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GI Flashcards

1
Q

layers of GI wall

A
  1. serosa: outer layer
  2. smooth muscle
    * outer longitudinal muscle
    * inner circular muscle
  3. submucosa
  4. mucosa = lumen ➞ continous w/ external envir
    1. acidic pH in stomach
    2. harsh enzymes that hydrolyze food = destructive to tissues ➞ synthesized in inctive form & converted to active form in lumen
    3. millions of microorganisms in GI ➞ detrimental if inside body
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2
Q

digestive processes

A
  1. motility: muscular contractions mix & move food
    1. propulsivemovements: moving forward/down
    2. mixing movements:
      1. aid digestion by mixing w/ digestive juices
      2. facilitate absorption by exposing food to absorbing surfaces
  2. secretion: juices secreted into lumen by exocrine glands (ex: pancreas)
  3. digestion: biochemical breakdown process where structurally complex food is converted to smaller absorbable units
  4. absorption: transfer of absorbable units + water + vit + electrolytes from GI into blood/lymph
    • mostly occurs in SI
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3
Q

salivary glands

A
  • cephalic phase response: produces saliva in anticipation of food prepares for food
    1. prepare food digestion & nutrient absorption efficiency
    2. w/out: delay in absorption, slow speed, longer digestion
    3. energy & homeostasis
  • secrete saliva in response to autonomic stimulation
  • contains:
    • mucus for moisture
    • lysozyme for bacteria
    • bicarbonate buffers for when stomach acid that comes back up
    • amylase: begins chemical digestion by cleaving polysaccharides to maltose
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4
Q

mouth

A
  • lips & tongue contain food in mouth ➞ guide food during chewing & swallowing
  • teeth begin mechanical breakdown by chewing
  • palate = roof of oral cavity
    • separates oral cavity from nasal passage
    • simultaneous chewing & breathing
  • uvula: soft tissues hangs from rear of mouth ➞ seals nasal passage during swallowing
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5
Q

pharynx + esophagus

A
  • pharynx: common passageway for GI & resp system in rear of mouth
  • esophagus: muscular tube that extends between pharynx & stomach
  • 2 sphincters:
    1. pharyngoesophageal sphincter = upper ➞ normally closed to prevent air from entering esophagus (excessive eructation)
    2. gastroesophageal sphincter = lower ➞ normally closed to prevent acid reflux
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6
Q

swallowing

A

entire process of moving food mouth ➞ esophagus ➞ stomach

  • brain stem
  • propulsive movements
  • all or none reflex: initiated when food bolus is pushed by tongue into pharynx ➞ cannot be stopped after initiated
    • pressure stimulates receptors that activate swallowing center in medulla
    • swallowing center reflexively activates appropriate muscles to swallow
  • 2 stages
    1. oropharyngeal stage: 1 sec ➞ moving bolus from mouth through pharynx into esophagus
    2. esophageal stage: 5-9 sec ➞ peristaltic movement through esophagus into stomach
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7
Q

stomach: general anatomy & fxs

A
  • fundus: above gastroesophageal sphincter
  • body = middle portion
  • antrum = bottom portion
    • thick layer of smooth muscle
    • connected to SI by pyloric sphincter
  • fxs:
    1. stores ingested food & delivers to SI at appropriate rate
    2. secretes HCl, pepsinogen, & intrinsic fators to begin protein digestion
    3. produces chyme: thick liquid mixture made by pulverizing ingested food & mixing w/ gastric secretions
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8
Q

mucus

A
  • alkaline
  • from surface epithelial cells
  • covers surface of gastric mucosa to protect from:
    • mechanical injury: lubrication
    • self-digestion: inhibits pepsin
    • acid injury: mucus = alkaline ➞ neutralizes HCl
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9
Q

gastric mucosa

A

lining of stomach divided into 2 areas
1. oxyntic mucosa: lines body & fundus ➞ secretes mucus, pepsinogen, HCl, & intrinsic factor
2. pyloric gland area (PGA): lines the antrum

