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BRS 8: Gastroenterology > 8.2: The pancreas and small bowel > Flashcards

8.2: The pancreas and small bowel Flashcards

1
Q

Pancreatic embryology (3 steps)

A

1) abdominal accessory organs arise as foregut outgrowths
2) proximal duodenum rotates clockwise
3) ventral and dorsal pancreatic buds and ducts fuse. Bile and pancreatic ducts join to drain together at major papilla

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2
Q

What is the uncinate process of the pancreas originated from?

A

Ventral bud and duct

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3
Q

What do ventral and dorsal ducts emerge as respectively?

A

Ventral - main pancreatic duct at the major papilla
Dorsal - accessory pancreatic duct at major papilla

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4
Q

Is the accessory pancreatic duct present in all individuals?

A

No, In most adults it has been degenerated

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5
Q

What 2 ducts join at the major papilla?

A

Main pancreatic duct
Bile duct

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6
Q

What is meant by the pancreas having a retroperitoneal structure

A

It does not exist within the abdomen
It is behind the posterior to the peritoneum

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7
Q

What vein is formed by joining of the splenic vein and superior masenteric vein?

A

Portal vein

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8
Q

What two diagnostic techniques are used when looking at the pancreas

A

CT scan
MRCP

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9
Q

What is MRCP used for and how does it work?

A

Procedure can be used to determine wether gallstones are lodged in any ducts surrounding gallbladder
Uses MRI to visualise biliary pancreatic ducts non-invasively

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10
Q

What is pancreas divisum and why does it cause the patient to have recurrent episodes of pancreatitis?

A

Ventral bud and dorsal buds fail to fuse and so the ventral duct which usually has a large enough capacity to cope with the flow of the pancreatic juice can no longer do so

The large flow has to therefore go through a minor duct and so they get recurrent episodes of pancreatits

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11
Q

Define endocrine secretion

A

Secretion into the blood stream to have effect on distant target organ

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12
Q

Define exocrine secretion

A

Secretion into a duct to have direct local effect

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13
Q

3 main endocrine secretions of pancreas

A

Insulin
Glucagon
Somatostatin

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14
Q

4 actions of insulin

A

Anabolic hormone

Promotes glucose transport into cells and storage as glycogen

Decreased blood glucose

Promotes protein synthesis and lipogenesis

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15
Q

2 actions of glucagon

A

Increases gluconeogenesis and glycogenolysis
Increases blood glucose

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16
Q

What percentage of the pancreatic secretion is endocrine and what part of the pancreas does this?

A

2%
Islets of Langerhans

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17
Q

What percentage of the pancreatic secretion is exocrine and what does this involve?

A

98%
Secretion of pancreatic juice into duodenum vis MDP/sphincter of Oddi/ampulla

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18
Q

Describe the structure of acini in the pancreas

A

Attached to ducts
Grape-like clusters of secretory units

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19
Q

What do acini secrete into ducts?

A

Pro-enzymes

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20
Q

Describe the structure of islets

A

Derived from branching duct system but lose contact with the ducts to become islets

Differentiate into alpha and beta cells secreting into the blood

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21
Q

Are there more islets in the head or tail of the pancreas?

A

Tail

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22
Q

Why are the pancreatic islets highly vasculated?

A

Very good blood supply
Ensures that all endocrine cells have a site for close access to a site for secretion

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23
Q

What proportion of pancreatic islets are composed of alpha, beta and gamma cells respectively

A

alpha = 15-20%

beta = 60-70%

gamma = 5-10%

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24
Q

Compare the structures of the secretory acinar cells and the duct cells in the acini of the pancreas

A

Secretory acinar cells - large with apical secretions
Duct cells - small and pale

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25
Q

Describe the components of pancreatic juice formed by Acinar cells

A

Low volume
Viscous
Enzyme - rich

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26
Q

Describe the other components of pancreatic juice formed by the Duct and Centroacinar cells

A

High volume
Watery
HCO3 - rich

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27
Q

What are the functions of the bicarbonate secretion in the pancreatic juice?

A

Neutralises acid chyme from stomach - prevents damage to duodenal mucosa, raises pH to optimum range for pancreatic enzymes to work
Washes low volume enzyme secretion out of pancreas into duodenum

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28
Q

Why does HCO3- secretion stop increasing when pH goes below 3?

