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104 Theme 1 - The Alimentary System > Secretions of the intestine, liver, gallbladder and pancreas > Flashcards

Secretions of the intestine, liver, gallbladder and pancreas Flashcards

1
Q

What is the role of the small intestine?

A

Governs the majority of chemical digestion and absorption of nutrients, electrolytes and water

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2
Q

What are the primary secretions of the intestine?

A
  • intestinal juice (mucus/HCO3-)
  • pancreatic juice ( digestive enzymes)
  • bile (bile salts)
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3
Q

How are bile and pancreatic secretions regulated?

A

Key endocrine hormones secretin, cholecystokinin (CCK), glucose dependent insulinotrophic peptide * (GIP) regulate bile and pancreatic secretions

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4
Q

What are the secretory cells of the small intestine?

A

Villi: absorptive enterocytes and mucus secreting goblet cells

Intestinal glands: Enterocytes secreting isotonic fluid, entero-endocrine cells, paneth cells

In the duodenum only: Brunner’s glands secrete mucus and HCO3-

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5
Q

How doe stem cell renewal of epithelial cells work?

A

Derive from daughter cells at bottom of crypt

Start at bottom and move up during differentiation

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6
Q

What is the turnover of epithelial cells and what is the consequence of this?

A
  • rapid turnover of epithelial cells every 3-6 days
  • vulnerable to radiation, chemotherapy
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7
Q

Summarise the secretions of the small intestine

A
  • Intestinal juice (1.2 L):
  • fluid containing electrolytes and water (secretory enterocytes), lysozyme (Paneth cells), mucus (goblet cells), alkaline mucus containing fluid (submucosal duodenal Brunner’s glands)
  • Key endocrine hormone secretion (by enteroendocrine cells) into vasculature
  • CCK (I cells)– stimulate pancreatic and gallbladder secretion
  • Secretin (S cells)–stimulate pancreatic and biliary bicarbonate secretion
  • GIP (K cells) – may inhibit acid secretion/ stimulate insulin release
  • Exocrine pancreatic juice (1.5 L): bicarbonate / digestive enzymes
  • Bile (0.5-1 L) : bile salts for lipid emulsification (liver hepatocyte synthesis, gall bladder storage)
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8
Q

What does the pancreas secrete?

A

•Exocrine pancreas secretes pancreatic juice containing bicarbonate rich secretion (pH 8) and digestive enzymes essential for normal digestion and absorption

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9
Q

What is the structure of the pancreas?

A

•The pancreas consists of glandular epithelial clusters

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10
Q

What are the 2 types of glandular epithelial clusters found in the pancreas?

A
  • 99% exocrine acinar clusters secreting pancreatic juice (water, electrolytes, sodium bicarbonate and pro-enzymes)
  • 1% endocrine pancreatic islets (Islets of Langerhans) of 4 types secreting glucagon (alpha), insulin (beta), somatostatin (delta), pancreatic polypeptide (F cell)
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11
Q

Complete the diagram on the exocrine acinar cluster

A
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12
Q

Complete the diagram on acinar and ductular secretion

A
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13
Q

What is acinar and ductular secretions made of?

A

Ductular - NaHCO3, fluid

Acinar - enzymes, NaCl, fluid

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14
Q

How is acinar enzyme production regulated?

A
  • Acetylcholine released via P/S vagus stimulation
  • CCK – trigger is chyme containing fat and protein products
  • Produces lower volume enzyme rich pancreatic juice
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15
Q

How is ductal bicarbonate and water secretions regulated?

A
  • Secretin - trigger is H+ in highly acidic chyme
  • Produces copious, HCO3- rich, low enzyme pancreatic juice
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16
Q

What are the 5 pancreatic enzymes?

A
  • Proteolytic enzymes secreted in inactive form, convert proteins to peptides
  • Amylase hydrolyses starch, glycogen and other carbohydrates other than cellulose to form di and trisaccharides
  • Lipases hydrolyse fat into fatty acids and monogylcerides
  • Nucleases digest RNA and DNA to nucleic acids
  • Trypsin inhibitor prevents activation of trypsin to prevent pancreatic digestion
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17
Q

How are proteolytic enzymes activated?

A
  • Proteolytic enzymes are produced as inactive precursors called zymogens
  • Small intestinal brush border enterokinase enzyme cleaves hexapeptide to form active trypsin from trypsinogen
  • Trypsin cleaves and activates other proteolytic enzymes
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18
Q

Why are proteolytic enzymes produced in the inactive form?

A

•Process prevents pancreatic autodigestion (+trypsin inhibitor)

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19
Q

What does secretin stimulate?

A

•Secretin stimulates high volume HCO3− rich pancreatic juice

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20
Q

Which cells are involved in duct secretion of sodium bicarbonate?

A

•HCO3− secretion out of cell into the duct lumen is via Cl−/HCO3− exchange at the apical cell membrane

21
Q

How does duct secretion of sodium bicarbonate occur?

A
  • Cl− is recycled out of the cell via the cystic fibrosis transmembrane conductance regulator (CFTR) Cl− channel under secretin stimulation via cAMP
  • Na+ is secreted transcellularly into the duct lumen following HCO3− secretion down electrochemical gradient, water follows by osmosis
22
Q

What does this diagram show?

A

•HCO3− secretion out of cell into the duct lumen is via Cl−/HCO3− exchange at the apical cell membrane

23
Q

Complete the diagram with which ions each line represents

A
24
Q

How does ionic composition of pancreatic juice depends on secretory rate when it is unstimulated and stimulated?

A
  • Unstimulated
  • Low secretion rate - electrolyte content is similar to that of plasma
  • Stimulated

Higher secretion rate and rise in HCO3- from ductal cells inversely related to reduced concentration of Cl- in pancreatic juice

25
Q

At all rates pancreatic juice is ____tonic with plasma

A

Isotonic

26
Q

How is pancreatic juice secretion regulated?

