2. Secretions of GI Tract and Pancreas Flashcards
What are the key functions of saliva
- Initial digestion of starches and lipids
- Dilution and buffering of ingested food
- Lubrication of ingested food with mucus
What is the serous salivary gland and what is its function
Parotid Gland
- Secrete fluids composed of water, ion and enzymes (rich in AMYLASE)
- 25% of daily saliva
What are the mixed salivary glands and what are their functions
Submandibular and Sublingual
- Secrete aqueous fluid and mucin glycoprotein for lubrication
- Secrete MOST of the saliva (75%)
What cells secrete the initial saliva and saliva with similar composition to plasma
Initial- acinus
same composition as plasma- intercalated duct
What cells modify the initial saliva to produce the final saliva
In the striated duct, Ductal cells modify the intial salica into the final HYPOTONIC saliva
(Ductal cells are H2O impermeable)
How does saliva compare to plasma
Hypotonic
- Increased potassium and bicarb
- Decreased sodium and chloride
What are the two main steps in the formation of saliva
- Formation of isotonic plasma-like solution by acinar cells
- Modification of the isotonic solution by the ductal cells
What is the net movement of molecules for salivary secretion
Net absorption of solute with NaCl being absorbed and secretion of K and HCO3
In salivary secretion how does bicarb leave the cell
HCO3 leaves the cell to the lumen via cAMP activated CFTR Cl- channel or via the Cl-/HCO3- exchanger
What are the innervations of the salivary glands
Parasympathetic- facial and glossopharyngeal
Sympathetic- T1 to T3 via superior cervical ganglion
What is the role of ADH/Aldosterone in saliva compositon
They modify the composition of saliva by decreasing its Na+ concentration and increasing its K+ concentration
What are the two unusual features of salivary secretion regulation
- Salivary is EXCLUSIVELY under the control of the ANS
- Salivary secretion is INCREASED by BOTH parasympathetic and sympathetic stimulation
The gastric mucosa is divided into the oxyntic gland area and pyloric gland area. Describe the oxyntic gland
- Located in the proximal 80% of the stomach (body and fundus)
- Secretes acid
The gastric mucosa is divided into the oxyntic gland area and pyloric gland area. Describe the pyloric gland
- Located in the distal 20% of the stomach (antrum)
- Synthesizes and releases gastrin
What is the major function of parietal cells
SECRETION of HCl and absorption of HCO3-
(catalyzed by carbonic anhydrase)
Low pH is required to convert pepsinogen to pepsin
What does Omeprazole do
- Inhibits the H+/K+ ATPase
- Used in the treatment of ulcers to reduce H+ secretion
What is important about the parietal component of gastric secretion
As the secretion rate increase, the concentrations of electrolytes begin to approach those of pure parietal cell secretion
What agents stimulate H+ secretion by gastric parietal cells
- Acetylcholine
- Gastrin
- Histamine
What agents inhibit H+ secretion by gastric parietal cells
- Somatostatin (acts on G cells to inhibit gastrin release)
- Prostaglandins
Does atropine work on the direct or indirect pathway of vagal stimulation of HCl secretion by parietal cells
Direct- atropine blocks the direct pathway of vagal stimulation
Indirect- atropine will not block the vagal effects on gastrin secretion because the neurotransmitter at the synapse on G cells is GRP
What does Cimetidine do
Antagonist of H2 receptors used to treat duodenal and gastric ulcers, GERD
What are the three phases of Gastric HCl secretion
- Cephalic phase via vagus
- Gastric phase
- Intestinal phase
Describe the cephalic phase of Gastric HCl secretion and how to abolish this phase
- Accounts for 30% of total HCl secreted in response to a meal
- Stimulated by smelling, tasting, chewing and swallowing
Vagotomy abolishes this phase
Describe the gastric phase of Gastric HCl secretion and how to abolish this phase
- Accounts for 60% of total HCl secreted in response to a meal
- Stimulated by distension of the stomach
(coffe stimulates gastric HCl secretion)
When is pepsinogen secreted
- ONLY when the gastric pH is acidic enough to convert it to pepsin
- Vagus nerve stimulation is the most important stimulus
How does pH change pepsin
Optimal = 1.8 - 3.5
Reversibly inactivated = 3.6 - 5.0
Irreversibly inactive = pH greater than 7
What is Intrinsic Factor required for and what secretes it
- Required for absorption of Vitamin B12 in the Ileum
- Secreted by parietal cells
What leads to pernicious anemia and what are its common causes
Failure to secrete IF leads to pernicious anemia
Common causes:
- Atrophic gastritis (loss of parietal cells)
- Autoimmune metaplastic atrophic gastritis (immune cells attack IF protein)
What procedures can disrupt the absorption of vitamin B12
Gastrectomy- loss of parietal cells
Gastric Bypass
What protects the gastric mucosa
- HCO3 and mucus
- Prostaglandins (misoprostol)
- Mucosal blood flow
- Gastrin and growth facotrs
What damages the gastric mucosa
- H+ and Pepsin
- NSAIDs
- Helicobacterpylori
- Alcohol and Smoking
- Bile and stress
What is Zollinger-Ellison Syndrome
- Large secretion of gastrin by gastrinomas
- When excessive H+ arrives to duodenum it overwhelms the buffer capacity of HCO3- in pancreatic juice creating an ulcer
What is used to diagnose a gastrin-secreting tumor
Secretin stimulation test
What are the predominant causes of Peptic Ulcer Disease and what are they the result of
Causes- H Pylori infection and NSAIDs
Result of- loss of protective mucosal barrier, excessive H+ and pepsin secretions
What allows H Pylori to colonize gastric mucosa
The Enzyme UREASE
When comparing disorders of gastric H+ secretion what is different between gastric ulcers, duodenal ulcers and Zollinger-Ellison syndrome
Gastric- decreased H+ secretion
Duodenal- increased H+ secretion
Zollinger- VERY INCREASED H+ secretion
All have increased gastrin levels
How is the exocrine pancreas innvervated differently from salivary glands
Parasympathetics stimulate still but sympathetics inhibit
What are the two components of pancreatic secretions
Active enzymes- amylases and lipases
Inactive enzymes- proteases
converted into active form in lumen of duodenum
What is the first organ to fail in Cystic Fibrosis
Pancreas
What are the key secretions from the three phases of pancreatic secretion
Cephalic- mainly enzymatic secretion
Gastric- mainly enzymatic secretion
Intestinal- 80% of secretion, enzymatic and aqueous secretions
