Digestive system - Mt. San Antonio College · 2012-01-24 · Digestive systemDigestive system Dr....
Transcript of Digestive system - Mt. San Antonio College · 2012-01-24 · Digestive systemDigestive system Dr....
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Digestive systemDigestive systemDigestive system
Dr. Carmen E. RexachDr. Carmen E. RexachPhysiologyPhysiology
Mt San Antonio CollegeMt San Antonio College
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Functions
• Motility– ingestion– mastication– deglutition– peristalsis
• Secretion– exocrine– endocrine
• Digestion• Absorption
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Tunics
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Innervation• Autonomic nervous system• Parasympathetic (vagus nerve)
– motility– secretion
• Sympathetic– decreases peristalsis– secretion– contraction of sphincters
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Esophagus• Collapsible tube• first 1/3
– Skeletal muscle• last 2/3
– Smooth muscle• gastroesophageal
sphincter
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Stomach• Function
– food storage– initiates protein
digestion– kills bacteria– moves food to SI– intrinsic factor– Digestion and
absorption• proteins• alcohol/aspirin
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Gastric Glands
• Goblet cells– mucus
• Parietal cells– HCl
• Chief cells– pepsinogen
• Argentaffin cells– Seratonin (correct)
• G cells– gastrin
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pH• 3 functions of pH in
stomach– denature ingested
protein– convert pepsinogen to
pepsin– destroy bacteria
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Protective mechanisms• Impermeability of gastric mucosa to
CO2 and NH3
• alkaline mucus• tight junctions• rapid cell division and replacement• protective effects of prostaglandins• In doudenum
– bicarbonate from pancreatic juice – Brunner’s glands
• Secrete mucinous alkaline solution
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Peptic ulcers– Gastric ulcers
• Weakened defense mechanisms– Duodenal ulcers
• Increased acid and pepsin production– Helicobacter pylori
• Major factor in both types• Present in 100% of gastric ulcer patients and 95% of
duodenal ulcer patients– Exceptions: Those whose ulcers are the result of overuse of
NSAIDS = inhibits prostaglandin production• Produces NH3 and urease
– damages epithelium and allows H+ to permeate• Also linked to increased acid secretion
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Small intestines
• Length• 3 divisions
– duodenum– jejunum– ileum
• absorption– doudenum + jejunum– ileum
• system of folds– microvilli, villi, plicae circularis
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Villi
microvillivillus• Columnar epithelium
• goblet cells• crypts of Lieberkuhn
– Glands that secrete intestinal juice at base of the villi
• brush border– surface area– enzymes– enterokinase
capillaries
nerve
lactealarteriole
arteryMuscularismucosa
veinLymphduct
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Intestinal motility• Major types of
contractions– peristalsis– segmentation
• Pacesetter potential– smooth muscle cells– can lead to a/p– parasympathetic and
sympathetic influences
• Relaxation– NO
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Large Intestine
• Structure• Haustra• Fluid and electrolyte absorption
– 90% in SI– passive osmosis– water secretion
• Defecation
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Liver
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Liver lobules: functional unit
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Functions of liver• Exocrine• Endocrine• Clotting functions• Plasma proteins• Organic metabolism• Cholesterol metabolism• Excretory and degradative functions
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Exocrine & Endocrine• Synthesis and
secretion of bile salts– 250-1500ml/day
• Adds bicarbonate rich solution to bile
• Secretes IGF-1 = promotes cell division
• Forms T3 from T4• Secretes
Angiotensinogen• Metabolizes hormones• Secretes immune
cytokines
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Clotting and Plasma proteins• Produces
prothrombin & fibrinogen
• Produces bile salts needed for vitamin K absorption
• Produces plasma albumin, acute phase proteins, binding proteins, and lipoproteins
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Organic metabolism• Converts plasma glucose to glycogen and
triglycerides• Converts amino acids to fatty acids• Produces triglycerides and secretes them
as lipoproteins• Gluconeogeneisis and glycogenolysis• Converts fatty acids into ketones• Produces urea
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Cholesterol metabolism/excretory & degradative functions
• Synthesizes cholesterol
• Secretes plasma cholesterol into bile
• Converts plasma cholesterol into bile salts
• Secretes bile pigments
• Excretes toxins via bile
• Destroys old erythrocytes
• …and lots, lots more!
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Gallbladder & pancreas
Pancreaticduct
Pancreas
Doudenum
Duodenalpapillae
gallbladder
Commonbile duct
Hepatic ducts
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Pancreas
• Endocrine = islets of Langerhans– insulin and glucagon, etc.