A

Bile also contains HCO3 and helps neutralise acid chyme
Burners glands secrete alkaline fluid

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29
Q

Describe how HCO3- is produced and secreted by the pancreatic duct cells

A
  • CO2 enters pancreatic duct cell from blood and catalysed by carbonic anhydrase it reacts with H20 to form H+ and HCO3-Na+ moves down gradient via paracellular junctions and H20 follows from the blood to the lumenAE1 transporter exchanges HCO3- from duct cell into lumen with Cl- into the duct cellNa+/H+ antiporter active on the basolateral membrane and Na+ enters the cell down gradientTo maintain Na+ concn. gradient in the cell, Na+/K+ATPase transports Na+ into blood and K+ into cellTo maintain K+ concn. gradient in cell, K+ returns to blood via K+ channelsCl- returns to lumen via Cl- channel (CFTR) to maintain concn. gradient established by AE1 transporter
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30
Q

What makes gastric venous blood alkaline and pancreatic venous blood acidic?

A

Stomach
H+ → Gastric Juice
HCO3- → blood

Pancreas
HCO3- secreted → juice
H+ → blood

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31
Q

Which classes of digestive enzymes are present in the Acinar cell enzyme secretion?

A

Lipases - fat
Proteases - protein
Amylase - carbohydrates

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32
Q

Why are proteases released as inactive pro-enzymes?

A

Protects acini and ducts from auto-digestion

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33
Q

What other protective mechanisms against auto-digestion of the pancreas are there?

A

Pancreas contains trypsin inhibitor to prevent trypsin activation
Enzymes only activated in duodenum where they have to start digesting food

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34
Q

What enzyme is secreted by the duodenal mucosa and what does it do?

A

Enterokinase (enteropeptidase)
Converts trypsinogen into trypsin

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35
Q

What is the function of trypsin?

A

Converts all other proteolytic and some lipolytic enzymes into their active form

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36
Q

Why is it not dangerous to secrete lipase in its active form?

A

Requires colipase for action and the presence of bile salts for effective action

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37
Q

What can a lack of pancreatic enzymes lead to?

A

Malnourishment

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38
Q

What is a side effect you might expect from anti-obesity drug Orlistat which inhibits pancreatic lipases?

A

Steatorrhoea

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39
Q

What are the 3 phases involved in the control of pancreatic juice secretion?

A

Cephalic phase
Gastric phase
Intestinal phase

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40
Q

Describe the Cephalic phase of pancreatic juice secretion

A

Reflex response to sight/smell/taste of food

Enzyme-rich component only

Low volume - ‘mobilises’ enzymes

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41
Q

Describe the gastric phase of Pancreatic juice secretion

A

Stimulation of pancreatic secretion originating from food arriving in the stomach

Same mechanisms involved as for cephalic phase

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42
Q

Describe the intestinal phase of pancreatic juice secretion.

A

Hormonally mediated when gastric chyme enters duodenum

Both components of pancreatic juice stimulated enzymes & HCO3- juice flows into duodenum

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43
Q

What 2 components is the pancreatic juice enzyme secretion controlled in the acini by?

A

Vagus nerve (Cholinergic,
Vagal stimulation of enzyme secretion (& communicates info from gut to brain))

Cholecystokinin (CCK) (Ca2+/PLC)

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44
Q

What hormone controls the pancreatic juice bicarbonate secretion in the duct and centroacinar cells?

A

Secretin (cAMP)

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45
Q

What compounds stimulate the release of CCK from the duodenal I cells?

A

Amino acids and fatty acids

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46
Q

What effect does trypsin have on the release of CCK from duodenal I cells?

A

Inhibitory

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47
Q

Is acinar fluid isotonic?

A

Yes and it resembles plasma in its concentrations of Na+, K+, Cl- and HCO3-

48
Q

Why is secretin-stimulated secretion richer in HCO3- than acinar secretion?

A

Because of the Cl-/HCO3- exchange in the extralobular duct

As it flows down into the larger pancreatic ducts, it becomes richer in HCO3- due to there being more duct and centroacinar cells

49
Q

Describe the negative feedback loop involved in the control of HCO3- secretion in ducts?

A

Decrease in pH in duodenum activates S cells to release Secretin

Secretin stimulates pancreatic ductal HCO3- secretion which increases pH

Since pH is increased, the S cells cannot be stimulated to release more secretin, controlling the secretion of HCO3-

50
Q

Does CCK alone have any effect on HCO3- secretion?

A

No

51
Q

When does CCK have an effect on HCO3- secretion?

A

It markedly increases HCO3- secretion that has been stimulated by secretin

52
Q

What effect does secretin have on enzyme secretion?

A

None

53
Q

What effect does bile and Brunei’s gland secretions have on pH?

A

Increase pH

54
Q

What is the general function of the small bowel?

A

To absorb nutrients, salt and water

55
Q

How long is the small bowel ?

A

6m long
3.5cm in diameter

56
Q

What are the functions of the mesentery?

A

Suspends small and large bowel from the posterior abdominal wall
Provides a conduit for blood vessels, nerves and lymphatic vessels

57
Q

What are the 3 main regions of the small intestine/bowel?

A

Duodenum

Jejunum

Ileum

58
Q

What are the 3 layers of the small intestine from superficial to deep?

