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Alimentary system > 4. Pancreas > Flashcards

4. Pancreas Flashcards

1
Q

Describe the embryology of the pancreas.

A

Pancreas develops as a ventral bud (part of the hepatobiliary bud) and the dorsal bud.
Duodenum rotates to form a C shape, the ventral bud swings around and both buds fuse.
Ventral bud duct becomes main pancreatic duct

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2
Q

What are the 5 parts of the pancreas?

A 
Uncinate process
Head
Neck
Body 
Tail
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3
Q

Where is the pancreas positioned? What are its posterior relations?

A

Extends from C of the duodenum to the hilum of the spleen

Posterior to the pancreas is the IVC, abdominal aorta and left kidney

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4
Q

Describe the blood vessels around the pancreas.

A

Coeliac trunk is just superior to the pancreas

Superior mesenteric arteries arise from in between the uncinate process and the body of the pancreas

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5
Q

How do pancreatic juice reach the duodenum?

A

Via main (and accessory) pancreatic ducts

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6
Q

Why are close relations with (and supply from) coeliac and superior mesenteric arteries important for the pancreas?

A

Needs blood supply for endocrine component (to carry hormones away) and to fuel activity during digestion

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7
Q

Define Endocrine

A

Secretion into the blood stream to have effect on distant target organ (Autocrine/Paracrine)
Ductless Glands

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8
Q

Define Exocrine

A

Secretion into a duct to have direct local effect

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9
Q

What are the main endocrine products of the pancreas?

A

Insulin: promotes glucose transport into cells and storage as glycogen, reduces BG
Glucagon
Somatostatin
Pancreatic polypeptide

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10
Q

What proportion of the pancreas is endocrine and what proportion is exocrine?

A

2% endocrine

98% exocrine

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11
Q

Why is somatostatin described as endocrine cyanide?

A

It suppresses many other endocrine processes

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12
Q

Which are the endocrine cells of the pancreas, and where are the majority of these found?

A

Islets of Langerhans

Tail of pancreas

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13
Q

What is the exocrine role of the pancreas?

A

Secretes (Pancreatic Juice) into duodenum via pancreatic duct/common bile duct.
Digestive function

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14
Q

What may pancreatic disease effect?

A

Both endocrine and exocrine function e.g. Cystic fibrosis

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15
Q

Describe the development of the endocrine and exocrine parts of the pancreas from ductal budding.

A

Endocrine and exocrine parts of the pancreas develop from the ductal budding
Endocrine components lose their connection with the duct whereas the exocrine parts retain their connection.

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16
Q

Describe 3 characteristic features of the exocrine pancreatic cells

A

Ducts
Acini (grape-like clusters of secretory units)
Acinar cells secrete pro-enzymes into ducts

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17
Q

Describe 4 characteristic features of the endocrine pancreatic cells

A

Derived from the branching duct system
Lose contact with ducts: become islets
Differentiate into α- and β-cells, secreting into blood
Tail > head

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18
Q

What percentage of the islets of Langerhans are alpha cells, beta cells and delta cells? What do each of these secrete?

A

Alpha cells: 15-20%, secrete glucagon
Beta cells: 60-70%, secrete insulin
Delta cells: 5-10%, secrete somatostatin

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19
Q

How does the staining of the islets of Langerhans differ from the rest of the pancreas?

A

Islets of Langerhans stain lighter than the other cells

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20
Q

Describe the structure of pancreatic acini

A

Consist of a blind ended tubule surrounded by acinar cells and a duct lined by duct cells.
Between the duct cells and the acinar cells are centroacinar cells.

21
Q

Describe the difference in appearance between duct and acinar cells

A

Acinar cells have large apical granules full of enzymes

Duct cells release different components so don’t have granules and are smaller and paler

22
Q

What are the 2 components of pancreatic juice and what are they produced by?

A

Acinar cells: secrete viscous, low volume, enzyme rich component
Duct (and centroacinar cells): secrete high volume, watery, bicarbonate rich component

23
Q

What are the 3 roles of the bicarbonate produced by the duct and centroacinar cells?

A

Bicarbonate neutralises acid chyme, hence preventing damage to duodenal mucosa
Also raises the pH to optimum range for pancreatic enzymes
Washes low volume enzyme secretion out of pancreas into duodenum

24
Q

What is the concentration of bicarbonate and pH of the fluid secreted by duct and centroacinar cells?

A

Bicarbonate: 120mM

pH 8

25
Q

How does bicarbonate secretion change with duodenal pH?

A

As duodenal pH decreases, bicarbonate secretion increases to a maximum at ~pH 3.

26
Q

Why does bicarbonate secretion stop at pH 5 (i.e. when it is still acidic)?

A

Neutralisation of the acid chyme is also affected by the bicarbonate in bile and the Brunner’s glands secreting alkaline fluid

27
Q

Describe the movement of sodium in the pancreas. What is the consequence of this?

A

Na+ moves down its gradient via the paracellular pathway (tight junctions) from the blood to the lumen.
This increases osmotic potential, so H2O follows

28
Q

What happens when CO2 enters cells?

