral nerves
• the vagus nerve (a parasympathetic nerve)
• the splanchnic nerves (sympathetic postganglionic
  nerves)

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Neural control of GI processes
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• Four hormones important in the control of
  motility and secretion
• Gastrin
• cholecystokinin (CKK)
• Secretin
• gastric inhibitory peptide (GIP)

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• 2. Chewing, Salivary Secretion, and Swallowing
• A. Chewing reduces the size of food particles and
  mixes them with saliva
• Chewing is accomplished by combined actions of
  the skeletal muscles of the jaws, lips, cheeks,
  and tongue
• Chewing is partly reflexive in nature and aids
  digestion
• B. Salivary secretion - lubrication of the mouth
  and initiation of the digestion of starch
• Salivary glands
• The parotid glands
• Submaxiilary glands
• Sublingual glands

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• Major stimuli chewing
• Major components of saliva water, mucus, and
  amylase
• Regulation of salivary secretion - by long
  reflexes involving sympathetic and
  parasympathetic nerves
• Salivary secretion serves a number of functions,
  including
• Softening and lubricating food
• Neutralizing acids produced by oral bacteria
• decrease of salivary secretion ? sensation of
  thirst
• salivary amylase degrades complex carbohydrates

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• C. Swallowing moves food rapidly from the mouth
  into the stomach
• Long reflexes mediated through a swallowing
  center in the brain coordinate actions of smooth
  and skeletal muscles involved in swallowing the
  bolus of food
• Food is prevented from entering the glottis by
  the epiglottis
• Role of the esophagus in swallowing
• upper esophageal sphincter (skeletal muscle)
• lower esophageal sphincter (smooth muscle)
• pressure gradient in the esophagus
• The all-or-none swallowing reflex is activated
  when a bolus stimulates mechanoreceptors in the
  pharynx

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• 3. The Processes of Motility and Secretion in
  the Stomach
• A. Muscular activities of the stomach and their
  regulation
• Peristalsis
• The motor activity of the stomach serves the
  major functions of
• mixing food with gastric secretions
• conversion of large solid food particles to
  liquid suspension of fine particles
• emptying of gastric contents into the duodenum
• storage

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• The storage function of the stomach
• reduction of the tone in the muscular wall of the
  stomach
• accommodate large volumes with minimal changes in
  intragastric pressure
• a limit (capacity) of a completely relaxed
  stomach 1.5L
• Regulation of stomach motility
• myogenic mechanism basic electrical rhythm
• excitatory stimuli à action potentials à more
  vigorous contraction
• neural mechanism
• mechanorecptors in the gastric wall distention
• short reflex
• long reflex

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• hormonal mechanisms
• gastrin - increase peristalsis
• inhibition of stomach motility - by duodenal
  inhibitory mechanisms
• stimuli fat, acidic, hyperosmotic or bulky
  contents
• sensors osmoreceptors, mechanoreceptors,
  chemoreceptors
• secretin
• gastric inhibitory peptide
• enterogastric inhibitory reflex

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• B. Gastric secretion
• The exocrine secretary glands contain three types
  of secretary cells
• Chief cells pepsinogen
• Parietal cells - HCI, intrinsic factor (necessary
  for vit.B12 absorption)
• Mucous cells - alkaline mucus (for protection of
  the mucosa)
• Transport of H and Cl- - active transport

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Cell types of gastric mucosa
Stomachsecretion integration
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• Transport of H and Cl- - active transport

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• Regulation of secretion of HCl in three phases
• cephalic phase
• gastric phase
• intestinal phases
• Both short and long reflexes via the vagus nerve
  are involved
• role of gastrin (G-cells)
• role of histamine in secretion of HCl

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Intestinal Phase of gastric function
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• Secretion of pepsinogen (the precursor of pepsin)
• synthesized and stored in the chief cells
• secretion of pepsinogen simulated by Ach. via
  short and long reflexes
• conversion to pepsin by HC1 and pepsin itself
• substrate of pepsin proteins end products
  peptides
• ideal pH for pepsin pH 2-3
• Both peptides and acetylcholine play important
  roles in the activation of pepsin and in the
  maintenance of the highly acidic environment
  required for optional activity of pepsin

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• B. Vomiting
• Vomiting - a reflex act coordinated by a center
  located in the medulla
• The force for expulsion
• an increase in intragastric pressure
• an increase in abdominal pressure
• Stimuli
• emetics - chemical agents that can stimulate
  chemoreceptors in the stomach and the small
  intestine or brain
• disturbances in the vestibular apparatus
• stimulation of sensory nerve endings in the
  throat
• Consequences of prolonged vomiting
• Dehydration
• loss of electrolytes (Na, K, Cl-, HCO3-)
• metabolic alkalosis

