Secretions of the Small Intestine, Liver, Gall Bladder and Pancreas Flashcards
The small intestine
GIP is
Gastric Inhibitory peptide/ glucose dependent insulinotrophic peptide
The small intestine governs
majority of chemical digestion,, absorption of nutrients, electrolytes and water
Primary Secretions of the small intestine are:
- intestinal juice (mucous/HCO3-)
- pancreatic juice (digestive enzymes)
- bile (bile salts)
Key endocrine hormones in the small intestine:
- secretin
- cholecystokinin (CCK)
- glucose dependent insulinotrophic
peptide (GIP) - regulates bile and pancreatic
secretion
Secretory cells of the small intestine
Secretory cells of the small intestine:
- Villi: houses absorptive enterocytes
and mucus secreting goblet cells - Intestinal Glands:
- Enterocytes (isotonic fluid)
- Enteroendocrine cells
(hormones) - Paneth cells (lysozymes)
- In the duodenum only:
- brunner’s glands (only in
duodenum) secrete alkaline
mucus and HCO3-
Stem cell renewal of epithelial cells:
insert diagram
- rapid regeneration rate (3-6 days)
- rapid regeneration rate makes the
cells vulnerable to radiation and
chemotherapy
CCK (I cells) function
stimulate pancreatic and gallbladder secretion
Secretin (S cells) function
stimulate pancreatic and biliary bicarbonate secretion
GIP (K cells) function
may inhibit acid secretion/ stimulate insulin release
Exocrine pancreatic juice consists of
bicarbonate and digestive enzymes
Pancreas
Pancreas only has endocrine function.
True or False?
False
Endocrine and Exocrine
Pancreas is regulated by both hormonal and neural mechanisms via
subdivision of cranial nerve X (vagus)
Exocrine pancreas secretes
pancreatic juice containing bicarbonate rich secretion (pH8) and digestive enzymes essential for normal digestion and absorption
The pancreas consists of
glandular epithelial clusters
99% of pancreatic structure is
exocrine acinar clusters secreting pancreatic juice
Pancreatic Juice consists of
- water
- electrolytes
- sodium bicarb
- pro-enzymes
1% pancreatic structure consists of
endocrine pancreatic islets
(Islets of langerhans) of 4 types of secreting cells: alpha, beta, delta, F cells
Alpha cells in the Islets of Langerhans secrete
glucagon
Beta cells in the Islets of Langerhans secrete
insulin
Delta cells in the Islets of Langerhans secrete
somatostatin
F cells in the Islets of Langerhans secrete
pancreatic polypeptide (PP cells)
Exocrine Acinar clusters
Regulation of exocrine pancreatic secretion
Acinar enzyme production:
- Ach released via parasympathetic
CNX (vagus) stimulation of CCK -
trigger is chyme containing fat and
protein products - produces lower volume of enzyme
rich pancreatic juice
Ductal Bicarb and Water:
- Secretin - trigger is H+ in highly
acidic chyme - produces plentiful, HCO3- rich, low
enzyme pancreatic juice
Pancreatic Proteolytic Enzymes are secreted in —— form:
- active
- inactive
inactive
Pancreatic Proteolytic enzymes are responsible for
the conversion of proteins to peptides
Amylase hydrolyses
starch, glycogen and other carbs to form di/trisaccharides
Lipases hydrolyse
fat into fatty acids and monoglycerides
Nucleases digest
RNA and DNA to nucleic acids
Trypsin inhibitor prevents
activation of trypsin to prevent pancreatic digestion
Activation of Proteolytic Enzymes: how are they produced?
inactive precursors called zymogens
Activation of Proteolytic Enzymes: how is active trypsin formed?
small intestinal brush border enterokinase cleaves hexapeptide to form active trypsin from trypsinogen
Activation of proteolytic enzymes: active trypsin purpose:
trypsin cleaves and activates other proteolytic enzymes
What prevents autodigestion?
- trypsin inhibitor
- process of pancreatic enzymes
destroying its own tissues, resulting
in inflammatory response
Duct Secretion of Sodium Bicarbonate
Duct secretion of sodium bicarbonate:
- secretin stimulates a high volume
of HCO3- rich pancreatic juice - HCO3- secretion out of cell into the
duct lumen is via CL-/HCO3-
exchange at the apical cell
membrane - 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
Regulation of Pancreatic Juice Secretion:
- Ach: vago-vagal gastro-pancreatic
reflex, gastrin, secretin,
CKK/pancreozymin
Cystic Fibrosis and the Pancreas
- lack a functional Cl- channel in the
luminal membrane, which results in
defective ductal fluid secretion
(CFTRE Cl-) - the ducts become blocked with
precipitated ezymes and mucus
and the pancreas undergoes
fibrosis - blocked ducts impair secretion of
needed pancreatic enzymes for
digestion of nutrients, resulting in
malabsorption
Treatment of cystic fibrosis pancreas effects
- treatment of malabsorption
includes oral pancreatic enzyme
supplements taken with each meal
Pancreatitis:
- is
- common cause
- inflammatory disease where
pancreatic enzymes are activated
within the pancreas and
surrounding tissues, resulting in
autodigestion - the most common causes:
gallstones, alcohol misuse, which
obstruct the pancreatic duct
The role of bile in digestion
- bile salts emulsify fats for digestion
by pancreatic lipase, solubilise fat
digestion products into
aggregations for absorption across
the mucosa - elimination of waste products
- bile pigment bilirubin from haem in
RBC degradation (breakdown
product stercobilin gives faecal
brown) - cholesterol
- drugs
Synthesis and Secretion of Bile in the Liver
- 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 - it drains into the blind ended
canaliculi and into the L/R hepatic
bile ducts of the liver
Bile is stored in the gall bladder
Bile Secretion Regulation
- CCK released in response to fat
content of duodenum leads to:- gall bladder contraction
- sphincter of hepatopancreatic
ampulla - sphincter of oddi relaxation
- secretin release in respons to acidic
chyme - liver ductal secretion of HCO3-.
H2O - minor role for vagal stimulation:
bile flow, gall bladder contraction
Enterohepatic circulation of bile salts
- bile salts are continuously recycled through active reabsorption within the ileum
- 94% bile salts return via the portal vein to drive bile synthesis in the liver
- many hydrophobic drugs are
deactivated by the liver and
excreted into bile; enterohepatic
recycling frequently occurs, slowing
the rate of drug elimination
Enterohepatic circulation of bile salts
Gall Bladder Disease:
Occurs in several forms, ranging from asymptomatic cholelithiasis [gallstones] to biliary colic [blockage of the cystic duct]. These disorder can affect different areas of the biliary tract
Gallstones common causes:
- Excessive water and bile salt
reabsorption from bile
[supersaturation ofbile] - Excessive cholesterol in bile
causing precipitation [high fat diet]. - Inflammation of epithelium [low
grade chronic infection].
Gall Bladder Disease
