Digestive System

• • Digestion breaks polymers (carbs, fats, and proteins) into monomers (monosaccharides, fatty acids, amino acids)
  • digestion occurs via hydrolysis reactions
• 90 percent of nutrient absorption occurs in small intestine
• Digestive tract functions

  • motility - ingestion (taking food into the mouth), mastication, deglutination (swallowing), peristalsis, segmentation (churning/mixing)
• Digestive tract function

  • secretion - exocrine ( digestive enzyme, acid, mucus), endocrine (hormones to regulate digestion)
  • digestion - breaking down food to be absorbed
  • absorption - passing broken down food into blood or lymph
• Digestive tract function

  • storage and elimination
  • immune barrier - simple columnar epithelial w tight junctions prevent swallowed pathogens from entering body.
• Digestive system

  • composed of alimentary canal (the whole tube from mouth to butt)
  • organs include - oral cavity, pharynx, esophagus, stomach, small and large intestine
  • accessory organs - teeth, tongue, salivary glands, liver, gallbladder, and pancreas
• GI motility regulated by parasympathetic activity arising from vagus and spinal nerve. also from pancreas and hormonal signals
• MOUTH

  • mastication - chew break down food and mix with saliva for deglutition
  • saliva - contains mucus ( antimicrobial agent (lysozyme)) and salivary amylase to start digestion of carbs
• Stomach functions

  • store food
  • churns food to mix w gastric secretions
  • begin protein digestion via hydrochloric acid (break down peptide bonds) and pepsin (break down actual protein)
  • mechanical digestion of nutrients
  • kill bacteria via hydrochloric acid
  • move small portions of food at a time to small intestine in form of chyme
• Stomach cells

  • goblet cells - secrete mucus
  • parietal cells - secrete HCI
  • chief cells - secrete pepsinogen
  • enterochromaffin-like cells - secrete histamine and serotonin
  • g cells - secrete gastrin
  • d cells - secrete somatostatin
• HCI in stomach

  • vagus nerve > g cells > gastrin > ECL cells > histamine > parietal cells > HCI > stomach lumen > denatures proteins
• HCI in stomach

  • makes gastric juice acidic which denatures proteins to be digested
  • also makes pepsinogen into pepsin
• Stomach defenses

  • acid and pepsin could eat thru the stomach lining but don’t bc of: layer of mucus with bicarbonate, tight junctions between epithelial cells, and rapid epithelial mitosis (3 days)
• Digestion and absorption in stomach

  • proteins are partially digested by pepsin
  • carbs digested by salivary amylase is inactivated by acidity
  • water, alcohol, and aspirin are commonly ingested substances stomach
• Small intestine functions

  • complete digestion of carbs, proteins and fats
  • absorption of sugars, lipids, amino acids, calcium, and iron in duodenum and jejunum
  • bile salts, vitamin b12, water, electrolytes in ileum
  • absorption is quick due to villi and microvilli
• Large Intestine Function

  • has no digestive capabilities
  • absorbs water, dissolved electrolytes, vitamin k, and some b vitamins
  • vitamin k and b are are made by microbial organisms in large intestine
  • storage of feces
• Large Intestine

  • intestinal flora contains large population of microflora
  • 400 different species of commensal (helpful) bacteria.
  • which produce folic acid and vitamin k
  • ferment indigestible foods to produce fatty acids
  • reduce ability of pathogenic bacteria to infect colon
  • antibiotics can kill commensals
  • probiotics can replace commensals
• Functions of Liver

  1. lipid metabolism: lipolysis, lipogenesis, synthesis of cholesterol
  2. protein metabolism: synthesizes plasma proteins (albumin, fibrinogen, alpha and beta goblins, immunoglobulin, and prothrombin). Breaks them down to carbs or lipids for storage
  3. Carbohydrate metabolism: helps maintain normal glucose levels by
  4. breaking glycogen to glucose then putting it in blood
  5. converting serum glucose into glycogen and triglycerides for storage
• Function of the Liver

