Transcript Gastro-intestinal tract

Gastro-intestinal
tract
Sophie Young
27th June 2007
Objectives
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Anatomy
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Physiology
 Motility
 Secretory
 Digestion
 Absorption
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Physiology of vomiting
Anatomy
Taken from http://human.freescience.org/
htmx/parts/organ_systems/gastrointestinal_tract.php
GI Motility
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Basic contractile unit
Na/K/ATPase pump – transmembrane potential
Basic electrical rhythm – slow wave activity
Rhythmical depolarisation + repolarisation
Gap junctions (stomach vs colon)
Frequency + amplitude of slow wave activity
affected by:
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Autonomic nervous system
Hormones
Drugs
Control of GI Motility
Intrinsic neuronal control
 Myenteric plexus (Auerbach’s)
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Cholinergic (+ve), adrenergic (+ve), NANC neurones (-ve)
Integrates all information
Extrinsic neuronal control
 Somatic and autonomic control
Chemical control
 Neurocrine, paracrine and endocrine
Localised GI Motility
Swallowing
 Motor reflex- voluntary, pharyngeal + oesophageal
stages
Gastric motility
 Fundus – prolonged tonic contractions
 Antrum – slow wave activity, pylorus affected by
fasted and fed states (?hormonal + neuronal control),
composition of chyme in duodenum
Bowel
 Small – slow wave activity, various types of activity
 Large – slow propulsion + mass movements
Secretory function of GI tract
Salivary glands – under Autonomic control
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Lubrication
Digest starch (amylase pH dep)
Digest fat (lipase, pH indep)
Gastric
 Oxyntic glands
– chief cells (pepsinogen)
except lesser curve)
– mucus cells
– parietal cells (HCl + intrinsic factor)
Pyloric glands
– G cells (gastrin)
(pylorus)
– mucus cells
(all gastric mucosa
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 Buffer
 Antibacterial/viral
Control of gastric secretions
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PICTURE - pg 463
Cephalic stage - gastrin
Gastric stage - gastrin
Intestinal stage - gastrin
Secretory function of GI tract
Pancreatic
 Stored as proenzymes
 Fat/protein – stimulus for release
 Secretin, cholecystokinin and vagus mediated
Biliary
 Emulsification of fat globules in SB
 Secretin, cholecystokinin and vagus mediated
 Enterohepatic circulation of 94% bile salts
Digestion
Carbohydrates
Carbohydrates
Salivary and Pancreatic Amylases
Oligo-/Disaccharides
Brush border enzymes eg lactase/sucrase
Monosaccharides
Glucose Fructose Galactose
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Cellulose -  glucose linkage, indigestible
Digestion
Protein
Protein
Pepsin (stomach)
Trypsin/chymotrypsin (SB)
Polypeptides
Carboxypeptidase (pancreas)
Aminopeptidase (brush border)
Amino acids Monopeptides Dipeptides
Hydrolysis in epithelial
cell after absorption
Amino acids
Digestion
Fat
Triglycerides (90% dietary fat)
Lingual + pancreatic lipases
Free fatty acid Monoglycerides
Combine with bile salts + cholesterol
Micelles
Absorption
Carbohydrates
Taken from http://www.colorado.edu/kines/Class/IPHY3430-200/image/figure16c.jpg
Absorption
Protein
 Secondary active transport
across apical membrane
 Di/tripeptides hydrolysed to
amino acids
 Facilitated diffusion into
blood
Taken from http://instruct1.cit.cornell.edu/courses/biog105/pages/demos/105/unit6/media/villus.structure.jpg
Absorption
Fat
Taken from http://www.colorado.edu/kines/Class/IPHY3430-200/15digest.html
Absorption
Vitamins
 Fat Soluble – micelles
 Water Soluble: B1- active in jejunum, C - active +
passive in SB, Folic acid – active in SB, B12 –
combines with IF ?pinocytosis in terminal ileum
Sodium and Chloride
 Na – cotransporter mechanism, Cl – colonic uptake
Water
 SB + colon – ANS and hormone control
Iron and Calcium
Physiology of Vomiting
Protective physiological function against ingested toxins
3 distinct phases
 Nausea: unpleasant sensation that immediately
proceeds vomiting.
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Retching: follows nausea and comprises laboured
spasmodic respiratory movements against a closed
glottis with contractions of abdominal muscles, chest
wall and diaphragm without any expulsion of gastric
contents.
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Vomiting: as for retching but with the opening of
gastric cardia, resulting in rapid + forceful
evacuation of stomach contents up to and out of the
mouth
Vomiting
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Reflex mechanism – sensory afferents, central
integrative centre, motor efferents
Sensory afferents
 GI – vagally mediated mechanoreceptors and
chemoreceptors
 CTZ – area postrema 4th ventricle, outside bloodbrain barrier, many receptors present:
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Muscarinic
Serotonergic (5HT3)
Histamine (H1) Dopaminergic (D2)
Opioid ()
Adrenergic (1 + 2)
Vestibular system – labyrinthine stimulation
Cortex
Taken from www.frca.co.uk
Vomiting
‘Vomiting Centre’
 Oversimplication, probable multiple discrete
interconnecting areas within medulla
Motor efferents
 Pre-ejection: nausea, sympathetic activity, vagal
mediation proximal stomach relaxation +
retrograde SB contraction
 Ejection: retching – perioesophageal diaphragm
contracted, vomiting – perioesophageal diaphragm
relaxes to expel contents with halting of respiration
Taken from http://www.nauseaandvomiting.co.uk/NAVRES001-2-NandV-general.htm
References
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Pinnock et al (2003) Fundamentals of Anaesthesia
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Vander et al (1998) Human Physiology
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Anaesthesia UK www.frca.co.uk