Gastrointestinal system

  • Created by: Zina_
  • Created on: 01-05-18 21:21

how is hypovolemic shock solved through redundancy

  • severe blood loss causes change in arteriole blood pressure 
  • loss of blood pressure due to loss of blood 
  • blood pressure rises back after a few hours 
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what mechanisms help against blood loss

  • reabsorption of tissue fluids into plasma 
  • baroreceptor reflexes - high/low pressure 
  • sympathetic surge 
  • release of other vasoconstriction substances 
  • renal conservation of salt/water 
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Re absorption of tissue fluids into the plasma

  • transcapillary refill/ autoinfusion 
  • fluid movement from interstitium(fluid between skin and organs) to vasculative 
  • fluid movement driven by hydrostatic pressure(difference between capillary and extravascular pressure) AND oncotic pressure(differences across capillary wall)
  • hypotension during haemorrhage causes a drop in hydrostatic pressure of the capillaries = higher hydrostatic pressure in the interstitial fluid = water movement into the capillaries due to osmotic gradient 
  • albumin secreted by the liver re establishes osmolarity 
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high pressure baroreceptor reflexes

  • baroreceptors detect stretch in arteries 
  • barorecptors at the aortic arch and carotid sinus (type of mechanoreceptors detecting inc in stretch)
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low pressure barorecptors

  • located in the right atria
  • respond to dec in stretch 
  • dec in blood pressure = +afferent pathways = medulla = increased sympathetic output = increased vassopressin secretion and synthesis 
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sympathetic surge

  • baroreceptors activate sympathetic postganglionic neurons to release adrenaline/noradrenaline, triggered by the adrenal medulla 
  • sympathetic post granglionic neurons are differientiated by forming neurosecretory chromaffin cells which release adrenaline upon sympathetic stimulation 
  • nurotransmitter release proportional to haemorrhage severity 
  • sympotoms of sympathetic release: tachycardia,inotropy,arteriole constriction 
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how does sympathetic surge affect action potential

  • symathetic stimulation = stronger depolarization = more APs= ncreased HR 
  • adrenoceptor on heart muscle detects any adrenaline/noradrenaline causes increased contractile force 
  • ligand:adrenaline, receptor: B1 adrenoceptor 
  • APs cause Ca channels to open= Ca influx =  higher release of Ca from sarcoplasmic reticulum = positive inotropy 
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how does sympathetic surge affect blood vessel dia

  • noradrenaline release onto a1/B2 receptors causes increased sympathetic tone and vasoconstriction 
  • vasoconstriction- inc blood pressure and arteriole pressure 
  • sympathetic surge however DOES NOT cause vasoconstriction in supply to the heart/brain 
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how is blood supply to the brain/heart autoregulat

  • no vasoconstriction 
  • cerebral/ coronary vascular beds regulate blood flow through local mediatiors not neuronal influences 
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release of other vasoconstriction substances to re

  • renin release from juxtaglomerular apparatus 
  • reduction in blood flow= reduction in renin circulating = detected by baroreceptor in the afferent renal arterioles= renin release stimulated = renin cleaves to angiotensinogen which converts it to angiotensin 1 = angiotensin 1 converted to angiotensin 2 by ACE enzyme
  • angiotensin 2 is a vasoconstricter, stimulates aldesterone release from adrenal glands
  • aldosterone is a lipid soluble hormone regulating the expression of epithelial sodium channels 
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how does aldosterone conserve renal salt?

  • aldosterone release promotes sodium reabsorption in kidney collecting ducts 
  • controlling sodium amount in extracellular space 
  • sodium control is important as is the primary osmotically active particle, during hypovelmic shock you want to preserve fluid 
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how does decrease in atrial pressure stimulate vas

  • afferent atrial receptors synapse the medulla and paraentricular nucleus of the hypothalamus 
  • medulla then increases sympathetic output 
  • vassopressin is a vasoconstricter and increases water rebasorption 
  • vassopressin also increases the number of water channels in the collecting duct 
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what are the symptoms of hypovalemic shock?

  • rapid and weak pulse caused by tachycardia due to sympathetic heart stimulation 
  • feeling cold - due to vasoconstriction and sympathetic stimulation of sweat glands 
  • low urine output - due to vassopressin secretion 
  • sodium retention - due to aldosterone affect causing thirst 
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what is the general organisation of the GI tube?

