secretions of the small intestine

Question 1

what does the small intestine do?

Answer 1

• governs the majority of chemical digestion and absorption of nutrients, electrolytes and water

Question 2

what are the primary secretions of the small intestine? 3

Answer 2

• intestinal juice (mucus/ HCO3-)
• pancreatic juice (digestive enzymes)
• bile (contains bile salts)

Question 3

what are the key endocrine hormones of the small intestine (3) and what do they do?

Answer 3

• secretin
• cholecystokinin
• glucose dependent insulinotropic peptide
• regulate bile and pancreatic secretions

Question 4

describe the secretory cells of the small intestine? 4

Answer 4

• 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

Question 5

describe the stems cell renewal of epithelial cells? 2

Answer 5

• rapid turnover of epithelial cells every 3-6 days

- vulnerable to radiation, chemotherapy

Question 6

what is intestinal juice? 6

Answer 6

• fluid containing
• electrolytes
• water (secretory enterocytes)
• lysozyme (Paneth cells)
• mucus (goblet cells)
• alkaline mucus containing fluid (submucosal duodenal Brunner’s glands)

Question 7

describe the endocrine hormone secretion into the vasculature of the small intestine? 3

Answer 7

• 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

Question 8

what does the pancreas do?

Answer 8

• exocrine pancreas secretes pancreatic juice containing bicarbonate rich secretion (pH8) and digestive enzymes essential for digestion and absorption

Question 9

what is bile? 3

Answer 9

• has bile salts for lipid emulsification
• synthesised by liver hepatocytes
• stored in the gallbladder

Question 10

what is the structure of the pancreas? 3

Answer 10

• 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)

Question 11

how is the acinar enzyme production up regulated? 3

Answer 11

• acetylcholine is released via parasympathetic vagus stimulation
• CCK- trigger is chyme containing fat and protein products
• produces a lower volume, enzyme rich pancreatic juice

Question 12

how are ductal bicarbonate and water up regulated? 2

Answer 12

• secretin trigger is H+ in highly acidic chyme

- produces copious HCO3- rich low enzyme pancreatic juice

Question 13

describe how proteolytic enzymes are secreted?

what do they do?

Answer 13

• in inactive form

- convert proteins to peptides

Question 14

what does amylase do?

Answer 14

hydrolyse starch, glycogen and other carbohydrates other than cellulose to form disaccharides and trisaccharides

Question 15

what do lipases do?

Answer 15

• hydrolyse fat into fatty acids and monoglycerides

Question 16

what do nucleases do?

Answer 16

• digest RNA and DNA to nucleic acids

Question 17

what does the trypsin inhibitor do?

Answer 17

• prevents activation of trypsin to prevent pancreatic digestion

Question 18

how are proteolytic enzymes activated? 4

Answer 18

• 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)

Question 19

explain the duct secretion of sodium bicarbonate? 4

Answer 19

• 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

Question 20

what does ionic composition depend on?

Answer 20

• 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

Question 21

how is pancreatic juice secretion regulated? 2

Answer 21

• neurotransmitter acetylcholine mediated vago-vagal gastro-pancreatic reflex
• hormone- gastrin, secretin, cholecystokinin (pancreozymin)

Question 22

explain dysfunction in the ductal CTFR Cl- channel? 4

Answer 22

• 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

Question 23

explain dysfunction in the enzyme activation process in the small intestine? 3

Answer 23

• 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

Question 24

what is the role of bile in digestion? 4

Answer 24

• 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)

Question 25

describe synthesis and secretion of bile in the liver? 3

Answer 25

• 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

Question 26

what does the gallbladder do to the bile?

Answer 26

• concentrates it
• water and electrolytes are reabsorbed across the gall bladder mucosa
• this concentrates the bile salts, bilirubin and cholesterol

Question 27

describe the excretion of bile pigment bilirubin in bile? 6

Answer 27

• 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

Question 28

what is jaundice? 6

Answer 28

• 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)

Question 29

describe the regulation of bile secretion? 5

Answer 29

• 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

Question 30

describe the enterohepatic circulation of bile salts? 3

Answer 30

• 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

Question 31

describe gall bladder disease?

Answer 31

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

Question 32

how are gallstones commonly caused? 3

Answer 32

• 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)