Secretions of the gastrointestinal tract Flashcards
What is the function of saliva?
Initial digestion of starches and lipids
dilution and lubrication of ingested food
Describe parotid glands
Serous cells
Secrete water, ions, & enzymes
25% of daily output
Describe submaxmillary and sublingual glands
Serous and mucous cells
secrete aqueous fluid & mucin glycoprotein for lubrication
75% of daily output
What secretes inital saliva?
Acinar cells
How do myoepithelial cells secrete saliva?
When stimulated by neural input, contract to eject saliva into mouth
In what space is saliva similar in ionic composition to plasma?
In the intercalated duct
How do ductal cells modify inital saliva to produce final saliva?
Ductal cells alter the concentration of various electrolytes to make it hypotonic
Describe saliva in comparision to plasma in the intercalated duct and the striated duct
Intercalated duct-isotnic to plasma
Striated duct- Hypotonic
More K+ and HCO3-
Less Na+ and Cl-
How does HCO3- leave the cell to the lumen?
Via CAMP-activated CFTR Cl- channel or the Cl-/HCO3- exchanger
Where do parasympathetic nerves originate for salivary glands?
Facial and glossopharyngeal
Where do parasympathetic postsynaptic fibers originate?
In autonomic ganglia and innervate individual glands
Where do sympathetic preganglionic nerve originate?
Cervical ganglion
Where do sympathetic postganglionic fibers go?
Extend to the glands in the periarterial spaces
What stimulates the parasympathetic nervous system to secrete saliva?
Food
Nausea
Smell
What inhibits the parasympathetic nervous system to secrete saliva?
Dehydration
Fear
Sleep
Describe the parasympathetic pathway of salivary secretion
CN VII and CN IX –> Ach –> mAChR –> IP3 and increase in Ca2+ at acinar or ductal cells –> saliva
Describe the sympathetic pathway of salivary secretion
T1-T3 –> NE –> Beta-adrenergic receptor –> cAMP –> Saliva
How do ADH and aldosterone modify the composition of saliva?
Decreasing Na+ and increasing K+ concentration
What are the main components of gastric juice?
HCl (H+)
Pepsinogen
Mucus
Intrinsic Factor
H20
Where is the oxyntic gland and what does it do?
Proximal 80% of the stomach (body and fundus)
Secretes acid
Where is the pyloric gland and what does it do?
Distal 20% of the stomach (antrum)
Synthesizes and releases gastrin
What determines the maximal secretory rate of HCl?
The number of parietal cells
What is the function of the low gastric pH?
To convert pepsinogen to pepsin
Where is HCl formed?
In the body of the stomach by parietal cells
At the villus-like membranes of the canaliculi
Name the cells in the stomach, what they secrete and where they are located
Parietal cells –> IF and HCl (Body)
Chief cells –> pepsinogen (Body)
G cells –> gastrin (Antrum)
Mucus cells –> mucus, HCO3- and pepsinogen (Antrum)
What is secreted at the luminal side and what is absorbed at the basolateral side of a gastric parietal cell?
H+ is secreted (Cl- follows)
HCO3- is absorbed (Cl- goes into the cell)
Which receptors stimulate Gq (IP3/Ca2+)?
M3 Receptor and CCKB
Which receptors impact cAMP?
H2 Receptor –> Gs (stimulatory)
Somatostatin–>Gi (Inhibitory)
What does atropine do?
Blocks direct pathway of vagal stimulation but does not block vagal effects on gastrin secretion
How is gastrin release regulated?
Somatostatin acts on G cells to inhibit gastrin release
Vagal activation stimulates gastrin release by GRP and inhibiting somatostatin
Gastrin itself increases somatostatin
What do antagonists of H2 receptors do?
Block the direct action of histamine and block potentiated effects of ACh and gastrin
What do antagonists of mAChRs do?
block the direct effects of ACh and the ACh-potentiated effects of histamine and gastrin
What does Cimetidine do?
