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BRS Term 2 > Pancreas and small bowel (39) > Flashcards

Pancreas and small bowel (39) Flashcards

1
Q

What are the 3 stages of pancreatic embryology?

A
  1. abdominal accessory organs arise as foregut outgrowths
  2. proximal duodenum rotates clockwise, taking ventral pancreatic bud and duct around
  3. ventral and dorsal pancreatic buds and ducts fuse… bile duct and pancreatic ducts join to drain together at major papilla
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2
Q

What is a landmark for the neck of the pancreas?

A

superior mesenteric vein

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3
Q

How do you define endocrine vs exocrine secretion?

A

endocrine: secretion into the bloodstream with an effect on a DISTANT target organ- DUCTLESS
exocrine: secretion into a DUCT to have a direct LOCAL effect

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4
Q

What are the main endocrine secretions of the pancreas and their actions?

A
  • insulin: dec. glucose, inc. glucose transport into cells for storage as glycogen, inc. protein synth. and lipogenesis
  • glucagon: inc. gluconeogenesis and glycogenolysis (for inc. blood glucose)
  • somatostatin: ‘endocrine cyanide’–> -ve effect on everything
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5
Q

What area of the pancreas contains endocrine cells?

A
  • islets of Langerhans (2% of gland)

- secrete hormones into blood (insulin, glucagon, somatostatin and pancreatic polypeptide)

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6
Q

What is the exocrine function of the pancreas?

A
  • covered by 98% of the gland
  • secretes pancreatic juice into duodenum via main pancreatic duct, sphincter of Oddi and ampulla
  • digestive function
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7
Q

What are the acini in the pancreas?

A
  • ducts
  • grape-like clusters of secretory units
  • acinar cells secrete pro-enzymes (becomes pancreatic juice) into ducts
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8
Q

What are the islets in the pancreas?

A
  • derived from branching duct system but lose contact w/ ducts (hence islets)
  • differentiate into alpha and beta cells–> secrete into blood
  • many more islets in tail of pancreas
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9
Q

What is the micro anatomy of the pancreatic acini?

A
  • intercalated pancreatic ducts w/in acini
  • acinar cells (large w/ apical secretion granules)
  • specialised centroacinar cells- small and pale (when leave acinus, become normal pancreatic duct cells)
  • intercellular canaliculi between acinar cells
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10
Q

What is the composition of the islets?

A
  • alpha cells: 15-20% of islet tissue, secrete glucagon
  • beta cells: 60-70%, secrete insulin
  • delta cells: 5-10%, secrete somatostatin
  • islets= highly vascular (so all endocrine cells close to bloodstream for secretion)
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11
Q

What are the 2 components of pancreatic juice and what cells produce them?

A
  • acinar cells produce low volume, viscous, enzyme rich component
  • duct and centroacinar cells produce high volume, watery, HCO3- rich component
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12
Q

What is the function of bicarbonate in pancreatic juice?

A
  • neutralises acid chyme from stomach–> prevents damage to duodenal mucosa and raises pH for pancreatic enzymes optimum (7.5-8)
  • washes low volume enzyme secretions out of pancreas into duodenum
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13
Q

What is the effect of duodenal pH on bicarbonate secretion rate?

A
  • when pH dec. from 5, there is linear inc. in pancreatic bicarbonate secretion
  • but when pH reaches 3, rate of secretion levels off bc bile also contains bicarbonate and Brunners gland secretes alkaline fluid
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14
Q

What is the mechanism of pancreatic bicarbonate secretion?

A

in pancreatic duct cells

  • carbonic anhydrase catalyses conversion of H2O+CO2–> H+ and bicarbonate
  • sodium moves down gradient through paracellular tight junctions into lumen–> water follows
  • chloride/bicarbonate exchange by anion exchanger–> bicarbonate goes into lumen
  • H+ out into blood and sodium into cell via NHE-1 antiporter
  • electrochemical gradient maintained by Na+/K+ exchange pump (primary active transport)
  • K+ returns to blood via K+ channel
  • Cl- returns to lumen via Cl- channel (CFTR)
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15
Q

What is the function of the small bowel?