A

CO2 reacts with H2O under the influence of carbonic anhydrase to produce H2CO3, which readily dissociates into H+ and HCO3-

29
Q

Describe the movement of bicarbonate and protons in the duct cells of the pancreas.

A

HCO3- is pumped into the lumen via exchange with Cl-
H+ are pumped into the blood via exchange with Na+
(both secondary active transport)
Na+ gradient is maintained by a Na+/K+ pump (primary active transport).
K+ channels allow K+ to return to blood
Cl- channels allow Cl- to return to the lumen

30
Q

What is the role of CFTR in this mechanism of bicarbonate secretion? What is the result of a defective CFTR and what is this disease called?

A

CFTR allows the outwards movement of Cl- so the gradient can be used to pump out HCO3-
Cystic fibrosis: defective CFTR doesn’t allow cl- out, results in thick mucous and thick pancreatic juice

31
Q

Same reaction in gastric parietal cells (acid) and pancreatic duct cells (alkaline): what causes different pH in venous blood?

A

Stomach: H+ goes into gastric juice, HCO3- into blood. Gastric venous blood is alkaline
Pancreas: HCO3- secreted into juice and H+ into blood. Pancreatic venous blood is acidic

32
Q

How are lipases, proteases and amylase synthesised and stored?

A

In zymogen granules

Zymogens= pro-enzymes

33
Q

How are proteases secreted by the acinar cells and how are they activated?

A

Proteases are secreted as zymogens.
Acinar cells also produce trypsin inhibitors to prevent activation of proteases in the pancreas.
Enterokinase is required to convert trypsinogen to trypsin, which then converts other zymogens into active proteases.
Enterokinase is only found in the duodenum.

34
Q

What happens if there is a blockage in the pancreatic duct?

A

Pancreatic juices accumulate and overload the protection
Trypsin becomes activated
Results in auto-digestion (acute pancreatitis)

35
Q

Describe the requirement for lipase to work properly.

A

Need co-lipases (which are produced in an inactive form)

Need bile salts to emulsify the lipids to be able to digest the lipids well.

36
Q

What does Orlistat do?

A

Pancreatic lipase inhibitor

Reduced digestion and absorption of fats leading to steatorrhoea

37
Q

What is the main nerve controlling pancreatic secretion?

A

VAGUS NERVE
Uses ACh
Efferent and Afferent innervation

38
Q

What happens in the pancreas during the cephalic phase of secretion?

A

Reflex response to sight/ smell/ taste of food

Sensory input channeled to pancreas to prepare it for enzyme release

39
Q

What happens in the pancreas during the gastric phase of secretion?

A

Food arriving in the stomach stimulates pancreatic secretion via the vagus nerve.

40
Q

What mediates the pancreatic secretion during the intestinal phase?

A

Hormones released by enteroendocrine cells when gastric chyme enters the duodenum

41
Q

What happens in the pancreas during the intestinal phase of secretion?

A

Release of pancreatic juice, both enzyme and watery component squirted into duodenum

42
Q

Which hormones stimulate the release of the bicarbonate component and the enzyme component of pancreatic juice?

A

Bicarbonate component:
Secretin hormone
Enzyme component:
Cholecystokinin hormone (CCK) and vagal nerve reflex

43
Q

What does CCK also stimulate? (other than the release of the enzyme rich component)

A

Stimulates bile secretions

44
Q

Describe the control of bicarbonate secretion from duct cells

A

H+ concentration is detected by the S cells in the duodenum.
Influx of H+ (from chyme) causes release of secretin (from S cells) into the systemic circulation
Secretin causes intracellular enzyme cascade in pancreatic duct cells, favouring the release of bicarbonate rich component
Bicarbonate rich component travels down duodenum and neutralises H+

45
Q

Describe the control of enzyme secretion from acinar cells

A

Peptides and fats are detected by the C cells in the duodenum
C cells release CCK into blood
CCK acts on acinar cells and makes them release pro-enzymes and trypsin inhibitor.
Vagus can also stimulate the release of proenzymes and trypsin inhibitor from acinar cells via ACh receptors

46
Q

What happens to bicarbonate secretion as pH starts to increase?

A

Increasing pH reduces stimulus for release of secretin
=Feedback cycle:
stop releasing secretin, stop releasing bicarbonate rich fluid

47
Q

How do you switch off CCK?

A

End of the cephalic phase (once the meal has been eaten) shuts down the vagal stimulation.
Absorption of fats and peptides removes the local stimulus for CCK release from enteroendocrine cells
There may also be other mechanisms

48
Q

How does CCK affect bicarbonate secretion? Does secretin affect enzyme secretion in the same way?

A

CCK potentiates the effects of secretin.
CCK alone will not trigger the production of any bicarbonate but CCK and secretin together will cause a massive increase in bicarbonate secretion.
Vagus nerve has a similar effect to CCK.
Secretin has NO effect on enzyme secretion.

49
Q

Why does CCK potentiate the effects of secretin but not trigger the production of bicarbonate when it is on its own?

A

Don’t want large amount of duodenum to be acidic before you are releasing pancreatic juice, as it would increase risk of damage.
Secretin acts as a “switch”, so other hormones can upregulate effect

Alimentary system (15 decks)

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