Image result for GI tube anatomy

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what is the mucosa layer of the GI tube?

  • made up of 3 layers:
  • 1) epithelium 
  • 2)lamina propria (loose connective tissue layer containing capillaries,lymph and immune mast cells) - underlying connective tissue layer 
  • 3) muscalaris mucosae (smooth outer layer)
  • inner layer of the GI tract
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what is the submucosa layer of the GI tube?

  • loose collagenous connective tissue which contains the larger blood vessels, lymphs, nerves and glands 
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what is the muscalaris externa layer of the GI tub

  •  contains longitudinal smooth muscle 
  • and contains circular smooth muscle on the inside 
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what is the serosa layer of the GI tube?

  • outermost layer 
  • eneveloping tough layer of connective tissue keeping everything within the GI tract 
  • serosa forms the outer wall of the GI tract
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provide an overview of the stomach structure and t

  • mucosa layer of the stomach forms gastric pits (gastric mucosa cells)
  • structure of the gastric pits are:
  • 1) superficial epithelial cells - HCO3- secretion 
  • 2) mucous neck cell - mucous secretion 
  • 3) stem/regenerative cell - for repair 
  • 4) parietal cell - for Hcl secretion 
  • 5) chief cell - for pepsinogen secretion 
  • 6) endocrine cell - for hormone secretion
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what are the 4 main functions of the stomach?

  • 1) digestion 
  • 2) secretion 
  • 3) defence 
  • 4) motility 
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describe the stomachs role in digestion

  • happens in the chief cells 
  • chef cells secrete pepsionogens 
  • pepsinogens hydrolyzed by acid stomach activating them into pepsins 
  • protein digestion by pepsin
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describe the stomachs role in secretion

  • tubolovesicular parietel cells change conformational shape into canalicular shape when stimulated, then secrete acid to kill bacteria/denautre proteins
  • change in shape as it increses the SA of the cells therefore increasing the SA for insertion of proton pumps for increased acid secretion 
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how are canalicular parietal cells adapted for sec

  • contain lots of mitochondria 
  • contain various transporters, channels and enzymes for hcl secretion 
  • Na/H exchanger and Na pump control pH of the cytoplasm 
  • K+ channel/ HCO3 CL exchanger allows HCO3 to leave the cell
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how is acid secretion regulated?


  • Ach,histamine stimulate parietal cell to secrete protons into the lumen


  • Ach, gastrin stimulate ECL cell which secretes histamine
  • histamine acts on parietal cell to secrete acid
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explain the mechanism leading to gastric acid secr

  • 1) vagus nerve stimulates parietel cell directly 
  • 2) vagal stimulation of ECL cells increases histamine release 
  • 3) histamine stimulates acid secretion 
  • 4)vagal stimulation of G cells triggers gastrin release 
  • 5)gastrin directly stimulates acid secretion from the parietal cell 
  • 6) gastrin indirectly stimulates acid secretion by increasing histamine release from ECL cells 
  • 7)luminal acid stimulates D cells to release somatostatin 
  • 8) somatostatin inhibits acid secretion directly at the parietal cell
  • 9)somatostatin inhibits gastrin release at the G cell 
  • 10)somatostatin inhibits histamine release at the ECL cell
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how is acid secretion stimulated directly?

  • histamine, gastrin, ach stimulate secretion directly through binding to receptors on parietal cells 
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how is acid secretion indirectly regulated?

  • gastrin and ach stimulate histamine secretion from ECL cells 
  • histamine release stimulates acid secretion from parietal cells 
  • somatostatin from D cells inhbits acid secretion 
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how is the stomach epithelium protected from acid?

  •  mucus secreted by mucous neck cells traps HCO3_  forming a protective mucus gel layer 
  • HCO3 secreted by surface epithelium cells 
  • mucus creates a diffusion barrier for H+ and pepsins = creates a neutralization zone which protects gastric epithelia from H+/inactive pepsins 
  • this maintains pH of surface epithelium at pH 7 
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what is the stomachs role in defence

  • stomach is the first line of defence as its the first hollow organ to recieve external food in the chyme 
  • acid pH of stomach kills most bacteria 
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what is the stomachs role in motility?

  • movement of food by propulsive movements along the GI tract 
  • mechanically mix food to maximise exposure of particles to digestive enzymes by churning movements 
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