GI Tract Secretions Flashcards
What are the 3 regulatory phases of GI function?
- cephalic
- gastric
- intestinal
What are the primary regulatory events in the cephalic phase?
- initiated by taste/smell of food
- vagal activity triggers increase in salivary secretions
- increase in stomach secretions (HCl, pepsin, mucus and gastrin)
- stimulates bile ducts, hepatocytes and pancreatic acinar cells
What are the primary regulatory events in the gastric phase?
- initiated by food entering stomach causing distension and release of gastrin
- G cells triggered by parasympathetic pathways
- decreased acidity in stomach from buffering of food
- distension of antrum
- breakdown products of gastrin (amino acids, peptides and proteins)
What are the primary regulatory events in the intestinal phase?
- initiated by presence of food in duodenum
- if pH below 2: gastric inhibition and intestinal stimulation
- if pH above 3: release of gastrin, acid causes release of secretin
What factors affect saliva secretion?
- sympathetic and parasympathetic NS increase saliva secretion:
- PSNS: produce large volume of watery, enzyme rich saliva
- SNS: produce small volume, thick, mucousy saliva
- sleep, dehydration and atropine decrease saliva secretion
Describe the process of saliva release
- acinar cells secrete primary secretions containing Na, Cl, K, bicarbonate, amylase and mucin
- myoepithelial cells are stimulated to eject saliva
- ductal cells cause secondary modification (resorbs Na, Cl and adds K)
How does flow rate affect bicarbonate concentration of saliva?
- high flow rate = high bicarbonate concentration
- so after a meal, bicarbonate can protect teeth from bacteria
What are the different salivation reflexes?
- simple: pressure receptors in mouth activated in presence of food
- acquired: smell/see/hear in preparation of meal
How does the oesophagus produce secretions?
- the main body is lined by simple mucous glands for mechanical damage protection
- gastric end has compound mucus glands to protect against chemical damage
Where are the different cell types found in the stomach?
- body: parietal and chief cells
- antrum: G cells and mucous cells
What are the different gastric secretions and their functions?
- HCL: small protein digestion and activation of pepsinogen
- pepsinogen: protein digestion
- intrinsic factor: vitamin B absorption
- mucous: protection and lubrication
What increases the surface area of the stomach?
- gastric pits that contain mucous cells, parietal and chief cells
When is HCl secreted and describe its process
- to protect parietal cells
- at basolateral membrane, bicarbonate ions exchanged for Cl-
- at apical membrane H+ are secreted into lumen by H/K-ATPase and Cl- ions then diffuse across membrane
- in the lumen they form HCl
What are the proportions of secretions that occur at each gastric stage?
- cephalic: 30% by stimulation of parietal cells by vagus directly or gastrin indirectly
- gastric: 60% by vagal stimulation direct by distension or local reflexes that cause gastrin release
- intestinal: 10% from presence of breakdown products
What is HCl secretion inhibited and how?
- when HCl is no longer needed to convert pepsinogen into pepsin
- somatostatin directly inhibits release by binding to receptors on parietal cells
- inhibiting histamine or gastrin release indirectly inhibits it
How is pepsinogen secreted?
- by chief and mucous cells in oxynitic glands in response to vagal stimulation
- H+ ions trigger local reflexes that stimulate secretion
What makes up exocrine pancreatic secretions and what are its functions?
- made up of solution of enzymes and bicarbonate ions
- bicarbonate ions neutralise stoamch acid
- enzymes digest carbs, proteins, lipis
What increases exocrine pancreas secretions?
- secretin
- CCK
- parasympathetic NS
How are pancreatic secretions formed?
- aqueous component released from centroacinar and ductal cells
- ductal cells cause high bicarbonate content
- acinar cells release enzymes
- amylase and lipase are active
- protease is inactive and needs activation in duodenum
What are the 3 major proteolytic enzymes?
- trypsinogen
- chymotrypsinogen
- procarboxypeptidase
- allinactive in zymogen cells to prevent self digestion
How are the proteolytic enzymes activated?
- trypsinogen activated by enterokinase/enteropeptidase to form trypsin
- trypsin then activates more trypsinogen
- chymotrypsinogen to form chymotrypsin
- and procarboxypeptidase to form carboxypeptidase
How does the pancreas respond to the intestinal phase of digestion?
- duodenal I cells secrete CCK, vagal release of ACh potentiates CCK action
- triggers acinar cells to produce enzymes
- secretin stimulus for bicarbonate secretion
- CCK and ACh potentiate secretion action
- triggers ductal cells to produce Na, K, Cl and HCO
What is the function of the gallbladder?
- stores bile produced by hepatocytes
- concentrates bile
- CCK major stimulus for bile ejection (released from I cells in duodenum and jejunum)
Describe the regulation of bile secretion
- between meals: bile salts recirculate to liver so more bile produced then stored in gallbladder
- cephalic phase: vagal stimulation increases bile flow
- after meal: chyme in duodenum stimulates release of CCK and secretin (triggers bile release)
Describe the secretions of the small intestine
- crypts of leiberkuhn and villi are covered in goblet cells (release mucus) and enterocytes that secrete water and electrolytes
- secrete water and electrolytes in crypts
- absorbs water and electrolytes in the villi
- secretions regulated by distension and irritative/tactile stimuli from chyme
Describe the secretions of the large intestines
- alkaline mucus high in potassium and bicarbonate
- protects and lubricates and neutralises H+ ions produced by gut bacteria
- secretion stimulated by distension and some parasympathetic input