  • acid ≠ secreted
  • secretes mucus, pepsinogen, & gastrin
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10
Q

gastrin

A
  • hormone secreted from pyloric antrum
  • circulates back to body & fundus of stomach
  • stimulates parietal & chief cells to produce highly acidic gastric juice & pepsinogen
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11
Q

pepsinogen

A
  • secreted by chief cells in stomach
  • precursor of pepsin that breaks down peptides
  • inactive form
  • cleaved by HCl to form pepsin:
    • can autocatalytically convert
    • initiates protein digestion by splitting peptide bonds
    • self-digestive: stored & secreted in inactive form & only active in gastric lumen
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12
Q

intrinsic factor

A

required for absorption of vit B12

  • only secreted from parietal cells
  • glycoprotein
  • binds B12 to form complex that binds to intestinal receptor cubulin triggering receptor-mediated endocytosis
  • vit B12 essential for normal RBC fx
    • no B12 = anemia
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13
Q

mucosal gland cells

A

mucous neck cells secrete thin watery mucus

chief cells secrete pepsinogen: precursor of pepsin that breaks down peptides

  • pepsinogen stored in zymogen granules: vesicles inside chief cells
  • HCl cleaves of pepsinogen forming pepsin

parietal (oxyntic) cells secrete HCl & intrinsic factors

  • H+ & Cl- are actively secreted by separate pumps to protect against HCl acid-degradation
  • intrinsic factor: required for absorption of vit B12
  • degeneration of parietal cells leads to:
    • loss of intrinsic factor ➞ most important: only secreted from parietal cells
    • loss of pepsinogen
    • loss of HCl
    • loss of pepsinogen & HCl can be compensated for by enzymes produced in SI & pancreas

surface cell epithelium secrete thick alkaline mucus

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14
Q

HCl

A
  • secreted by parietal (oxyntic) cells
  • H+ & Cl- are actively secreted by separate pumps ➞ protection against HCl acid-degradation
  • H+ actively transported against large [gradient]
    • derived from metabolic process: H2O mol form inside parietal cell
  • new H+ generated from carbonic acid when H+ secreted
  • Cl- transported against much smaller [gradient]
  • fxs:
    1. activates pepsinogen w/ acid envir
    2. acid breakdown of connective tissue
    3. kills harmful microorganisms from food
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15
Q

gastric filling

A

stomach can stretch up to 20x b/c of smooth muscle

  • stomach smooth muscle maintains constant tension of range of lengths by slow-wave potentials
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16
Q

gastric mixing & emptying

A

strong antral peristaltic contractions
* propulsive movements
* gastric mixing in antrum
* mix food w/ gastric secretions to produce chyme
* propel chyme to pyloric sphincter ➞ small amount goes in duodenum
* sphincter only allows some food to enter duodenum ➔ then closes & pushes remaining chyme back into stomach to prevent too much or too big of particles
* factors controlling gastric mixing & emptying
1. in the stomach:
* volume of chyme: distension stimulates motility & emptying via:
1. direct effects of stretch on smooth muscle
2. intestinal plexuses & vagus nerve
3. gastrin (stomach hormone)
* fluidity of chyme: contents must be in fluid form to exit ➞ more liquidy = faster emptying
2. in the duodenum: presence of fat, acid, hypertonicity, & distension inhibit gastric motility & emptying until duodenum has broken down & passed on/digested
* fat cannot empty from stomach until the fat in SI is processed (is only digested & absorbed w/in SI)
* acid: highly acidic chyme from stomach must be neutralized with NaHCO3 (sodium bicarbonate) from pancreas
* hypertonicity: high concentration of AA & glucose in duodenum ↑ osmolarity (more solute in water)
* more solute inside than outside (hypertonic) causes water to diffuse into duodenum & swell ➔ triggers reflex to reduce gastric emptying
* distention = distended (extended & expanded)

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17
Q

gastrin

A
  • hormone released from pyloric gland area in stomach into blood
  • stimulate parietal cells & chief cells to secrete HCl & pepsinogen
  • enhances gut motility
  • stimulates ileal motility
  • relaxes ileocecal sphincter
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18
Q

pancreas

A
  • located behind & below stomach
  • all pancreatic enzymes work best in alkaline envir
  • made of endocrine & exocrine tissue (exocrine mainly for GI, endocrine for hormones)
  • acinar cells: (at bottom) release 3 types of enzymes
    • all 3 pancreatic enzymes are packaged into common vesicles ∴ fat or protein triggers release of all 3 ➔ carbs do not trigger release
      1. proteolytic enzymes for protein digestion
      1. trypsinogen (inactive) ➞ trypsin (active) by enterokinase = enzyme in luminal border of duodenum
        • trypsin can autocatalytically trigger further conversion
          1. chymotrypsinogen ➔ chymotrypsin
          2. procarboxypeptidase ➔ carboxypeptidase
        • trypsin inhibitor: substance in pancreas that blocks trypsin in case trypsinogen inadvertently converts trypsin
          2. pancreatic amylase for carbohydrate digestion
            * salivary gland also releases amylase
            * not harmful ∴ secreted as active form
          3. pancreatic lipase = only enzyme in digestive tract that can digest fat
  • duct cells: release sodium bicarbonate (NaHCO3)
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19
Q

