gi system

• Stages of digestion


  1. Ingestion

  2. Peristalsis

  3. Digestion

  4. Absorption

  5. Defaecation
• Digestive organs

  • Gastrointestinal tract
  • Mouth & pharynx
  • Oesophagus & stomach
  • Small intestine (duodenum, jejunum, ileum)
  • Large intestine (colon, rectum)
  • Teeth and tongue
  • Salivary glands, liver, gallbladder, pancreas
• Mouth


  Food mixed with saliva as it is chewed
• Salivary glands


  • Parotid
  • Submaxillary
  • Sublingual
• Around 1500ml saliva secreted each day
• Components of saliva


  • Mucous to help lubricate food
  • α-amylase to initiate breakdown of carbohydrate
• Formation of saliva

  1. Isotonic fluid produced by acinar cells by secretion of electrolytes and water

  2. Fluid modified as it flows along salivary duct

  3. Final composition depends upon flow rate
• Regulation of salivary secretion


  • Parasympathetic (watery saliva, rich in amylase and mucus)
  • Sympathetic (promotes output of amylase, reduces blood flow to glands, overall reduction in secretion)
• Swallowing (deglutition)


  1. Voluntary stage (tongue pushes bolus backwards to ortho-pharynx)

  2. Pharyngeal stage (reflex action controlled by deglutition centre in medulla and pons of CNS, soft palate and uvula move up to seal off nasopharynx, larynx is raised and epiglottis covers and seal glottis and breathing suspended for 1-2 secs)
• Oesophagus


  Tube connecting to stomach, muscularis layer is striated for first third to assist swallowing, middle third is a mixture of striated and smooth muscle, lower third is all smooth muscle
• Oesophageal phase


  No absorption of food or secretion of digestive enzymes occurs but mucous produced to aid swallowing, food pushed along by peristalsis controlled by medulla of the brain and autonomic nerves, lower oesophageal sphincter relaxes to allow food to enter stomach, passage of bolus takes 4-8 seconds
• Peristalsis


  Contractile activity in GI tract
• GI anatomy


  • Mucosa (epithelial layer and connective tissues, capillaries, enteric neurones, immune cells, surface area greatly increased by villi/glands)
  • Submucosa (loose connective tissues & large blood vessels, contains glands for secretion)
  • Muscle layer (2 layers - circular/longitudinal)
  • Enteric nervous system
  • Serosa (connective tissue, squamous epithelia)
• Segmentation


  Occurs largely in small intestine to facilitate mixing of food
• Peristalsis


  Concerned mainly with propulsion of food along tract
• Innervation of the GI tract


  • Enteric nervous system (myenteric (Auerbach's) plexus largely motor in function, submucosal (Meissner's) plexus largely sensory, release range of substances like cholecystokinin (CKK), substance P, VIP, somatostatin, enkephalins)
  • Extrinsic innervation (nerve plexuses linked to CNS via afferent fibres activated by stretch, chemical simulation, receive efferent innervation from the ANS, parasympathetic input stimulates gut motility and secretory activity, sympathetic nerves cause pre-synaptic inhibition of parasympathetic-induced contraction)
• Hormonal regulation of GI tract

  Around 20 different regulatory peptides act through endocrine/paracrine pathways, endocrine (hormonal) peptides (e.g.gastrin) regulate secretions in stomach and pancreas, paracrine (local) agents (e.g. histamine) regulate secretion in salivary glands and stomach
• Functions of the stomach

  • Temporary storage of food
  • Chemical digestion of proteins
  • Mechanical digestion by stomach movements
  • Regulation of passage of chyme into small intestine
  • Secretion of intrinsic factor – essential for absorption of vitamin B12
• Gastric glands


  • Chief or peptic or zymogenic cells (secrete pepsinogen)
  • Parietal or oxyntic cells (secrete HCl and intrinsic factor)
  • Goblet cells (secrete mucous)
  • Enteroendocrine (G-cells) (secrete gastrin, VIP, GIP, CCK)
• Mechanical digestion in the stomach

  3 layers of muscularis enable food to be churned, food mixed with gastric juice to produce chyme, chyme passes through pyloric sphincter to duodenum
• Chemical digestion in the stomach

  Proteins broken down by pepsin (only active in acid environment, converted from pepsinogn by HCl), mucous protects stomach from gastric juices, gastric lipase (also from chief cells) breaks down fat at higher pH (5-6)
• Phases of gastric secretion

  • Cephalic phase (stimulated by sight, smell, taste, thought of food or decreased blood glucose)
  • Gastric phase (stimulated by stomach distension due to presence of food)
  • Intestinal phase (stimulated by digested proteins in the duodenum)
• Why doesn't the stomach digest itself?

  Mucus gel layer, mucosal barrier, tight junctions between mucosal epithelial cells prevents leakage of gastric juice onto underlying tissue, mucous secreted by epithelial cells has a higher pH, providing localised neutralisation and physical barrier to acid, prostaglandins increase mucosal thickness and stimulate bicarbonate production
• Inhibition of gastric secretion


  Neuronal inhibition (partially digested in duodenum inhibits secretion by "enterogastric reflex", reflex mediated by medulla and leads to decreased parasympathetic stimulation), hormonal inhibition (secretin and cholecystokinin released from duodenum, other inhibitors include motilin, GIP, gastrone, glucagon, VIP)
• Gastric motility


  Results from contraction of the 3 layers of muscle in the stomach wall to grind, churn, knead, twist and propel contents, co-ordinated by myenteric plexus which receives input from ANS, parasympathetic stimulation increases motility, sympathetic stimulation decreases motility
• Gastric emptying


  Stimulated and inhibited in a similar manner to gastric secretion, emptying occurs at a rate proportional to gastric volume, physical and chemical nature of contents influence rate, involves constriction of lower oesophageal sphincter, contraction of gastric muscularis, relaxation of pyloric sphincter
• Absorption in the stomach


  Stomach wall impermeable to a range of compounds- through water, alcohol and some drugs and electrolytes can be absorbed, absorption usually only starts when contents reach small intestine where food is acted upon by products of pancreas, liver, gallbladder
• Vomiting


  Sudden and forceful oral expulsion of contents of stomach (and sometimes duodenum), occurs due to reflex coordinated by reticular formation of medulla, actived by stretch, chemical agents, bacteria & toxins, sight, smell, motion, balance, pain, emotions, cytotoxic drugs, prolonged vomiting can cause metabolic alkalosis through loss of gastric acid
• Emesis (and nausea)

  Triggered by afferent impulses to the emetic centre (from chemoreceptor trigger zone, vestibular system or periphery), afferent impulses integrated by the emetic centre, output to medullary control centres (vasomotor, respiratory) and subsequent somatic and visceral outputs to effector organs, nausea >> feeling of malaise, unease, discomfort in stomach, or urge to vomit
• Structure of the small intestine

  • Duodenum (first part, main role is to complete the first phase of digestion, food from stomach is mixed with enzymes from pancreas and bile from gallbladder)
  • Jejunum (2nd part, inside walls absorb the foods nutrients, many circular folds to increase surface area)
  • Ileum (3rd part, absorbs bile acids and vitamin B12)
• Layers of the small intestine


  • Mucosa (pits lined with glandular epithelium, intestinal glands secrete enzymes of digestive juices)
  • Submucosa (contains Brunner's glands which secrete alkaline mucous to protect intestinal wall and neutralise acid chyme)