A

to absorb nutrients, salt and water

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16
Q

What is the structure of the small bowel?

A
  • approx. 6m long and 3.5cm diameter
  • duodenum: 25cm
  • jejunum: 2.5m (thicker)
  • ileum: 3.75m (thinner)
  • no sudden transitions between them
  • all have same basic histological organisation
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17
Q

What are the functions of the mesentery?

A
  • suspends small and large bowel from posterior abdominal wall: anchoring them in place, whilst still allowing some movement
  • provides a conduit for blood vessels, nerves and lymphatic vessels
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18
Q

What is the structure of the epithelium of the small bowel?

A
  • serosa: tough, protective layer
  • longitudinal muscle
  • circular muscle
  • submucosa
  • mucosa
  • plicae circulares: folds of mucous membrane, have villi on them–> microvilli
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19
Q

What are the villi?

A
  • only in small intestine
  • motile
  • rich blood supply and lymph drainage for absorption of digested nutrients
  • good innervation from submucosal plexus
  • simple epithelium: 1 cell thick, dominated by enterocytes
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20
Q

What types of cells line the villi (mucosa)?

A
  • absorptive: enterocytes

- secretory: goblet, enteroendocrine, tuft, paneth

21
Q

What types of cells line the Crypts of Lieberkuhn?

A
  • paneth cells

- stem cells

22
Q

What are enterocytes?

A
  • most abundant cells in small bowel
  • tall columnar cells w/ microvilli and basal nucleus
  • specialised for absorption and transport of substances from lumen–> bloodstream
  • short life span (1-6 days)
23
Q

What are microvilli?

A
  • make up brush border
  • several thousand per cell
  • microvilli surface covered w/ glycocalyx: rich carb layer on apical membrane, protects from digestive enzymes, traps a layer of water+mucous- “unstirred layer”
24
Q

What are goblet cells?

A
  • 2nd most abundant epithelial cell type
  • produce mucous–> large glycoprotein that facilitates passage of material through bowel
  • more goblet cells in colon vs duodenum (bc lubricant)
25
Q

What are enteroendocrine cells?

A
  • columnar epithelial cells
  • scattered amongst enterocytes
  • often found in lower part of crypts
  • secrete hormones for gut motility
26
Q

What are paneth cells?

A
  • found only in bases of crypts
  • contain large, acidophilic granules: contain lysozymes, zinc and glycoproteins
  • also engulf some bacteria and protozoa
  • may help regulate intestinal flora
27
Q

What are stem cells (GI tract)?

A
  • undifferentiated cells
  • pluripotent
  • exist in crypts of small bowel
  • continually divide by mitosis to replenish surface epithelium
  • migrate to top of villus, replacing older cells that die
28
Q

Why do enterocytes and goblet cells of the small bowel have such a short life span (36hrs)?

A
  • enterocytes= 1st line of defence against GI pathogens–> directly affected by toxic substances in diet
  • any lesions will be short-lived
29
Q

How is the duodenum distinguished?

A

Brunner’s glands: secrete alkaline fluid, open into base of crypts

30
Q

What are some structural differences between the jejunum and ileum?

A
  • jejunum= thicker walls than ileum bc more plicae circulares
  • different relationship to mesentery
  • jejunum has fewer arterial arcades, and longer branches
31
Q

What are the functions of small intestine motility?

A
  • mix ingested food w/ digestive secretions and enzymes
  • facilitate contact between contents of intestine and intestinal mucosa
  • propel intestinal contents along alimentary tract
32
Q

What is segmentation?

A

mixes contents of lumen by circular muscle contractions at intervals

33
Q

What is peristalsis?

A

propels chyme towards colon by sequential contraction of adjacent rings of smooth muscle

34
Q

What is the migrating motor complex?