acinar cells

A
  • cells of the pancreas
    1. proteolytic enzymes for protein digestion
    1. trypsinogen (inactive) ➞ trypsin (active) by enterokinase = enzyme in luminal border of duodenum
      • trypsin can autocatalytically trigger further conversion
        1. chymotrypsinogen ➔ chymotrypsin
        2. procarboxypeptidase ➔ carboxypeptidase
      • trypsin inhibitor: substance in pancreas that blocks trypsin in case trypsinogen inadvertently converts trypsin
        1. chymotrypsinogen ➔ chymotrypsin
        2. procarboxypeptidase ➔ carboxypeptidase
          2. pancreatic amylase for carbohydrate digestion
          * salivary gland also releases amylase
          * not harmful ∴ secreted as active form
          3. pancreatic lipase = only enzyme in digestive tract that can digest fat
20
Q

duct cells

A

cells of the pancreas that release sodium bicarbonate (NaHCO3)

21
Q

regulation of pancreatic secretions

A
  • hormonally regulated
  • duodenum releases 2 major enterogastrones in response to chyme:
    1. secretin release triggered by acidic chyme ➞ acts on duct cells to secrete NaHCO3 & inhibits gastric motility and secretion
    2. cholecystokinin (CCK) release triggered by fat & protein in chyme
      • inhibits gastric motility and secretion
      • stimulates pancreatic enzyme secretion
      • stimulates gallbladder contraction
      • acts as a satiety signal
22
Q

secretin

A
  • secreted from duodenum triggered by acidic chyme
  • acts on duct cells to secrete NaHCO3 & inhibits gastric motility and secretion
23
Q

cholecystokinin (CCK)

A

secreted from duodenum triggered by fat & protein in chyme
* inhibits gastric motility and secretion
* stimulates pancreatic enzyme secretion
* stimulates gallbladder contraction
* acts as a satiety signal

24
Q

biliary system: general

A
  • liver, gallbladder, & associated ducts
  • liver produces bile salts
    • much higher concentration emptied into duodenum each meal than in body
    • recycled through enterohepatic circulation
    • lost bile salts replaced by newly synthesized ones
  • bile secreted by liver ➔ enters duodenum during meals
  • between meals sphincter of oddi closes & bile is diverted to gallbladder for storage until next meal when it reopens
  • bile stored in gallbladder
  • gallbladder ≠ essential
25
Q

biliary system function

A
  1. aid in fat digestion: act as detergent to emulsify fat
    • detergent action: bile salts convert large fat globules to lipid emulsion of many small fat droplets suspended in aqueous chyme ➔ ↑ SA for action by pancreatic lipase
    • mechanism:
      * to digest fat lipase must come into direct contact with triglyceride
      * bile salts dissolve & absorb in surface of fat droplet leaving charged water-soluble part projecting from surface
      * negatively charged portion cause droplets to repel ➞ cannot recombine into big fat droplet
  2. facilitate fat absorption by forming fat micelles
    • micelle: bile salts + lecithin aggregate in small clusters forming hydrophobic core & hydrophilic shell
    • formed from bile salts + cholesterol + lecithin (also constituent of bile)
    • if cholesterol is out of proportion with bile salts/lecithin ➔ cholesterol precipitates to form gallstones
26
Q

SI: general

A
  • most digestion & absorption in SI ➞ no digestion occurs after SI
  • very large SA for absorption: inner surface has circular folds + surface of circular folds has villi + surface of villi have microvilli (brush border)
    • total ↑ SA 600x
27
Q

SI: movement

A
  • segmentation:
    • primary method of motility
    • oscillating ring-like contractions (not peristalsis)
    • absent between meals but strong during & after a meal
    • both mixes & propels chyme
  • chyme moves forward b/c frequency of contractions ↓ gradually along length of SI
28
Q

SI: digestion

A
  • inside lumen with pancreatic enzymes
  • fat digestion enhanced by bile secretions & completed extracellularly ➞ in lumen
  • protein & carb digestion completed intracellularly ➞ in brush border
  • does not synthesize digestive enzyme secreted into lumen
  • synthesizes digestive enzymes that act intracellularly w/in brush-border membrane of epithelial cells lining lumen
  • 3 brush-border enzymes:
    1. enterokinase: activates pancreatic enzyme trypsinogen
    2. the disaccharidases: maltase, sucrase, & lactase ➞ complete digestion of carbs
    3. the aminopeptidase complete digestion of proteins
29
Q

carb digestion & absorption in SI

A

carbohydrate digestion completed intracellularly

polysaccharides broken down into monosaccharides & enter blood stream via passive diffusion
* glucose & galactose via passive carriers ↓[gradient]
* fructose via facilitated diffusion