A

Serosa, longitudinal muscle layer and circular muscle layer.

59
Q

Are villi found in the large bowel?

A

No; only occur in the small intestine

60
Q

What innervates villi?

A

Innervation from submucosal plexus (part of the enteric nervous system)

61
Q

How thick is the epithelium of villi?

A

1 cell thick

(dominated by enterocytes - simple columnar absorptive cells)

62
Q

What are villi (mucosa) lined with?

A

Simple columnar epithelium consisting of:

Primarily enterocytes

Scattered goblet cells

Enteroendocrine cells

63
Q

What types of cells does the epithelium of the Crypts of Leiberkuhn include?

A

Paneth and stem cells

64
Q

What is the most abundant type of cell in the small bowel?

A

Enterocytes (tall columnar cells with microvilli and a basal nucleus)

65
Q

What are these cells specialised for and what are their lifespans?

A

Absorption and transport of substances

1-6 days (relatively short)

66
Q

What is the cylindrical internal surface area of the small bowel?

A

0.4m^2

67
Q

What increases the surface are of the small bowel and by how much?

A

Villi and microvilli

~500 fold ^

68
Q

What structure makes up the ‘brush border’?

A

Microvilli - several thousand microvilli per cell

69
Q

What is glycocalyx?

A

Rich carbohydrate layer on apical membrane of microvilli

70
Q

What is the purpose of glycoalyx?

A

Covers surface of microvilli

Serves as protection from digestional lumen yet allows for absorption

Traps a layer of water and mucous known as ‘unstirred layer’ → regulates rate of absorption from intestinal lumen

-

71
Q

What is the second most abundant epithelial cell type?

A

Goblet cells

72
Q

What is mucous and what does it do?

A

Large glycoprotein that facilitates passage of material through bowel

73
Q

What happens to the number of goblet cells along the entire length of bowel?

A

Abundance increases along entire length of bowel

Low in duodenum and high in colon

74
Q

What classification of epithelial cells are enteroendocrine cells and where are they most often found?

A

Columnar epithelial cells

Lower parts of crypts

75
Q

What is the function of enteroendocrine cells?

A

Hormone secretion for example to influence gut motility

76
Q

What do enteroendocrine cells have a high affinity for in terms of staining?

A

Chromium/silver salts

77
Q

Where are Paneth cells exclusively found?

A

Bases of crypts

78
Q

What do Paneth cells contain that are important for their functions and what are some of these functions?

A

Large, acidophilic granules containing anti-bacterial lysozyme(protects stem cells) and glycoproteins and zinc (essential trace metal for a number of enzymes)

Engulf some bacteria and protozoa

May have a role in regulating intestinal flora

79
Q

Why are epithelial stem cells essential in the GI tract?

A

To continually replenish the surface epithelium as cells such as enterocytes have a very short life span

They continually divide by mitosis and migrate to the top of villus, replacing older cells that die by apoptosis → digested and reabsorbed

80
Q

How long is the lifespan of enterocytes and goblet cells present in the small bowel?

A

~36 hours (relatively short)

81
Q

List reasons why enterocytes and goblet cells in the small bowel have this lifespan

A

Enterocytes are first line of defence against GI pathogens and may be directly affected by toxic substances in diet

Effects of agents which interfere with cell function, metabolic rate will be diminished

Any lesions → short-lived

If escalator-like transit of enterocytes is interrupted through impaired production of new cells, severe intestinal dysfunction will occur

82
Q

What structure distinguishes the duodenum from the jejunum and ileum?

A

Brunners glands

83
Q

What does the brunners gland secrete and why?

A

Alkaline fluid to neutralise chyme from stomach, protecting proximal small bowel

And to help optimise pH for action of pancreatic digestive enzymes

84
Q

Compare the walls of the jejuum and the ileum and what is the reason for this difference

A

Jejunum is wider and thicker walled and redder than the ileum

The plicae circulares in the jejunum are larger and more numerous and so they are more closely set

85
Q

Compare the jejunal and ileal mesenteries

A

Jejunal mesentery forms 1-2 arterial arcades whereas ileal forms 3-4, but these 3-4 are shorter

Jejunal mesentery is above and to left of aorta whereas ileal mesentery is attached below and to the right of the aorta

86
Q

Where in the small intestine are Peyer’s Patches found?

A

Lower part of the ileum

87
Q

List the functions of small intestine motility?

A

Mix ingested food with digestive secretions and enzymes

Facilitate contact between contents of intestine and intestinal mucosa

Propel intestinal contents along alimentary contact

88
Q

Outline the 3 steps of small bowel motility

A

Segmentation (mixing)
Peristalsis (propelling)
Migrating motor complex

89
Q

Outline the step of segmentation

A

Mixes contents of lumen

Occurs due to stationary contraction of circular muscles at intervals. More frequent contraction in duodenum

Contractions allow pancreatic enzymes and bile to mix with chyme

Although chyme moves in both directions, net effect is movement into the colon

90
Q

Outline the step of peristalsis

A

Involves sequential contraction of adjacent rings of smooth muscle

Propels chyme towards colon. Most waves of peristalsis only travel about 10cm. Segmentation and peristalsis result in chyme being segmented, mixed and propelled into the colon.

