PHYSIOLOGY FINAL: digestive

• Digestive system functions

  • Motility
  • Secretion
  • Digestion
  • Absorption
  • Storage and elimination of indigestible food
  • Acts as an immune barrier
• Motility


  Movement of food through the digestive tract via various processes
• Secretion

  Release of substances to the outside of a cell via an exocrine or endocrine route. Examples include enzymes, HCl, bile salts, mucus, hormones
• Digestion


  Breakdown of complex food molecules into smaller subunits which can be absorbed
• Digestion products


  • Carbohydrates - Simple sugars
  • Proteins - Amino acids, small peptides
  • Fats - Glycerol + fatty acids
• Absorption


  Movement of molecules from opening of digestive tract into the blood or lymph
• Regulation of the GI tract

  • Intrinsic sensory neurons (enteric nervous system) in walls of digestive tract coordinate local activity
  • Extrinsic nerves - Nerves from the ANS that regulate the activity of the GI tract
  • Gastrointestinal hormones (Gastrin, Ghrelin, Secretin, CCK, GIP)
• Functions of the mouth

  • Ingestion (taking food into mouth)
  • Taste (gustation)
  • Chewing (mastication) - grinding, breaking up, and mixing food with saliva
• Saliva


  Secreted by salivary glands (controlled by the ANS) and has many functions, including starting carbohydrate digestion with salivary enzymes (amylases)
• Digestion in the mouth is minimal and involves no absorption
• Burping (eructation)


  Expulsion of air from the mouth
• Vomiting (emesis)


  A protective reflex originating in a vomiting center in the medulla oblongata
• Swallowing (deglutition)


  A sequentially programmed response coordinated by a swallowing center in the medulla oblongata
• Swallowing


  1. Propels mass of food (bolus) forward by peristalsis (contractions of the circular smooth muscle that push the bolus ahead of the contraction)

  2. Gastroesophageal sphincter regulates passage of food into stomach and prevents reflux out of the stomach
• No digestive enzymes secreted by esophagus
• Stomach


  • Capable of filling, storing, mixing, and emptying
  • Movement of material here occurs by peristalsis
  • Secretes mucus, HCl, pepsinogen, intrinsic factor (IF)
  • Pepsinogen, in the presence of HCL, is converted to pepsin which digests protein
  • Does not absorb food, but can absorb alcohol and aspirin
• Gastrin


  Hormone that stimulates various activities in the stomach
• Ghrelin


  Hormone that may stimulate appetite and food intake. Found to be higher before meals and lower after meals
• Small intestine segments


  • Duodenum (D)
  • Jejunum (J)
  • Ileum (I)
• Small intestine


  • Most digestion and absorption occur in the D and J
  • Most contractile activity occurs by segmentation = simultaneous contractions and relaxations of smooth muscle that mix the contents and propel chyme short distances along the SI
  • Digestion in the SI lumen is accomplished partly by pancreatic enzymes
  • Well adapted for absorption with modifications like villi and microvilli to increase surface area
  • Ileum - Some absorption (bile salts, Vitamin B12, water, and electrolytes) occurs here
• Secretin


  Hormone that inhibits gastric function and stimulates bicarbonate secretion from pancreas
• Cholecystokinin (CCK)


  Hormone that inhibits gastric function, stimulates pancreatic enzyme secretion, and stimulates gallbladder contraction to release bile
• Gastric inhibitory peptide (GIP)


  Hormone that inhibits gastric function
• Large intestine


  • Some absorption occurs here (electrolytes, water, and vitamins)
  • One way material moves is via a wave of contraction called a mass movement that triggers defecation
  • Significant function is to dry, by extracting water, and store feces
  • Elimination of feces (defecation) is under reflex control and occurs when rectal pressure rises to a particular level
  • Does not secrete digestive enzymes as digestion is complete before chyme reaches the colon
  • Rich source of bacteria (gut microbiome) that synthesize Vitamin K and Vitamin B
• Flatulence


  Passage of intestinal gas derived either from swallowed air or generated from the actions of bacteria
• Accessory digestive organs


  • Pancreas (exocrine portion)
  • Liver
  • Gallbladder
• Pancreas (exocrine portion)


  • Secretes enzymes (e.g., amylase, trypsin, lipase) - most in inactive zymogens
  • Secretes bicarbonate which neutralizes stomach acid in the small intestine and facilitates enzyme function
  • Secretion is hormonally regulated by the hormones CCK and secretin
• Liver


  • Produces and secretes bile and bile salts (derived from cholesterol) which aid in the emulsification of fats
  • Bile also contains bilirubin (derived from the breakdown of heme)
  • Most bile salts are reabsorbed in the small intestine and returned to the liver via the enterohepatic circulation
  • Processes nutrients after absorption from the digestive tract via the hepatic portal system
  • Stores glycogen and fats; releases glucose and fatty acids into blood
  • Produces and secretes ketones from fatty acids
  • Synthesizes plasma proteins (for steroid and cholesterol transport)
• Gallbladder


  • Located under the liver
  • Stores and concentrates bile
  • Target organ of the hormone CCK