Blocks histamine from reaching H2 Receptor
Used to treat duodenal and gastric ulcers, GERD etc.
What does Omeprazole do?
Inhibits H+/K+ ATPase, used in the treatment of ulcers to reduce H+ secretion
Describe the cephalic phase
Vagus nerve –> parietal cell–> secretion of HCl
Vagus nerve –> GRP to G cells –> Gastrin –> Parietal cell–> HCl
Describe gastric phase
distension
Vagus nerve –> parietal cell–> secretion of HCl
Vagus nerve –> GRP to G cells –> Gastrin –> Parietal cell–> HCl
Describe Intestine phase
Distension of small intestine
Stimulates acid secretion
What stimulates pepsinogen secretion by chief cells and mucus cells?
Vagus nerve
requires H+ secretion from parietal cells to lower pH
What is the purpose of pepsin?
Degrades food proteins into peptides
Optimal pH is between 1.8-3.5
What is the purpose of intrisic factor?
It is required for the absoprtion of Vitamin B12 (cobalamin) in the ileum
Failure to secrete IF is associated with achlorhydria and absence of parietal cells
What does B12 originally bind to?
R-protein in the stomach
Switches to IF in the duodenum due to pancreatic proteases
What is the cause of pernicious anemia?
Stomach does not produce enough IF, low vitamin B12
What are common causes of pernicous anemia?
Atrophic gastritis
Autoimmune metaplastic atrophic gastritis
What can cause disruption in the absoprtion of vitamin B12?
Gastrectomy: loss of parietal cells
Gastric bypass: Exclusion of stomach, duodenum, and proximal jejunum
What are protective factors of the gastroduodenal mucosa?
HCO3-
Mucus
Prostaglandins
Mucosal blood flow
GF
What are damaging factors of the gastroduodenal mucosa?
H+ and pepsin
H. pylori
NSAIDS
Stress, smoking, and alcohol
What is zollinger-Ellison syndrome?
Large secretion of gastrin by duodenal or pancreatic tumors
increase in H+ secretion by parietal cells and increase in parietal cell mass
What are the effects of zollinger-ellison syndrome?
Inactivates pancreatic digestive enzymes
Interferes with the emulsifcation of fat
Damages intestinal epitheial cells and villi
Leads to maldigestion and malabsoprtion
How does the secretion stimulation test work?
Normal conditions: secretin inhibits gastrin release
Gastrinoma: secretin causes an increase in gastrin release
What are predominant causes of peptic ulcer disease?
H. Pylori and NSAIDS
What does H. Pylori do?
Releases cytotoxins that breadown the mucosal barrier
Cytotoxins coverts urea to ammonia (NH3) which alkalinzes the local environment
The resulting ammonium (NH4) causes mucosal damage
H. Pylori uses urease so you can test for urease to determine if H. Pylori is present
What is the main presentation of gastric ulcers?
Low H+ secretion
High gastrin levels
What is the main presentation of dudenal ulcers?
Increased H+ secretion
Increased gastrin levels
What is the sympathetic and parasympathetic innervation of the pancreas?
Sympathetic: Postganglionic nerves from celiac and superior mesenteric plexuses
Parasympathetic: Vagus nerve
- Preganglionic fibers synapse in the ENS
- Postganglionic fibers synapse on the exocrine pancreas
What do acinar cells in the pancreas secrete?
Amylases and lipases in active form
proteases in inactive forms which are converted into active form in the duodenum
What is secreted by centroacinar and ductal cells in the pancreas?
Secrete a HCO3- liquid via the Cl-/HCO3-
What channel has a mututation in cystic fibrosis?
Cystic Fibrosis Transmembrane conductance regulator (CFTR)
What induces release of pancreatic enzymes into the duodenal lumen?
CCK
What induces the secretion of HCO3- from pancreatic cells into the duodenum?
Secretin
