28 Secretory Functions of the GI Tract Flashcards
what are the acinar cells?
produce primary secretion which is isotonic to plasma and includes most of the protein
what are the ductal cells?
smaller and modify saliva…results in a hypotonic, alkaline solution
Absorbs NaCl
Excretes K+ and HCO3
what is the regulation of saliva? what nerves are important?
almost exclusively neural with parasympathetic predominating; CN 7 and 9 are important for saliva excretion
What are the functions of saliva?
- Lubrication for speech and swallowing
- Neutralization of acid
- Initiation of digestion
- Antimicrobial
- Tooth homeostasis
- Enhances taste
what are the functions of gastric secretion?
- Fluid: Lubricating and liquefying chyme
- Acid: Sterilization and activation of pepsin
- Mucous and bicarbonate: Protection
- Intrinsic factor: Vitamin B12 absorption
- Pepsinogen: protein digestion
what are the parietal cells?
located in the body of the stomach and secretes HCl and intrinsic factor
describe the chief cells?
located in the body of the stomach and stimulated by ACh and gastrin, they release pepsinogen which begins protein digestion (~20% will occur in stomach)
also secretes gastric lipase which does minimal fat digestion
what is pepsinogen activated by?
Pepsinogen is activated by pepsin and H+
Deactivated in the duodenum by high pH
what are the ECL cells?
Influence parietal cells through paracrine secretion by releasing histamine
what are the G cells?
Located in antrum of the stomach
Release gastrin to come back in hormonal pathway back to parietal cells to increase release of HCl
Low pH in stomach inhibits G cells
where are parietal cells stimulated? what causes this stimulation?
basolateral membrane
ACh from vagus binds M3 receptor
Gastrin from bloodstream
Histamine
how do somatostatin and prostaglandins affect cAMP?what are the clinical correlations?
inhibit H+ secretions by decreasing cAMP
patients on NSAID drugs for an extended period of time run the risk of developing ulcers
what is the intracellular mechanism of parietal cell stimulation?
- ACh and Gastrin increase calcium levels
- Histamine increases cAMP
- Increased Ca and cAMP lead to tubulovesicular fusion
what is the effect of parietal cells secreting intrinsic factor?
which binds to vitamin B12 and is absorbed in the terminal ileum
IF secretion is the only VITAL FUNCTION performed by the stomach
what is the Pharmacological inhibition of acid secretion?
- Omeprazole irreversibly inhibits the proton pump
- Atropine blocks ACh release
- Cimetidine blocks histamine release
Discuss the role of the enzymes released once chyme enters the duodenum?
Acidic chyme comes in and distension of duodenum triggers release of pancreatic juices and bile
Exocrine functions of pancreas release enzymes for digestion
Combined bile and pancreatic juices are released through sphincter of Oddi
Increased amylase or lipase in blood may indicate pancreatic issue (such as pancreatitis)
what are the release enzymes for digestion? describe them
acinar and ductal cells (just like saliva)
Isotonic and alkalinic (due to high bicarbonate levels)
Secretin increases secretions of pancreas
-CCK (released by duodenum) potentiates secretin release
Increased amylase or lipase in blood may indicate?
pancreatic issue like pancreatitis
Acidic chyme comes in and distension of duodenum triggers release of?
pancreatic juices and bile
what is saliva characterized by?
high volume high K+ and HCO3- concentrations low Na+ and Cl- concentrations hypotonicity presence of the alpha amylas, lingual lipase, kallikrein
what stimulates saliva?
both para(major- increases transport processes, muscurinic receptors) and sympatheti(B-adrenergic)c nervous systems
what three major glands produced saliva?
the parotid, submandibular, and sublingual glands
this produces the initial saliva with composition similar to plasma?
the acinus, it also isotonic and has the same NA, K, Cl, and HCO3 concentrations as the plasma
what increases saliva production?
smells, conditioned reflexes, and nausea
what do parietal cells secrete?
HCI and intrinsic factor
what do chief cells secrete?
secrete pepsinogen
what do G cells secrete?
secretes gastrin
what are the three gastric cell types?
parietal cells, chief cells, G cells
H+ is secreted into the lumen of the stomach by H+-K+ pump, what drug inhibits this?
omeprazole inhibits this pump
in terms of gastric H+ secretion, what happens when vomiting occurs?
gastric H+ never arrives in the small intestine, there is no stimulus for pancreatic HCO3- secretion and the arterial blood becomes alkaline (metabolic alkalosis)
what stimulates gastric H+ secretion?
vagal stimulation
gastrin
histamine
potentiating the effects of the ACh, histamine, and gastrin in H+ secretion
in terms of gastric H+ secretion and vagal stimulation, what are the two pathways? What can inhibit this?
direct and indirect where the direct pathway uses the vagus nerve to innervate the parietal cells via NT ACh utilizing muscurinic recpetors
the indirect pathway uses the vagus nerve to innervate the G cells and stimulate gastrin secretion. The NT for this pathway is GRP
Atropine, a cholinergic muscarinic antagonist, inhibiting H+ secretion blocking the direct pathway using ACh as the NT. It does not inhibit the indirect pathway. Vagotomy can eliminate both pathways
H+ secretion can be potentiated using these two routes?
Histamine potentiates the actions of ACh and gastrin
H2 receptor blockers like cimetidine can block this for treating ulcers
ACh potentiates the action of histamine and gastrin
this can be blocked by atropine
what are peptic ulcers? the types
ulcerative lesion of the gastric or duodenal mucosa
gastic ulcers
duodenal ulcers
zollinger ellison syndrome
what is the effect of gastric ulcers?
