small intestine

  • Created by: Zina_
  • Created on: 03-05-18 00:06

what is the gross anatomy of the small intestine?

  • SI forms in circular folds called the folds of kerckring 
  • folds increase SA 
  • mucosa has villi rather than pits 
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what are the different cells found in the small in

  • absorptive cell 
  • goblet cell - secretory cells that produce mucin 
  • entric endocrine cells 
  • stem/progenitor cell - differentiation and migration to replace sloughed( shredded dead cells) 
  • paneth cells- secretion of antibacterial enzymes like lysosomes and defensins 
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how is the small intestine structure adapted for m

  • small intestine forms folds of kerckring
  • mucosa(membrane) forms villi and crypts increasing SA
  • villi contains capillaries and lacteals(lymphatic vessels) for absorption 
  • cells in the epithelial layer have specialised functions, e.g. absorptive epithelia cells have microvilli 
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what are the 6 main functions of the small intesti

  • 1) digestion 
  • 2)absorption 
  • 3)secretion 
  • 4)defence 
  • 5)motility 
  • 6)excretion 
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what is the small instestines role in digestion?

  • pancreatic juices/bile enzmes released into the duodenem 
  • lipases break down fats 
  • amylases breakdown carboydrates 
  • protease breaks down protein s
  • enzmes present on the transporting epithelia vili brush border
  • enzymes are alkali to neutralise stomach acidity 
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absorption of carbohydrates/protein in the small i

  • Na/glucose linked symporter on the villi as secondary transporter 
  • glucose smaller unit of carbohydrates 
  • gradient for this symporter created through primary active transport of 3Na/2K pump 
  • amino acids  absorbed into SI by Na driven amino acid symporter 
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absorption of water in the small intestine

  • water passively flows down the osmotic gradient generated by secretion and absorption of osmotic substances such as glucose,a acids, Na, cl 
  • water then absorbed into the SI through two pathway:
  • 1) paracellular pathway (between cells)
  • 2) transcellular pathway ( through cells(aquaporins))
  • water balance sheet is created by the amount of water absorbed/secreted in different areas of the body
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absorption of sodium/fats in the SI

  • Na/H secondary active exchanger on apical membrane of SI driven by Na/K primary active transport 
  • fats degraded by lipases secreted from the pancreas = simple lipids 
  • 1) lipases secreted from the pancreas break down fats into lipids 
  • 2)using bile salt as a detergent, micelles are formed 
  • 3)small micelles migrate through the SI at the same time Na is absorbed in exchange of H+ creating a slightly acidic environment
  • 4) acid environment breaks down the micelles releasing the fatty acids 
  • 5)free fatty acids now able to be absorbed by cells through fatty acid transport protein 
  • 6)once fatty acids are inside the epithilium of the SI, the fatty acids are now absorbed and the free fatty acids re synthesize into triglycerides,phospholipids,cholesterol ester 
  • 7) intracellular micelles called chylemicrons now formed, which have a lipoprotein coat 
  • 8)chylemicrons leave basolateral membrane(of epithelial cell) by exocytosis and diffuse into the villi lacteals, then leave the intestine with lymphs 
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SI role in defence

  • GI mucosa is in contact with the external environment therefore must protect the body against viruses,toxins,bacteria 
  • 2 different types of defences -
  • 1) IMMUNE DEFENCE: gut associated lymphoid tissue(immune cells in the villi and peyers patches)
  • peyers patches contain B+T cells,
  • 2) NON IMMUNE DEFENCE:secretion of mucous (by goblet cells), secretions of defensins(by paneth cells), tight junctions between cells 
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what are the 3 different motility techniques in th

  • = movement of material in the GI tract as a result of coordinated contractions of the smooth muscle by the longtitudanal and circular layers of the muscalaris externa
  • 3 types of motility 
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what is migrating motor complex?

  • happens during fasting stage betwen meals 
  • ryhthmic contractions of the stomach/si 
  • housekeeping between meals by sweeping remnants into intestine 
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what is peristalsis?

  • contractions moving food bolus forward to next GI tract area 
  • circular muscles contract, whilst longtitudanal muscles relax which pushes food into recieving segment 
  • in the recieving segment, circular muscles relax whilst longitudinal muscles contract 
  • antiperistalsis = vomitting
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what are segmental movements

  • mixing and churning 
  • occur randomly 
  • churning occurs as a result of circular muscles contracting in segments 
  • contraction rate increases as a result of membrane potential variation = slow waves= slow waves exceeding electric threshold = firing of APs= increased smooth muscle contraction 
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