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• 4. Secretion of the Exocrine Pancreas
• A. Secretion of pancreatic bicarbonate
• The epithelial cells lining of the intercalated
  ducts secrete an alkaline juice that contains a
  high concentration of sodium bicarbonate

• The bicarbonate protects the duodenum from damage
  by gastric acid and provides an optimum pH for
  pancreatic digestive enzymes

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• B. Secretion of pancreatic digestive enzymes
• protein-digesting enzymes
• trypsin - converted by enterokinase from
  trypsinogen

• chymotrypsin - converted by tripsin from
  chymotrypsinogen
• carboxypeptidase - converted by trypsin from
  procarboxypetidase
• Pancreatic lipase - triacylglycerol fats
• Other enzymes phospholipase, cholesterol
  esterase, amylase, ribonuclease, and
  deoxyribonuclease

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• C. Regulation of pancreatic secretion
• Hormonal regulation
• secretin released from the duodenal mucosa
• increase secretion of bicarbonate-rich juice
• the most important stimulus acid
• cholecystokinin released from the duodenal mucosa
  
• increase secretion of digestive enzymes from
  acinar cells
• stimuli protein and fat products

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• 5. The Secretion in the Biliary System
• A. Important structures in biliary system
• hepatocytes - bile producing cells
• bile canaliculi
• the hepatic duct
• the cystic duct
• Gallbladder
• the common bile duct
• sphincter of Oddi

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• B. Secretion of the bile
• Bile secreted by hepatocytes
• Stored in the gallbladder
• Major functions help in the digestion and
  absorption of fat
• Bile composition bile salts, pigments,cholesterol
  , lecithin, and inorganic ions such as sodium,
  chloride, and bicarbonate
• C. Enterohepatic circulation of bile salts
• Bile salts are recycled between the liver and
  small intestine
• D. Regulation of the secretion of bile
• Concentration of bile salts in the portal blood
• Secretin stimulates epithelial cells lining of
  the bile ducts to release sodium bicarbonate to
  neutralize gastric acid in the small intestine

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• 6. Motility, Secretion, and Absorption in the
  Small Intestine
• A. The muscular movements of the small intestine
• Segmenting contractions (mixing movements)
• Peristalsis
• Gastrin increases motor activity of the ileum and
  relaxes the ileocecal sphincter
• B. Secretion of water and solute from blood to
  small intestinal lumen
• Fluid moves in response to the hyperosmotic
  contents of the lumen
• Mucosal exocrine cells secrete water, inorganic
  ions, and mucus into intestinal lumen

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Daily mass balance in the digestive system
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• C. Absorption of nutrients
• Villi, and microvilli increase the surface area
  of the small intestine
• Water absorption by osmosis - secondarily to the
  absorption of dissolved minerals
• Sodium absorption - actively transported
• Other inorganic ions (e.g., calcium, potassium,
  zinc, chloride, phosphate, bicarbonate) and iron
  and iodide are actively or passively transported
  into the blood

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• Carbohydrates digested enzymatically (by maltase,
  lactase, sucrase) into monosaccharide units,
  which may be absorbed by different mechanisms
• Facilitated diffusion (fructose)
• Active transport by carrier-mediated process
  (glucose)
• Co-transport (glucose, galactose coupled with
  sodium)
• Amino acids - transported by active processes
• specific carrier for each class of amino acid
• transport coupled with sodium transport
• dipeptides and tripeptides also absorbed
• In the newborns of some species, proteins are
  absorbed by pinocytosis

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Carbohydrate digestion
Fat digestion
Endopeptidases cleave interior peptide bonds
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Peptide absorption
Carbohydrate absorption
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• Triacylglycerols
• emulsification by bile salts - Bile salts render
  monoglycerides and free fatty acids water soluble
  by facilitating the formation of micelles
• digetstion by pancreatic lipase into a
  monoglyceride and free fatty acid molecules

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• 7. Processes of Absorption, Secretion, and
  Motility in the Large Intestine
• A. Water and sodium are absorbed and potassium
  and bicarbonates are secreted by the large
  intestine
• The active transport of sodium from the colon -
  accompanied by the osmotic absorption of water
• The colonic mucosa secretes an alkaline fluid
  that protects the mucosa from chemical and
  mechanical trauma and lubricates the fecal mass,
  thus facilitating its passage
• B. Motility of the large intestine
• Segmenting contractions promote absorption of
  water and inorganic ions by exposing the contents
  to the mucosa
• Propulsive mass movements may be stimulated by
  gastrin, the gastrocolonic reflex, and distention
  of colonic walls

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• C. The defecation reflex
• Mass contractions move the contents into the
  rectum, whose distention elicits the defecation
  reflex
• Constipation, the accumulation of feces in the
  large intestine above the normal capacity, occurs
  when the muscular activity of the large intestine
  is decreased

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Colonocytes NaCl reabsorption
Colonic crypt cells NaCl