Secretions Of The GI Tract Flashcards
What are the functions of the salivary glands?
Initial digestion of starches and lipids
Dilution and buffering of ingested food
Lubrication of ingested food with mucus
Describe the parotid gland
Composed of serous cells
Secrete fluids composed of water, ions and enzymes (rich in amylase which digests starches)
Secretes 25% of the daily output of saliva
Describe sub axillary and sublingual glands
Mixed glands composed of both serous and mucous cells
Secrete aqueous fluid and mucin glycoprotein for lubrication
Secrete the remainder 75% of daily output saliva
The acinar cells located in an acinus of a salivary gland secrete what?
The initial saliva
What are myoepithelial cells?
Located in salivary glands
Have motile extensions
When stimulated by neural input they contract to eject saliva into the mouth
Describe the intercalated duct of a salivary gland
Saliva in the intercalated duct is similar in ionic composition to plasma
Describe the striated duct of a salivary gland
Lined by columnar epithelial cells (ductal cells)
Ductal cells modify the initial saliva to produce the final saliva (hypotonic)
Ductal cells alter the concentration of various electrolytes
What is saliva composed of?
H2O, electrolytes, alpha-amylase (sensitive to pH and is deactivated in the stomach), lingual lipase (begins lipids digestion), kallikrein (proteases and vasodilator that increases blood flow to the salivary gland and increase production of saliva) and mucous
Compared to plasma, saliva is what?
Hypotonic
Greater K+ and HCO3- concentrations
Lower Na and Cl concentrations
Describe the parasympathetic innervation of salivary glands
Presynaptic nerves originate at facial and glossopharyngeal nerves
Postsynaptic fibers in autonomic ganglia innervate individual glands
Describe the sympathetic innervation of salivary glands
Preganglionic nerves originate at the cervical ganglion whose postganglionic fibers extend to the glands in the periarterial spaces
What are two unusual features in the regulation of salivary secretion?
Exclusively under the control of the ANS
Salivary secretion is increased by both parasympathetic and sympathetic stimulation
What are the main components of gastric juice?
HCl, pepsinogen, mucosa and intrinsic factor (and H2O)
What is the role of HCl in gastric juice?
Necessary for the conversion of pepsinogen to pepsin
Kills a large number of bacteria that enter the stomach
What is the role of pepsinogen in gastric juice?
Inactive precursor
Gets cleaved into pepsin and works with HCl to initiate protein digestion
What is the role of mucus in gastric juice?
Lines the wall of the stomach and protects it from damage
Lubricant
Together with HCO3 it neutralizes acid and maintains the surface of the mucosa at neutral pH
What is the role of intrinsic factor in gastric juice?
Required for the absorption of vitamin B12 in the ileum
What is the role of water in gastric juice?
Medium for the action of HCl and enzymes
Solubilizes much of the ingested material
What are the two types of glands in gastric mucosa?
Oxyntic and pyloric
What are oxynitc glands?
Located in the proximal 80% of the stomach (body and fundus)
Secretes acid
Contains parietal cells, enterochromaffin like cells, chief cells and mucous cells
What are pyloric glands?
Located in the distal 20% of the stomach (Antrum)
Synthesizes and releases gastrin into the circulation
Contain D cells, G cells (secrete gastrin) and enterochromaffin cells
What do parietal cells secrete?
HCl and intrinsic factor
Located in the body
Contains lots of mitochondria (needed for energy to secrete HCl)
What do chief cells secrete?
Pepsinogen
Located in the body of the stomach
What do G cells secrete?
Gastrin into the circulation
Located in the antrum of the stomach
What do mucous cells secrete?
Mucous, HCO3 and pepsinogen
Located in antrum
The number of parietal cells determines what?
The maximal secretory rate
What is the function of the low gastric pH (1-2)?
To convert pepsinogen to pepsin
Where is HCl formed?
At the villus like membranes of the canaliculi of the parietal cell
Gastric juice can be seen as a mixture of what two separate secretions?
Parietal and non-parietal secretions
What are non-parietal secretions?
Basal alkaline secretion of constant and low volume
It’s primary constituents are Na and Cl; K is present at the same concentration as plasma
HCO3 is secreted at a concentration of ~30mEq/L
What are parietal secretions?
Slightly hyperosmotic
Contains 150-160 mEq/L of h and 10-20 mEq/L of K
Cl is the only anion present
As the secretion rate increases the concentrations of electrolytes begin to approach those of pure parietal cell secretion
Pepsinogen is only secreted during what conditions?
When the gastric pH is acidic enough to convert it to pepsin
What is the most important stimulus for pepsinogen secretion?
Vagus nerve stimulation
What triggers local cholinergic reflexes that stimulate chief cells to secrete pepsinogen?
H+ ions
Pepsin can convert what?
More pepsinogen into pepsin Proteolytic enzyme (splits interior peptide linkages)
What is the optimal pH for pepsin?
Between 1.8-3.5
Reversibly inactivated at > pH 3.5-5
Irreversibly inactivated >pH 7-8
What is intrinsic factor?
Mucoprotein secreted by parietal cells that is required for the absorption of vitamin B12 in the ileum
Binds to vitamin B12
Only secretion by the stomach that is required (essential)
Failure to secrete IF is associated with achlorhydria (no acid production) and with absence of parietal cells
Describe growth of the gastric mucosa
Gastric epithelium secretes HCO3 and mucus to form the gel like mucosal barrier
Mucous cells secrete mucous and surface epithelial cells secrete HCO3
The mucosal barrier protects the gastric mucosal epithelium against the HCl and pepsin
What factors protect the gastric mucosa?
HCO3, mucus, prostaglandins (e.g. Misoprostol), mucosal blood flow, gastrin, and growth factors
Which factors damage gastric mucosa?
Acid (H+), pepsin, NSAIDs (e.g. aspirin), H pylori, alcohol, bile, stress and smoking
What is pernicious anemia?
Results from the absence of intrinsic factor which leads to decreased absorption of vitamin B12
What are the common causes of pernicious anemia?
Atrophic gastritis (chronic inflammation of the stomach mucosa that leads to loss of parietal cells) Autoimmune metaplastic atrophic gastritis (immune system attaches IF protein or gastric parietal cells)
What can disrupt the absorption of vitamin B12?
Gastrectomy (loss of parietal cells which is the source of IF) Gastric bypass (exclusion of the stomach, duodenum, and proximal jejunum alters absorption of vitamin B12
What does pancreatic juice contain?
HCO3- for the neutralization of H from the stomach and enzyme secretions to digest carbs, proteins and lipids into absorbable molecules
What is the sympathetic innervation of the exocrine pancreas?
Postganglionic nerves from the celiac and superior mesenteric plexus
Inhibits pancreatic secretion
Describe the parasympathetic innervation of the exocrine pancreas
Vagus nerve
Preganglionic fibers synapse in the ENS
Postganglionic fibers synapse on the exocrine pancreas
Stimulates pancreatic secretion
Describe the organization of the exocrine pancreas
Organized like salivary glands
Acinus contains acinar cells that synthesize and secrete major enzymes for digestion
Ducts are lined by ductal epithelial cells; extends to the region in the acinus containing centroacinar cells