• Exocrine = pancreatic acini– pancreatic juice– composition
• water• bicarb• digestive enzymes
– role of enterokinase
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Neural and endocrine regulation• Long reflex
– preconditioning• Short reflex
– more local• GI hormones• Extrinsic controls
– 3 phases (cephalic, gastric, intestinal)• some events without neural or
endocrine control
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GI hormones• Gastrin• Secretin• CCK• glucose-dependent insolinotropic
peptide (GIP)• Glucagon-like peptide 1 (GLP-1)• Guanylin
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Gastrin• endocrine cell location
– antrum of stomach• stimulus for release
– amino acids, peptides in stomachparasympathetic nervous system
• actions– stimulates secretion of HCl and
pepsinogen
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Secretin• Endocrine cell location
– small intestine• stimulus for release
– acid in small intestine• action
– stimulate pancreatic bicarbonate secretion
– potentiate CCK-stimulated pancreatic enzyme secretion
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Cholecystokinin (CCK)• Endocrine cell location
– small intestine• Stimulus for release
– amino acids, fatty acids in small intestine
• Action– contraction and emptying of the gall
bladder– stimulates release of pancreatic
enzymes– (in the brain: acts as satiety hormone)
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Glucose-dependent insulinotropic peptide (GIP)
• Endocrine cell location– small intestine
• Stimulus for release– glucose, fat in the small intestine
• Action– inhibits gastric emptying– potentiates insulin release
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Glucagon-like peptide 1 (GLP-1)
• Endocrine cell location– ileum and colon
• Stimulus for release– oral ingestion of nutrients– glucose, fats, amino acids (mixed meals)
• Action– inhibits gastric motility– stimulates insulin secretion
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Guanylin
• Endocrine gland location– ileum and colon
• Action– stimulates intestinal Cl- release– result: increased NaCl and water in
feces
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Cephalic phase• First 30 minutes
– Smell, sight, and taste of food stimulates vagus nuclei of brain (long reflex)
• vagus nerve stimulates:– parietal and chief cells– Gastrin secretion by G cells
• Also stimulates parietal and chief cells
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Gastric phase• Begins when food arrives in stomach• Stimulus
– distension– chemical composition of chyme
• short polypeptides and amino acids in stomach– Positive feedback, cells secrete gastrin– HCl and pepsinogen are released
• Glucose– No effect
• Lipids– Inhibition of gastrin secretion– pH<2.5 negative feedback inhibition of gastrin
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Intestinal phase• Begins when chyme enters small intestines• neural inhibition of gastric emptying and acid
secretion due to:– increased osmolality– stretching
• enterogastrone– stimulus: fat in chyme– inhibits gastric acid secretion
• GIP = insulin secretion if glucose is present• CCK = inhibits gastric emptying when chyme
in duodenum
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Intestinal reflexes
• Gastroileal reflex– increased gastric activity = increased
motility of ileum = increased chymethrough sphincter
• ileogastric reflex– ileal distension = decreased gastric
motility• intestino-intestinal reflex
– one segment overdistends = rest relax
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Regulation pancreatic juice & bile• Neural & hormonal• Pancreatic juice
– secretin• pH <4.5 in duodenum• stimulates bicarbonate production by pancreas• cAMP as a second messenger
– CCK• fat content in chyme• production of pancreatic enzymes• Ca++ as second messenger
• Secretion of bile– continuous secretion– gallbladder contraction under influence of CCK and
secretin
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Digestion and absorption of carbohydrates
• Mouth– salivary amylase
• Stomach– enzymes denatured
• Small intestines– pancreatic amylase = maltose, maltriose– brush border enzymes =
monosaccharides– secondary active transport with Na+
– Into capillaries in villus
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Mouth:Salivary amylase
Stomach:Enzymes denatured
Duodenum:Pancreatic amylaseBrush border enzymes
Digestion of Carbohydrates
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Absorption of carbohydrates
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Digestion and absorption of proteins
• Mouth– nothing
• Stomach– Pepsin = short chain polypeptides
• Small Intestines– trypsin, chymotrypsin, elastase– carboxypeptidase– aminopeptidase = from brushborder– across into blood– Whole proteins
• babies• botulinum toxin
Free amino acidsDipeptidestripeptides
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Digestion of proteins
Stomach:Pepsin and HCl
Small intestines:Trypsin, chymotrypsinCarboxypeptidaseBrush border:aminopeptidases
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Protein absorption
-Amino acids move into enterocytes by counter-transport, in exchange for two Na+. -They are absorbed into a capillary bed on the basal side of the cell and taken to the liver via the hepatic portal system.
aa
Na+
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Digestion and absorption of lipids
• Emulsification• pancreatic lipase• Mixed micelles to brushborder• transport
– across into epithelial cells– can be moved inside cell– chylomicron into lacteal
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Digestion of Fat
Small intestines:Pancreatic lipase
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lacteal
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Lipid transport• From lymphatics to thoracic duct• Triglycerides removed by lipoprotein lipase
on endothelial membranes• Free fatty acids and glycerol into tissues• Leftovers to liver
– Remnant particles contain cholesterol– Combined with apoproteins to produce VLDL’s– Deliver triglycerides to other organs– Later converted to LDL’s when triglyceride has
been removed
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Lipoproteins• What are they?
– Lipid & protein complexes– Transport cholesterol & triglycerides in blood– Protein allows hydrophobic lipids to remain in
suspension • Five classes: Based on density, molecular weight, size,
chemical composition• Chylomicrons• VLDL• IDL• LDL
– High levels associated with increased risk CVD• HDL
– Low levels associated with increased risk of CVD– Best profile = high HDL, low LDL
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Treatment for morbid obesity
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Lapband
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Roux-en-Y Gastric Bypass