A

cycles of smooth muscle contractions sweeping throughout the gut, prevents migration of colonic bacteria into ileum

35
Q

How are carbohydrates digested?

A
  • begins in mouth–> salivary alpha-amylase
  • small intestine–> pancreatic alpha-amylase continues digestion of starch and glycogen–> acts mainly in lumen, but digestion of amylase products and simple carbs occurs at brush border
36
Q

What does MRCP stand for?

A

magnetic resonance cholangiopancreatography

37
Q

Where are the enzymes of digestion secreted from in the pancreas?

A
  • acinar cells
  • lipases, proteases, amylase
  • synthesised and stored in zymogen granules (zymogens=pro-enzymes)
38
Q

How does the pancreas attempt to prevent autodigestion?

A
  • proteases released as inactive pro-enzymes–> protects acini and ducts from auto digestion
  • trypsin inhibitor prevents trypsin activation
  • enzymes only activated in duodenum
    N.B. blockage of main pancreatic duct may overload protection–> auto digestion–> acute pancreatitis
39
Q

How are digestive enzymes activated in the duodenum?

A
  • duodenal mucosa secretes an enzyme: enterokinase/enteropeptidase–> converts trypsinogen TO trypsin
  • trypsin then converts all other proteolytic and some lipolytic enzymes
    (N.B. lipase secreted in active form but requires colipase and requires presence of bile salts for effective action)
40
Q

What are the phases of pancreatic juice secretion?

A
  1. cephalic phase: reflex to sight/smell/taste of food, ONLY enzyme component released
  2. gastric phase: stimulation from food arriving in stomach
  3. intestinal phase (70-80% of pancreatic secretion): hormonally mediated when gastric chyme enters duodenum, BOTH components of pancreatic juice stimulated
41
Q

How is pancreatic juice enzyme secretion controlled in acini?

A
  1. vagus nerve- ACh

2. cholecystokinin- secreted by I cells (CCK)

42
Q

How is pancreatic juice bicarbonate secretion controlled in duct and centroacinar cells?

A
  • secretion of acinar fluid (isotonic) and its proteins is stimulated by CCK
  • secretion of water and bicarbonate from duct cells is stimulated by secretin (cAMP)
    N.B. secretin-stimulated secretion= richer in bicarbonate vs acinar secretion bc of Cl-/HCO3- exchange
43
Q

What is the negative feedback loop involved in bicarbonate secretion in ducts?

A

dec. luminal pH in duodenum–> S- cells produce secretin–> causes pancreatic ductal bicarbonate secretion–> feedsback: inc. pH

44
Q

How do CCK and secretin react and affect the rate of bicarbonate secretion?

A
  • CCK alone: no effect on secretion

- CCK can inc. bicarbonate secretion a lot along w/ secretin (symbiotic relationship)

45
Q

What transporters are used in the absorption of monosaccharides?

A
  • glucose and galactose: SGLT-1 carrier protein on apical membrane (2y active transport)
  • fructose: GLUT-5 carrier protein on apical membrane (facilitated diffusion)
  • all exit by GLUT-2 on basolateral membrane
46
Q

How are proteins digested?

A
  • begins in lumen of stomach by pepsin (then inactivated in alkaline duodenum)
  • trypsinogen activated by enterokinase–> trypsin in duodenum
  • trypsin activates other proteases, which hydrolyse proteins to single AAs
47
Q

How are lipids digested?

A
  • secretion of bile salts and pancreatic lipases
  • emulsification (inc. SA for digestion)
  • enzymatic hydrolysis of ester linkages by lipase+colipase
  • solubilisation of lipolytic products in bile salt micelles
48
Q

With what 2 pathways can fatty acids and monoglycerides be resynthesised into triglycerides?

A

monoglyceride acylation (major) and phospatidic acid pathway (minor)

in enterocytes

49
Q

What is the ileocaecal valve?

A
  • separates the ileum from the large intestine
  • relaxes and contracts to control passage of material into colon
  • prevents back flow of bacteria into ileum

BRS Term 2 (36 decks)

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