GI Physiology Flashcards
(35 cards)
Composition of Saliva
Hypotonic fluid
Tonicity and composition changes with flow rate
Difficult to have exact list of constituents
99.5% water
Function of Saliva
Lubrication and protection (barrier function) - mucins
Buffering action and clearance - bicarbonate
Maintenance of tooth integrity - Ca + Ph
Antibacterial activity - Immunoglobulins (IgA)
Taste and swallowing - libricating food bolus, dissolve molecules for test sensation
Digestion - Amylase begins to act on carbs
Stimulation and regulation of Saliva
90% of saliva flow is stimulated by chewing and eating.
Almost none when sleeping
Secretion regulated by autonomic nervous system.
Medullary nuclei
Stimulated by mechanical, gustatory and olfactory stimuli -> smell, taste and movement of food.
Both PNS and SNS innervation
Goblet cells
Secrete Mucous
Alkaline mucous
Lines surface of stomach and protects from HCL and pepsin causing autodigestion.
Function of Mucous in stomach
Gastroprotection
Lubrication
Bacteriocidal
Secreted by goblet cells
Chief Cells
Secrete Pepsinogen and gastric lipase
Pepsinogen is converted into pepsin in the low PH stomach environment
Involved in proteolysis
Parietal Cells
HCL + Intrinsic Factor
Located in body and fundus of stomach.
Pepsinogen
Activated to pepsin in presence of H+
Involved in proteolysis
Function of HCL in stomach
Secreted by parietal cells
Provides optimal pH for pepsin function -> Proteolysis
Acidic environment -> Bacteriocidal
Inhibits further gastrin secretion -> neg feedback
Stimulates secretion of bile and pancreatic juices
Facilitates iron absorption in duodenum
Enterochromaffin Cells
Secrete Histamine
Histamine stimulates parietal cells to secrete HCL
Function of Histamine in Stomach
Stimulates parietal cells to secrete HCL
G Cells
Secrete Gastrin
Gastrin:
- Stimulates parietal cells to secrete HCL (directly and indirectly)
- Increases GI motility
- Stimulates pancreatic secretion and gall bladder contraction
Gastrin function in stomach
Stimulates Parietal cells to secrete HCL indirectly and directly - via histamine
Increases GI motility
Stimulates pancreatic secretion and gall bladder contraction
D Cells
Secrete Somatostatin
Antagonise the stimulatory effect of histamine on acid secretion
Intrinsic Factor Function in stomach
Co factor - binds vitamin B12, protects B12 from acidic environmentt and facilitates B12 absorption in the terminal ileum.
Secretions of Glands in the cardia
Mucous mostly
Dominated by Goblet cells
Secretions of glads in the fundus
oxyntic
Main production is HCL
Secretions of glands in the pylorus
Mainly Gastrin
Function of Gastric Secretions
- Immune function
- pH decontaminates bacteria.
- pH maintained by HCL secretion.
- HCL secreted by Parietal cells, stimulated by Histamine (ECL cells) and Gastrin (g cells.) - Barrier function.
- Gastric mucous acts as a barrier and neutralising agent against gastric acid. Prevents auto digestion.
- Mucous secreted by Goblet Cells. - Macronutrient digestion.
- Gastric acid
- proteolytic enzymes begin digestion of protein. Pepsinogen is secreted by Chief cells, and is converted to pepsin by acidic environment. Pepsin = 15% total protein breakdown.
- Gastric Lipase: Hydrolyses the ester bonds of triglycerides. 30% of total lipid catabolism - Macronutrient digestion
- Pepsin: ferric iron (Fe3) to more soluble ferrous (FE2)
- Intrinsic factor: protects B12. Binds B12 creating a complex that can be absorbed in the terminal ileum. - Endocrine and paracrine
- Gastrin + somatostatin + histamine affect rate of gastric acid secretion.
Factors that Increase Gastric Acid Secretion
- PSNS Via vagus.
- Mainly during cephalic phase of digestion.
- Stimulated by the food bolus entering mouth.
- mediated by acetylcholine via the M3 receptors - Gastrin
- during cephalic and gastric phases.
- Released due to central signals + mechanical effects of food in the stomach.
- Mediated by binding to gastrin receptors on parietal cells.
- Also binds CCK-B receptors on enterochromaffin cells = histamine release. - Histamine
- during cephalic and gastric phases.
- released due to central signals as well as stimulated by gastrin.
- Binds to H2 receptors on parietal cells to mediate the release of acid. - Mechanical stretch
- vago vagal and local smooth muscles stretch reflexes. - Peptides, caffeine, alcohol
- Sensed by mucosal chemoreceptors of the astral G cells, which then release gastrin.
Factors that Decrease gastric acid secretion
- Somatostatin
- If luminal pH decreases below 1.5 - Small intestine hormones
- during the intestinal phase of digestion, composition of content entering duodenum is sensed by duodenal chemo and osmoreceptor.
- Release of inhibitory hormones. Delay gastric emptying.
- Carbs = release of GIP (gastric inhibitory peptide) = inhibit emptying.
- Protein degradation products = release CCK (cholesteokinin) = inhibit emptying
- Fat = Release CCK = Inhibit emptying.
- Rate (fastest - slowest): Carbs > Protein > fat.
- Increased H+ = secretin release = inhibit emptying.
- > Oslolarity = ?unknown mediator = decreased emptying.
- Prostaglandins PGE2 and PGI2
- Promote increased HCO3 and mucus secretion in gastric mucosa
- these are products of COX 1 activity, the loss of which = ulcers.
Function of HCL
Secreted by Gastric parietal cells
Aids in protein digestion
optimal pH for pepsin activity
Bactericidal
Facilitates Iron absorption
Parietal Cell structure
Apical secretory canaliculi which increase the surface area
Numerous vesicles containing proton pump proteins
With stimulation these vesicles fuse with the apical canaliculi to rapidly increase the secretory capacity of these cells.
Basal Membrane Ion transport in parietal cells
CO2 and water are able to diffuse into parietal cells passively
Carbonic Anhydrase converts the CO2 and H2O into HCO3 and H.
HCO3 is then exchanged for chloride at the basal membrane
