secretions of the small intestine Flashcards
1
Q
what does the small intestine do?
A
- governs the majority of chemical digestion and absorption of nutrients, electrolytes and water
2
Q
what are the primary secretions of the small intestine? 3
A
- intestinal juice (mucus/ HCO3-)
- pancreatic juice (digestive enzymes)
- bile (contains bile salts)
3
Q
what are the key endocrine hormones of the small intestine (3) and what do they do?
A
- secretin
- cholecystokinin
- glucose dependent insulinotropic peptide
- regulate bile and pancreatic secretions
4
Q
describe the secretory cells of the small intestine? 4
A
- villi= absorptive enterocytes and mucus secreting goblet cells
- intestinal glands= enterocytes secreting isotonic fluid, entero-endocrine cells, panted cells
- in the duodenum only= Brunner’s glands secrete music and HCO3-
- paneth cell in the submucosa= secretes lysozyme and is capable of phagocytosis
5
Q
describe the stems cell renewal of epithelial cells? 2
A
- rapid turnover of epithelial cells every 3-6 days
- vulnerable to radiation, chemotherapy
6
Q
what is intestinal juice? 6
A
- fluid containing
- electrolytes
- water (secretory enterocytes)
- lysozyme (Paneth cells)
- mucus (goblet cells)
- alkaline mucus containing fluid (submucosal duodenal Brunner’s glands)
7
Q
describe the endocrine hormone secretion into the vasculature of the small intestine? 3
A
- 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
8
Q
what does the pancreas do?
A
- exocrine pancreas secretes pancreatic juice containing bicarbonate rich secretion (pH8) and digestive enzymes essential for digestion and absorption
9
Q
what is bile? 3
A
- has bile salts for lipid emulsification
- synthesised by liver hepatocytes
- stored in the gallbladder
10
Q
what is the structure of the pancreas? 3
A
- consists of glandular epithelial cells
- 99% are exocrine acinar clusters secreting pancreatic juice (water, electrolytes, sodium bicarbonate and pro-enzymes)
- 1% are endocrine pancreatic islets (islets of langerhans) of 4 types secreting glucagon (alpha), insulin (beta), somatostatin (delta) and pancreatic polypeptide (F cell)
11
Q
how is the acinar enzyme production up regulated? 3
A
- acetylcholine is released via parasympathetic vagus stimulation
- CCK- trigger is chyme containing fat and protein products
- produces a lower volume, enzyme rich pancreatic juice
12
Q
how are ductal bicarbonate and water up regulated? 2
A
- secretin trigger is H+ in highly acidic chyme
- produces copious HCO3- rich low enzyme pancreatic juice
13
Q
describe how proteolytic enzymes are secreted?
what do they do?
A
- in inactive form
- convert proteins to peptides
14
Q
what does amylase do?
A
hydrolyse starch, glycogen and other carbohydrates other than cellulose to form disaccharides and trisaccharides
15
Q
what do lipases do?
A
- hydrolyse fat into fatty acids and monoglycerides
16
Q
what do nucleases do?
A
- digest RNA and DNA to nucleic acids
17
Q
what does the trypsin inhibitor do?
A
- prevents activation of trypsin to prevent pancreatic digestion
18
Q
how are proteolytic enzymes activated? 4
A
- proteolytic enzymes are produced as inactive precursors called zymogens
- small intestinal brush border enterokinase enzymes cleaves hexapeptide to form active trypsin from trypsinogen
- trypsin cleaves and activates other proteolytic enzymes
- this process prevents pancreatic auto digestion (+ trypsin inhibitor)
19
Q
explain the duct secretion of sodium bicarbonate? 4
A
- secretin stimulates high volume HCO3- rich pancreatic juice
- HCO3- secretion out of the cell into the duct lumen is via Cl-/HCO3- exchange at the apical membrane
- Cl- is recycled out of the cell via the cystic fibrosis transmembrane conductance regulator (CTFR) 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
20
Q
what does ionic composition depend on?
