GI tract is also known as the alimentary canal GI tract is a barrier to entry of harmful bacteria into the body and produces immune components that destroy microorganisms in the gut Nutrients must pass through walls of GI tract to be absorbed 4 layers Mucosa innermost layer lined with epithelial cells folded and covered with villi that project into the lumen Submucosa connective tissue glands blood vessels and nerves Muscle inner layer of circular smooth muscle and outer layer of longitudinal muscle fibers stomach has third layer that runs diagonally Serosa outermost protective layer secretes fluid that cushions GI tract and reduces friction Sphincters ring like muscles that open and close like valves to control the flow of GI Peristalsis Coordinated wave of contraction and relaxation that propels chewed food contents Controls food transport down GI tract Occurs in Esophagus Stomach Small Intestine Large Intestine mixes them with digestive juices Occurs in Small Intestine Segmentation Back and Forth action that breaks chewed food into smaller pieces and Mouth oral cavity Where digestion begins Contains the salivary glands o Saliva dissolves taste forming compounds in food Produce 4 cups a day Teeth grind food into smaller particles Tongue mixes food with saliva creating a bolus mass of food that is swallowed Functions Chew food Perceive taste Lubricate food Release starch digesting enzyme amylase Initiate swallowing reflex Taste Smell Esophagus Stomach Flavor sensations are created by a combination of a food s aromatic smell and taste compounds texture and certain chemicals e g capsaicin in chili peppers Taste buds on papillae of tongue and soft palate contain specialized cells that can detect taste compounds in foods Moves bolus chewed food down GI Tract via peristalsis At entrance of esophagus is the epiglottis Flap like structure that prevents food from entering trachea windpipe Food passes from esophagus into stomach through lower esophageal sphincter Little digestion occurs in stomach Some fat digestion 20 of alcohol absorbed Stomach Secretions Secretes 8 cups gastric juice aids in digestive process Gastric juice Gastrin Hormone that controls release of HCl and pepsinogen Secretion highest at beginning of meal and declines as meal progresses Mucus Secreted from mucous cells on gastric mucosa Lubricates and protects stomach from being digested by HCl and pepsin Mixes food with gastric secretions Chyme Soupy acidic mixture of stomach secretions and partially digested food Regulates emptying of chime into small intestine Small Intestine 3 sections Duodenum digestion primarily occurs here upper part of Jejunum Jejunum Ileum Requires secretions from enterocytes pancreas liver and gallbladder Most nutrient absorption occurs absorbs 95 food energy Small Intestine Absorption Circular folds and fingerlike projections villi and microvilli Villi are lined with Goblet cells produce mucus Endocrine cells produce hormones and hormone like substances Enterocytes produce digestive enzymes and absorb nutrients Microvilli Make up brush border on enterocytes Covered with glycocalyx contains digestive enzymes Secretes 6 cups day of mucus enzyme and hormone containing fluid Absorptive cells migrate from crypts base of villi to surface of villi Mature as they migrate increase absorptive capabilities Shed into lumen when damaged by digestive enzymes every 2 5 days Nutrients absorbed in small intestine Calcium magnesium iron other minerals Glucose Amino acids Fats Vitamins Water 70 90 of total Alcohol 80 of total Bile acids Accessory Organs Secretions from these organs delivered through common bile duct and pancreatic duct Ducts come together and empty into duodenum at sphincter of Oddi also called hepatopancreatic sphincter Liver Produces bile 2 4 cups day Cholesterol containing yellow green fluid Emulsifies fat into micelles Reabsorbed in ileum and returned to liver enterohepatic circulation Small amount not reabsorbed released in feces only way to excrete cholesterol Gallbladder Pancreas Stores bile until needed Produces pancreatic juice 5 6 cups day Sodium bicarbonate neutralizes chyme to protect small intestine Pancreatic amylase digests starch Pancreatic lipase digests fat Proteases digest protein 4 Major Nutrient Absorptive Processes Passive Diffusion concentration of nutrient higher in lumen than in absorptive cells nutrient moves into absorptive cells by diffusion e g fats water some minerals nutrients move down concentration gradient but require carrier proteins to move into absorptive cells e g fructose Facilitated Diffusion Active Absorption nutrients move against a concentration gradient and require energy for absorption e g amino acids glucose Endocytosis Phagocytosis Pinocytosis absorptive cells engulf compounds form vesicle absorptive cells engulf liquids form vesicle Hormones Released by Ileum and Large Intestine in response to fat in large intestine o Gastric inhibitory peptide Hormone that slows gastric motility slows release of chyme into small intestine o Peptide YY inhibits gastric and pancreatic secretions Small intestine o Cholecystokinin CCK Released in response to dietary fat in chyme and stimulates release of pancreatic enzymes and bile from gallbladder o Secretin Released in response to acidic chyme and stimulates release of pancreatic bicarbonate o Motilin in response to gastric distension and dietary fat and it regulates motility od the gastrointestinal tract Gastrin Released by stomach duodenum in response for food reaching the stomach and triggers the stomach to release HCl stimulates gastric and intestinal motility Somatostatin Released by stomach small intestine and pancreas It inhibits the release of GI hormones slows gastric emptying GI motility and blood flow to the intestine Large Intestine 3 sections colon rectum anus Colon separated into 5 sections Cecum Ascending colon Transverse colon Descending colon Sigmoid colon 3 main functions Houses bacterial flora Absorbs water and electrolytes Forms and expels feces slower peristaltic waves propel fecal matter toward rectum for elimination Primary Nutrients Absorbed Sodium Potassium Some fatty acids Vitamin K Biotin Gases Water 10 30 Circulatory Systems Cardiovascular System o Components o Transports Lymphatic System o Components o Transports Heart blood vessels arteries capillaries veins blood Water soluble nutrients proteins carbohydrates B vitamins vitamin C Short and medium chain fatty acids Lymph flows