topic 10

• the alimentary canal or gastrointestinal tract digests and absorbs

  food
• the alimentary canal is made up of

  the mouth, pharynx, esophagus, stomach, small intestine, and large intestine
• accessory digestive organs


  teeth, tongue, gallbladder, digestive glands (salivary glands, liver, pancreas)
• activities of the GI tract

  ingestion

  propulsion

  mechanical digestion

  chemical digestion

  absorption

  defecation
• ingestion


  Intake of food
• propulsion


  swallowing + peristalsis
• mechanical digestion


  chewing, mixing, and churning of food, and segmentation
• chemical digestion


  catabolic breakdown of food
• absorption


  nutrients absorbed from gi tract into blood/lymph
• defecation


  discarding indigestible solid waste
• 4 tunics of the stomach

  mucosa

  sub mucosa

  muscularis externa

  serosa
• muscularis externa has an

  additional layer of smooth muscle, the oblique layer that allows the stomach to break down food into smaller fragments
• Mucosa: Epithelial Lining


  is simple columnar epithelium composed of mucous cells and gastric pits
• mucous cells


  produce a coat of alkaline mucus, the mucous surface layer traps a bicarbonate rich fluid beneath it
• gastric pits


  contain gastric glands that secrete gastric juice, mucus and gastrin
• surface mucous cell


  secretes alkaline fluid containing mucin
• mucous neck cell


  secretes acidic fluid containing mucin
• parietal cells


  secrete hydrochloric acid and intrinsic factor
• chief cells


  produce pepsinogen, which is activated to pepsin by HCl in the stomach and pepsin itself via positive feedback mechanism
• enteroendocrine cells


  secrete histamine, endorphins, serotonin, cholecystokinin, and somatostatin and gastrin into the interstitial fluid of lamina propria
• stimulatory and inhibitory events of gastric secretion occur in three phases


  - cephalic phase: prior to food entry

  - gastric phase: once food enters the stomach

  - intestinal phase: as partially digested food enters the duodenum
• release of gastric juice; stimulate events

  - cephalic phase via vagus nerve; sight of food and smell of food

  - gastric phase via vagus nerve; stomach distension
• release of gastric juice; inhibitory events

  - loss of appetite and depression

  - excessive acidity in the stomach

  - emotional upset

  - distension of the duodenum by food stuffs, or irritants can stimulate local reflexes and vagal nuclei of the medulla, or pressure at the pyloric sphincter can initiate the entero-gastric reflex

  - distension of the duodenum by food stuffs stimulates the release of intestinal hormones
• gastric contractile activity


  - this basic electrical rhythm is initiated by pacemaker cells

  - most vigorous peristalsis and mixing occurs near the pylorus

  - chyme is either delivered in small amounts to the duodenum or forced backward into the stomach for further mixing
• bile


  a yellow green sticky alkaline solution containing bile salts, bile pigments, cholesterol, neutral fats, phospholipids, and electrolytes
• bile salts are cholesterol derivates that


  emulsify fats

  facilitate fat and cholesterol absorption

  help solubilize excess cholesterol
• enterohepatic circulation recycles


  bile salts
• the chief bile pigment is

  bilirubin, a waste product of heme
• inactive trypsinogen turns into

  active trypsin with the help of membrane bound enteropeptidase
• inactive chymotrypsinogen turns into


  active chymotrypsin with help from trypsin
• inactive procarboxypeptidase turns into

  carboxypeptidase with help from trypsin
• regulation of bile and pancreatic juice release

  1) acidic, fatty chyme entering duodenum causes release of cholecystokinin and secretin from duodenal wall enteroendocrine cells

  2) cholecytokinin and secretin enter the blood stream

  3) bile salts and secretin transported via bloodstream stimulated live to produce bile more rapidly

  4) vagal stimulation causes weak contractions of gallbladder

  5) cholecystokinin causes gallbladder to contract and hepatopancreatic sphincter to relax; bile enters duodenum

  6) bile salts reabsorbed into blood
• small intestine: gross anatomy


  - runs from pyloric sphincter to the ilecocecal valve

  - has 3 subdivisions: duodenum, jejunum, and ileum

  - the bile duct and main pancreatic duct:

  ~ join the duodenum at the hepatopancreatic ampulla

  ~ enter the duodenum at the major duodenal papilla

  ~ are controlled y the hepatopancreatic sphincter

  - the ileum joins the large intestine at the ileocecal valve
• plicae circulares


  permanent circular folds of the mucosa and submucosa that force chyme to slowly spiral through lumen
• villi


  motive fingerlike extensions of the mucosa, villus epithelium is composed of simple columnar absorptive cells and goblet cells
• microvilli


  tiny projections of absorptive cells, bear brush border enzymes
• intestinal crypt epithelium


  secretory cells that produce intestinal juice, enteroendocrine cells
• duodenal glands


  secrete alkaline mucus
• peyers patches


  provide immunosurveillance
• intestinal juice


  secreted by cells in that line the intestinal glands, these glands are located in between the villi of the epithelium of the small intestine

  released in response to distension or irritation of the mucosa

  slightly alkaline and isotonic

  largely a pale yellowish watery substance that contains mucus, enzymes and hormones

  facilitates transport and absorption of nutrients