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
Functions
• Motility– ingestion– mastication– deglutition– peristalsis
• Secretion– exocrine– endocrine
• Digestion• Absorption
Tunics
Innervation• Autonomic nervous system• Parasympathetic (vagus nerve)
– motility– secretion
• Sympathetic– decreases peristalsis– secretion– contraction of sphincters
Esophagus• Collapsible tube• first 1/3
– Skeletal muscle• last 2/3
– Smooth muscle• gastroesophageal
sphincter
Stomach• Function
– food storage– initiates protein
digestion– kills bacteria– moves food to SI– intrinsic factor– Digestion and
absorption• proteins• alcohol/aspirin
Gastric Glands
• Goblet cells– mucus
• Parietal cells– HCl
• Chief cells– pepsinogen
• Argentaffin cells– Seratonin (correct)
• G cells– gastrin
pH• 3 functions of pH in
stomach– denature ingested
protein– convert pepsinogen to
pepsin– destroy bacteria
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
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
Small intestines
• Length• 3 divisions
– duodenum– jejunum– ileum
• absorption– doudenum + jejunum– ileum
• system of folds– microvilli, villi, plicae circularis
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
Intestinal motility• Major types of
contractions– peristalsis– segmentation
• Pacesetter potential– smooth muscle cells– can lead to a/p– parasympathetic and
sympathetic influences
• Relaxation– NO
Large Intestine
• Structure• Haustra• Fluid and electrolyte absorption
– 90% in SI– passive osmosis– water secretion
• Defecation
Liver
Liver lobules: functional unit
Functions of liver• Exocrine• Endocrine• Clotting functions• Plasma proteins• Organic metabolism• Cholesterol metabolism• Excretory and degradative functions
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
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
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
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!
Gallbladder & pancreas
Pancreaticduct
Pancreas
Doudenum
Duodenalpapillae
gallbladder
Commonbile duct
Hepatic ducts
Pancreas
• Endocrine = islets of Langerhans– insulin and glucagon, etc.
• Exocrine = pancreatic acini– pancreatic juice– composition
• water• bicarb• digestive enzymes
– role of enterokinase
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
GI hormones• Gastrin• Secretin• CCK• glucose-dependent insolinotropic
peptide (GIP)• Glucagon-like peptide 1 (GLP-1)• Guanylin
Gastrin• endocrine cell location
– antrum of stomach• stimulus for release
– amino acids, peptides in stomachparasympathetic nervous system
• actions– stimulates secretion of HCl and
pepsinogen
Secretin• Endocrine cell location
– small intestine• stimulus for release
– acid in small intestine• action
– stimulate pancreatic bicarbonate secretion
– potentiate CCK-stimulated pancreatic enzyme secretion
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)
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
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
Guanylin
• Endocrine gland location– ileum and colon
• Action– stimulates intestinal Cl- release– result: increased NaCl and water in
feces
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
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
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
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
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
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
Mouth:Salivary amylase
Stomach:Enzymes denatured
Duodenum:Pancreatic amylaseBrush border enzymes
Digestion of Carbohydrates
Absorption of carbohydrates
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
Digestion of proteins
Stomach:Pepsin and HCl
Small intestines:Trypsin, chymotrypsinCarboxypeptidaseBrush border:aminopeptidases
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+
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
Digestion of Fat
Small intestines:Pancreatic lipase
lacteal
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
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
Treatment for morbid obesity
Lapband
Roux-en-Y Gastric Bypass