  • detoxification - processes drugs and hormones; detoxifies substances like alcohol or excretes drugs such as the antibiotics into bile
  • synthesis of bile salts - salts used in S.I. for emulsification and absorption of lipids, cholesterol, phospholipids, and lipidproteins
  • storage - stores glycogen, vitamins, and minerals
  • phagocytosis - kupffer cells phagocytize RBCs, WBCs, bacteria, and toxins
• The Gallbladder

  • stores and concentrates bile produced by liver
  • when S.I. empty, sphincter of Oddi in common bile duct closes and bile is forced up into gallbaldder
  • when S.I. has food, the sphincter opens, gallbladder contracts, and bile is ejected thru ducts into duodenum
• The Pancreas

  • has both exo/endocrine functions
  • endocrine - performed by Islets of Langerhans cells, secrete insulin and glucagon
  • exocrine secretions - bicarbonate (HCO3-) and digestive enzymes
  • they pass in pancreatic duct to small intestine
• Cephalic phase of digestion

  • extrinsic control of gastric function is divided into cephalic, gastric, and intestinal phases
  • cephalic phase stimulated by sight, smell, though, and taste
  • activation of vagus nerve by this phase stimulates
  • salivary glands to secrete saliva
  • chief cells to secrete pepsinogen
  • g cells to secrete gastrin
  • ECL cells to secrete histamine
  • parietal cells to secrete HCI
• Gastric phase of digestion

  • the presence of proteins, polypeptides, and amino acids in stomach raises pH
  • the chemical change, and stomach distension activate the gastric phase
• Intestinal Phase of DIgestion

  • entergasterones (intestinal hormones) are secreted by intestines when chyme gets to duodenum
  • somatostatin - inhibits stomach acid secretion
  • cholecystokinin - stimulates secrete of pancreatic enzymes; contract of gallbladder; brings feeling of fullness; slows gastric motility and acid secretion
  • secretin - stimulates secrete of pancreatic bicarbonate and b8ile from liver; inhibits acid production and gastric motility
  • gastric inhibitory peptide - inhibits gastric motility and secretion; increases secrete of insulin from pancreas
• Carbohydrate digestion

  1. starts in mouth when salivary amylase liberates oligosaccharides/disaccharides from starch
  2. oligosaccharides and disaccharides pass stomach and enter S.I. in the form of chyme, where pancreatic enzymes further digest them into oligo/tri/disaccharides.
  3. oligo/tri/disaccharides continue thru S.I. and brush border enzymes liberate them into monosaccharides which are them absorbed into bloodstream
• Protein digestion

  1. begins in stomach where HCI denatures and pepsin liberates proteins into polypeptides
  2. go to S.I. (duodenum) where pancreatic enzymes turn polypeptides into oligopeptides some tri/dipeptides
  3. those continue going thru S.I. and brush border enzymes (animo peptidase) liberate tri/di and amino acids
  4. amino acids are them absorbed into bloodstream
• Fats digestion

  1. eat fats > lingual paste turns some globs of fat into triglycerides
  2. other globs of fat are turned into droplets of fat by bile from liver and gallbladder in S.I.
  3. droplets of fats are the turned into monoglycerides/fatty acids by pancreatic lipase in S.I.
  4. monoglycerides/fatty acids are absorbed into cells, and they are hooked together back into triglycerides which are packaged into chylomicrons
  5. chylomicrons are then absorbed into central lacteals (small lymphatic)and then travel thru lymphatic system and then enter blood at subclavian veins
• transport of lipids

  1. liver packages stored phospholipids, tryglycerides, cholesterol into VLDLs which are secreted into blood.
  2. they transport lipids to body cells
  3. when triglycerides are removed from VLDL they are not LDL - they transport remaining cholesterol and lipids to blood vessels and organs.
  4. an excess of these is associated w cardiovascular disease
  5. HDLs also made by liver, collect excess cholesterol from tissues and take it back to the liver for metabolism
  6. high ratio of HDL to total cholesterol is believed to confer protection against cardiovascular disease