H+ secretion is decreased because secreted H+ leaks back through the damaged gastric mucosa, gastrin levels are increased, H pylori is major cause which contain urease, 13C-ureas can be used a diagnostic measure
causes damage to protective barrier of gastric mucosa
what is the effect of the duodenal ulcers?
H+ secretion is increased and gastrin secretion in response to meal is increased and so H pylori is also a major cause inhibiting somatostatin secretion
increased parietal cell mass due to increased gastrin levels
this occurs due to a gastrin secreting tumor of the pancreas?
Zollinger-Ellison syndrome
what are the three major drugs that inhibit H+ secretion?
atropine - blocks H+ secretion inhibiting cholinergic receptors
cimetidine - blocks H2 receptors
omeprazole - proton pump inhibitor, inhibiting H+, K+ ATPase and H+ secretion
what are the functions of secretion?
Facilitate digestion and absorption
Protection
Excretion
what are the sources of secretion in the GI tract?
Accessory organs (salivary glands, liver and pancreas)
Cells lining the lumen of the GI tract
Glands in GI tract wall
secretory products include?
Fluid and electrolytes
Digestive Enzymes
Mucus
Protective molecules
in terms of the salivation and its mechanism, these cells modify saliva and this is the result?
ductal cells modify saliva into a hypotonic alkaline solution
what are the parasympathetic fibers that regulate salivary secretion? what about for the sympathetic NS?
cranial nerves 7 and 9 via NT ACh and through the muscarinic receptor there is upregulation of IP3/Ca2+ increasing the secretion
the sympathetic fibers travel with blood vessels, norepinephrine stimulates the glands via B-adrenergic receptor and there is upregulation of cAMP which increases secretion
what is the function of gastric secretion?
Fluid: Lubricating and liquefying chyme Acid: Sterilization and activation of pepsin Mucous and bicarbonate: Protection Intrinsic factor: Vitamin B12 absorption Pepsinogen: protein digestion
what secretes intrinsic factor? what does it bind to in the stomach? where is it then absorbed and what is the vital function performed by the stomach?
parietal cells; Vitamin B12; B12 is absorbed in the terminal ileum and intrinsic factor secretion is the only vital function performed by the stomach
pepsinogen is secreted by the ____cells?what stimulates this?
chief cells; ACh and gastrin
main pancreatic duct and bile duct join and enter the _____?
duodenum
how much does the pancreas weigh and the 2 components?
100 g, exocrine and endocrine components
what is the function of the exocrine pancreas?
Arranged similar to salivary glands with acini and ducts
Acinar cells secrete enzymes for digestion
Ductal cells secrete fluid and bicarbonate to neutralize H+
what is the function of the endocrine pancreas?
Secretes multiple hormones in to the bloodstream including insulin, glucagon and somatostatin
what cofactor is necessary for lipase function in pancreatic acinar secretion?
Colipase
what is Trypsin Inhibitor in pancreatic acinar secretion?
prevents early activation of trypsin in pancreas acinar cells and ducts
what is the major stimulus for acinar cell secretion in pancreatic acinar secretion?
CCK which is potentiated by ACh
what is the acinar of the pancreas?
the functional unit of the exocrine pancreas. It synthesizes, stores, and secretes digestive enzymes.
what could elevated serum amylase and lipase be indicative of?
pancreatitis
in relation to ductal cell secretion, cystic fibrosis patients can also get this?
pancreatitis
what are ductal cells? why is high flow important? what is important to note about the aqueous portion?
secrete the high volume aqueous portion of pancreatic juice.
important for flushing enzymes to the duodenum
Aqueous portion is isotonic and alkaline. HCO3- important for neutralizing H+ in duodenum for effective enzyme function.
what stimulates ductal cell secretion? what is the result?
secretin; : Net secretion NaHCO3 and H2O
in terms of pancreatic regulation, what are the hormones to note? why?
CCK - released from I cells and cause increase in acinar cell secretion
ACh - from vagus nerve and potentiates acinar and ductal cell secretion
Secretin - from S cells in the duodenum in response to low pH in duodenum and this increases ductal cell secretion
what are the functions of Biliary secretion?
digestion and excretory
what is the digestive function of biliary secretion?
Emulsifies fat
Forms micelles
Aids in fat absorption
Bile acids are key molecules for this function
what is the excretory function of biliary secretion?
Bilirubin
Cholesterol
Xenobiotics
what are the components of the biliary system?
Intrahepatic
Extrahepatic
what is the intrahepatic component of the biliary system?
Hepatocytes and intrahepatic ducts
what is the extra hepatic component of the biliary system?
Hepatic ducts, gallbladder, cystic duct and bile ducts
what regulates the bile flow in to the duodenum?
The sphincter of Oddi
what are bile acids synthesized from? what is the rate limiting step? where are primary bile acids synthesized?
cholesterol;7α-hydroxylase; in the liver
T/F, Bile acids are amphipathic molecules?
T
primary bile acids modified by intestinal bacteria become this?
Secondary bile acids
what are bile salts conjugated with?
glycine and taurine in the liver to make them more polar and lowers pKa
major stimulus for gallbladder contraction and sphincter of Oddi relaxation?
CCK
bile salts are used to aid in _____?
fat digestion
what percent of bile acids plus excretory molecules of bile enter the colon?
10%
what stimulates water and HCO3- secretion, bile is alkaline into intrahepatic ducts?
secretin
Gallbladder stores and concentrates bile by absorbing?
sodium
Most components of bile are actively secreted by?
hepatocytes
how are bile salts returned to liver?
portal circulation
where are the bile salts actively absorbed?
at the terminal ileum with sodium
what is the function of the intestinal secretion?
To continually provide fluid medium for digestion and absorption to occur and to lubricate passage of the bolus.