30
Q

protein digestion & absorption in SI

A
  1. hydrolyzed into small peptides & AA by pepsin & pancreatic proteolytic enzymes
  2. AA absorbed into cell via Na+ & active transport
  3. small peptides broken down into AA by intracellular peptidases & aminopeptidases
  4. AA enter blood ↓[gradiets]
31
Q

fat digestion & absorption in SI

A
  1. emulsified by detergent action of bile salts
  2. lipases hydrolyze triglycerides into monoglycerides & free fatty acids
  3. hydrophobic mol move interior of micelles to cell surface
  4. monoglycerides & free fatty acids diffuse into cell then re-synthesize into triglycerides
  5. triglycerides coated w/ lipoprotein & form chylomicrons that are exocytosed from cell
  6. chylomicrons enter lymphatic vessels
32
Q

LI: fxs

A

drying & storage

33
Q

LI anatomy

A
  • drying & storage organs
  • cecum: blind-ended pouch below ileocecal valve
  • appendix: finger-like projection of lymphoid tissue)
  • colon: ascending, transverse, descending, & sigmoid
  • rectum: straight, connected to anal canal
34
Q

LI: movement

A
  • one way flow of contents from ileum to cecum to keep bacteria out of SI
    • ileocecal velve opens in 1 direction only
    • ileocecal sphincter separates ileum & cecum ➞ opens/closes for regulation
  • haustral contractions slowly shuttle contents of large intestine to aid absorption of H2O & salt
    • gradually change location forming haustra: pouch
  • mass movements: large contractions in ascending & transverse colon that rapidly drive contents forward
    • occur 3-4x/d through gastrocolic reflex: triggered by gastrin & extrinsic autonomic nerves
  • defecation reflex causes internal anal sphincter (smooth muscle) to relax & rectum & sigmoid colon to contract
    • defecation if external anal sphincter is also relaxed
    • external anal sphincter = skeletal muscle ➞ voluntary control
  • constipation: defecation is delayed too long & too much water is absorbed from feces
  • appendicitis: if hardened feces gets lodged in appendix & obstructs normal circulation & mucus secretion
35
Q

trypsinogen

A
  • inactive form
  • converted to trypsin by enterokinase: enzyme in luminal border of duodenum
  • secreted by acinar cells in duodenum
  • proteolytic enzyme ➞ acts on proteins
  • optimal pH = alkaline
36
Q

amylase

A
  • active form
  • secreted by salivary glands & acinar cells in exocrine pancreas
  • acts on carbs
  • optimal pH = 7
37
Q

chymotrypsinogen

A
  • proteolytic enzyme ➞ acts on proteins
  • secreted by acinar cells in exocrine pancreas
  • inactive form
  • converted to chymotrypsin by autocatylitic conversion of trypsin
38
Q

enterokinase

A
  • enzyme in luminal border of duodenum
  • converts trypsinogen to trypsin
39
Q

pancreatic lipase

A
  • fat digestion
  • only enzyme in GI that can digest fat
  • released from acinar cells in exocrine pancreas
  • secreted in active form ➞ no risk of self-digestion
40
Q

bile salts

A
  • derivitives of cholesterol
  • produced by liver
  • recycled through enterohepatic circulation
    1. aid in fat digestion by acting as detergent (emulsifying fat)
    2. facilitate fat absorption by forming fat miscelles
    • convert large fat globules to lipid emulsion made of many small fat droplets suspended in aqueous chyme
    • ↑ SA for pancreatic lipase to digest fat
    • cause fat droplets to repel each other ensuring droplets don’t reform to 1 large one
41
Q

gallstones

A

precipitate that forms in gallbladder when amount of cholesterol is disproportionate to bile salts & lecithin

42
Q

sphincter of Oddi

A

muscle at the opening of the bile duct into the duodenum
* prevents bile from entering the duodenum except during digestion of meal
* when sphincter is closed bile is diverted back up into the gallbladder

43
Q

gallbladder

A
  • small, saclike structure tucked beneath but not directly connected to the liver
  • stores and concentrates bile between meals
44
Q

lecithin

A

component of bile that helps form micelles because of hydrophilic & hydrophobic properties

45
Q

enterogastrones

A

hormones secreted by the duodenal mucosa that regulate gastric emptying & pancreatic secretions
* inhibit gastric motility and secretion
* two hormones
1. secretin acts on duct cells to secrete NaHCO3 & stimulated by acidic chyme in duodenum
2. cholecystokinin (CCK) acts on acinar cells

NPB 101 (13 decks)

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