A
- secretory rate
- unstimulated= low secretion rate- electrolyte composition 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
21
Q
how is pancreatic juice secretion regulated? 2
A
- neurotransmitter acetylcholine mediated vago-vagal gastro-pancreatic reflex
- hormone- gastrin, secretin, cholecystokinin (pancreozymin)
22
Q
explain dysfunction in the ductal CTFR Cl- channel? 4
A
- patients with CF lack a functional Cl- CTRF 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
- blocked ducts impair secretion of needed pancreatic enzymes for the digestion of nutrients, resulting in malabsorption
- treatment of this type of malabsorption includes oral pancreatic enzyme supplements taken with each meal
23
Q
explain dysfunction in the enzyme activation process in the small intestine? 3
A
- pancreatitis
- inflammatory disease where pancreatic enzymes are activated within the pancreas (and surrounding tissues), resulting in auto digestion of the tissues
- the most common causes of pancreatitis include gallstones and alcohol abuse where obstruction of the pancreatic duct occurs
24
Q
what is the role of bile in digestion? 4
A
- required for digestion and absorption of fats from the small intestine
- bile salts (amphipathic with hydrophobic and hydrophilic regions) emulsify fats for digestion by pancreatic lipase, solubilise fat and digestion products into micelles for absorption across the mucosa
- elimination of waste products
- bile pigment bilirubin from heme in red blood cell degradation (the breakdown produce stercolbilin gives the brown colour in faeces)
25
Q
describe synthesis and secretion of bile in the liver? 3
A
- bile is constantly synthesised by hepatocytes lining sinusoidal blood vessels in the liver acinus
- hepatocytes are the key functional cell of the liver forming 80% of the liver mass
- bile drains into the blind ended canalculi and into then bile duct for storage in the gallbladder or direct drainage into the duodenum
26
Q
what does the gallbladder do to the bile?
A
- concentrates it
- water and electrolytes are reabsorbed across the gall bladder mucosa
- this concentrates the bile salts, bilirubin and cholesterol
27
Q
describe the excretion of bile pigment bilirubin in bile? 6
A
- haem from old faulty RBC is converted to bilirubin (orange) and is oxidised to biliverdin (green)
- this is then transported to the liver, bound to albumin in conjugated form
- bilirubin is conjugated with glucuronic acid to bilirubin diglucuronide by hepatocytes, excreted in bile
- gut bacterial hydrolysis deconjugates bilirubin to form urobilirubin
- urobilirubin is reduced to strecobilin which is secreted in faeces (brown colour)
- enterohepatic reabsorption of urobilinogen, most secreted in the bile, small amount secreted in the urine
28
Q
what is jaundice? 6
A
- the build-up of bilirubin in the blood
- in serious cases can lead to yellow discolouration of the skin
- may occur when underlying disease processes disrupt the production and excretion of bilirubin
- pre-hepatic jaundice= excessive RBC breakdown, build-up of unconjugated bilirubin due to overload of processing mechanisms (haemolytic anaemia)
- hepatocellular/ congenital jaundice= altered hepatocyte function (crigler-najjar syndrome, inborn error of metabolism, absence of hepatocyte bilirubin conjugating enzyme glucuronyl transferase results in increased unconjugated bilirubin)
- post-hepatic jaundice= obstruction to normal bile drainage, build up of conjugated bilirubin (gallstone obstruction of bile flow)
29
Q
describe the regulation of bile secretion? 5
A
- CCK released in response to fat content of duodenum
- gall bladder concentrates bile
- sphincter of hepatopancreatic ampulla (sphincter of oddi) relaxation
- secretin released in response to acidic chyme= liver ductal secretion of HCO3- and water
- minor role for vagal and enteric Act stimulation= bile flow, gall bladder concentration
30
Q
describe the enterohepatic circulation of bile salts? 3
A
- enterohepatic circulation= bile salts secreted by hepatocytes into bile and continuously recycled through active reabsorption from the ileum and the re-secretion into the bile
- 94% of bile salts return via the portal vein to drive bile synthesis int he liver
- many hydrophobic drugs are deactivated by the liver and excreted into the bile. enterohepatic recycling frequently occurs, slowing the rate of drug elimination
31
Q
describe gall bladder 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
32
Q
how are gallstones commonly caused? 3
A
- excessive water and bile salt reabsorption from bile
- excessive cholesterol in bile causing precipitation (high fat diet)
- inflammation of the epithelium (low grade chronic inflammation)