91
Q

Outline the steps of migrating motor complex

A

Cycles of smooth muscle contractions sweeping through gut. Begin in the stomach → small intestine → colon → next wave starts in duodenum.

Prevents migration of colonic bacteria into ileum.

92
Q

Describe the environment that digestion in the small bowel occurs in

A

Alkaline environment

93
Q

How do bile and pancreatic digestive enzymes enter the duodenum

A

Main pancreatic duct (MPD) and common bile duct (CBD)

94
Q

What does duodenal epithelium produce to aid digestion?

A

It’s own digestive enzymes

95
Q

Where does digestion of carbohydrates begin and by what enzyme?

A

Mouth by salivary alpha amylase, which is destroyed in stomach by acidic pH

96
Q

Where does most carbohydrate digestion occur

A

Small intestine

97
Q

Give examples of simple and complex carbohydrates

A

Simple: monosaccharides such as glucose and fructose; disaccharides such as sucrose and maltose.

Complex: starch, cellulose and pectin.

98
Q

What is secreted into the duodenum in response to a meal to aid digestion of carbs

A

Pancreatic-alpha-amylase

99
Q

What conditions does pancreatic-alpha-amylase need for optimal activity

A

Cl- and neutral or slightly alkaline pH

100
Q

Where does digestion of amylase products and simple carbs occur?

A

Brush border

101
Q

What carrier protein is used for absorption of glucose and galactose?

A

SGLT-1 on apical membrane (absorption is by secondary active transport)

102
Q

What carrier protein is used for absorption of fructose?

A

GLUT-5 on apical membrane (facilitated diffusion)

103
Q

What carrier protein facilitates exit of digestion products of carbs into the intestinal space at the basolateral membrane?

A

GLUT-2

104
Q

How much simple sugar can the human small intestine absorb daily?

A

10kg

105
Q

Where does protein digestion begin and by what enzyme?

A

Lumen of stomach by pepsin (pepsin inactivated in alkaline duodenum)

106
Q

List the 5 pancreatic proteases secreted as precursors into the lumen of the small bowel

A

Trypsinogen → Trypsin (via enterokinase)

Chymotrypsinogen → Chymotrypsin

Proelastase → Elastase

Procarboxypeptidase A - Carboxypeptidase A

Procarboxypeptidase B - Carboxypeptidase B

107
Q

Where is the enzyme enterokinase located?

A

Duodenal brush border

108
Q

What are oligopeptides?

A

Peptide whose molecules contain a relatively small number of amino-acid residues, but more than 2

109
Q

How do enterocytes directly absorb small amino acids and transport them into the interstitial space?

A
  • Via action of H+/oligopeptide cotransporter → PepT1 and other cotransporters, **some AAs will be absorbed into the epithelial cellsOther AAs are broken down to form smaller AAs by variety of peptidases on the brush borderThese AAs are then taken into epithelial cells by transporters and broken down by enzymes such as dipeptidases and tripeptidases so that the individual AAs can be transported into the interstitial space
110
Q

Why are lipids most complicated to digest?

A

They are poorly soluble in water

111
Q

Outline the 4 stage process of lipid digestion in the small bowel

A

Secretion of bile salts and pancreatic lipases

Emulsification of lipids to increase SA for digestion

Enzymatic hydrolysis of ester linkages via colipase-lipase complex

Solubilisation of lipolytic products in bile salt micelles (Holding station)

112
Q

What is the purpose of colipases in lipid digestion?

A

Prevents bile salts from displacing lipase from fat droplet

113
Q

Outline lipid absorption

A

Fatty acids (FAs) and monoglycerides (MGs) leave micelles and enter enterocytes.

FAs and MGs resynthesised into triglycerides (TGs) via 2 pathways:

Monoglyceride acylation (major)

Phosphatidic acid pathway (minor)

Chylomicrons synthesised and secreted across basement membrane by exocytosis

Chylomicrons enter a lacteal (lymph capillary) → lymph transports them away from bowel

114
Q

What are chylomicrons synthesised by and describe their composition

A

Golgi apparatus

80-90% Triglycerides

2% Cholesterol

2% Protein

Small traces of carbohydrate

115
Q

What separates the ileum from the colon and what is its purpose?

A

Ileocaecal valve

Relaxation and contraction controls passage of material into colon and it prevents backflow of bacteria into ileum

BRS 8: Gastroenterology (6 decks)

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