A

Neurotransmitter acetylcholine (Ach) mediated vago-vagal gastro-pancreatic reflex, hormones gastrin, secretin, cholecystokinin (pancreozymin)

27
Q

Complete the diagram on pancreatic juice regulation

A
28
Q

What goes wrong with cystic fibrosis?

A
  • Patients with cystic fibrosis lack a functional Cl− CFTR channel in the luminal membrane, which results in defective ductal fluid secretion
  • The ducts become blocked with precipitated enzymes and mucus and the pancreas undergoes fibrosis (hence the name of the disease)
  • Blocked ducts impair secretion of needed pancreatic enzymes for digestion of nutrients, resulting in malabsorption
29
Q

How is malnutrition caused by cystic fibrosis treated?

A

•Treatment of this type of malabsorption includes oral pancreatic enzyme supplements taken with each meal

30
Q

What is pancreatitis?

A

•Pancreatitis is an inflammatory disease where pancreatic enzymes are activated within the pancreas (and surrounding tissues), resulting in autodigestion of the tissues

31
Q

What is pancreatitis caused by?

A

•The most common causes of pancreatitis include gallstones and alcohol abuse where obstruction of the pancreatic duct occurs

32
Q

What is the role of bile in digestion?

A
  • Required for digestion and absorption of fats from the small intestine ( up to 1 L secreted /day)
  • Bile salts (amphipathic with hydrophobic/hydrophilic regions) emulsify fats for digestion by pancreatic lipase, solubilise fat digestion products into micelles for absorption across the mucosa
  • elimination of waste products
  • bile pigment bilirubin from heme in red blood cell degradation (breakdown product stercobilin gives faecal brown colour)
  • Cholesterol
  • drugs
33
Q

How is bile synthesised?

A
  • Bile is constantly synthesised by hepatocytes lining sinusoidal blood vessels in the liver acinus
  • Bile drains into the blind ended canaliculi and into the bile duct for storage in the gall bladder or direct drainage into duodenum
34
Q

What are hepatocytes?

A

•Hepatocytes are the key functional cell of the liver forming 80% of the liver mass

35
Q

Where is bile stored?

A

Bile is stored in the gall bladder

36
Q

What effect does secretin have on bile?

A

Liver duct epithelial cells add water, Na+, HCO3− to increase bile volume in response to hormone secretin

37
Q

How does the gallbladder concentrate bile?

A

Water and electrolytes (Na+, Cl-, HCO3-) are reabsorbed across the gall bladder mucosa to concentrate bile salts, bilirubin and cholesterol

38
Q

How is unconjugated bilirubin formed?

A

•Haem from old/faulty RBC converted to bilirubin (orange) and oxidised form biliverdin (green) (spleen, liver kupffer cells), transported to liver bound to albumin in unconjugated form

39
Q

How is bilirubin excreted?

A
  • Conjugated (made hydrophilic) with glucuronic acid to bilirubin diglucuronide by hepatocytes, excreted in bile
  • Gut bacterial hydrolysis (b glucuronidase) deconjugates bilirubin to form urobilinogen
  • urobilinogen reduced to stercobilin, excreted in faeces (brown colour)
40
Q

How is bilirubin reabsorbed?

A

•Enterohepatic reabsorption of urobilinogen, most re-secreted in bile (small amount excreted in urine)

41
Q

Complete the diagram on bilirubin

A
42
Q

What causes jaundice?

A
  • The build up of bilirubin (serum bilirubin >30-60 mmol.L-1) (yellow discoloration of skin in severe jaundice)
  • May occur when underlying disease processes disrupt the production and excretion bilirubin
43
Q

What are the 3 types of jaundice?

A
  • Pre-hepatic– excessive RBC breakdown, build up of unconjugated bilirubin due to overload of processing mechanisms eg haemolytic anaemia
  • Hepatocellular/congenital –altered hepatocyte function eg Crigler-Najjar syndrome (inborn error of metabolism - absence of hepatocyte bilirubin conjugating enzyme glucuronyl transferase results in increased unconjugated bilirubin)
  • Post-hepatic– obstruction to normal bile drainage, build up of conjugated bilirubin eg gallstone obstruction of bile flow
44
Q

How is bile secretion regulated?

A
  • CCK released in response to fat content of duodenum
  • Gall bladder contraction
  • sphincter of hepatopancreatic ampulla (Sphincter of Oddi) relaxation
  • Secretin released in response to acidic chyme
  • Liver ductal secretion of HCO3− , H20
  • Minor role for vagal and enteric ACh stimulation
  • Bile flow, gall bladder contraction
45
Q

What is enterohepatic circulation?

A

•bile salts secreted by hepatocytes into bile and continuously recycled through active reabsorption from the ileum and then re-secretion into bile

46
Q

•94% bile salts return via _______ to drive _________

A

•94% bile salts return via portal vein to drive bile synthesis in the liver

47
Q

What is the effect of enterohepatic circulation on drugs?

A

•Many hydrophobic drugs (e.g., acetaminophen) are deactivated by the liver and excreted into bile; enterohepatic recycling frequently occurs, slowing the rate of drug elimination

48
Q

Name an asymptomatic and symptomatic gallbladder disease

A

•Occurs in several forms, ranging from asymptomatic cholelithiasis (gallstones) to biliary colic (blockage of the cystic duct) affecting different areas of the biliary tract

49
Q

What are gallstones caused by?

A
  • Excessive water and bile salt reabsorption from bile
  • Excessive cholesterol in bile causing precipitation (high fat diet)
  • Inflammation of epithelium (low grade chronic infection)

104 Theme 1 - The Alimentary System (17 decks)

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