26-01-23 - GI Tract Secretion Flashcards
Learning outcomes
- Recount the timings and events of gastro-intestinal tract (GIT ) function control
- List the important secretory products of and into the GIT and their main stimuli
- Explain the mechanism of secretion in the mouth, oesophagus, stomach, pancreas, small intestine and large intestine
- Recall the sites of secretion and explain the physiological role(s) of the (named) secretion
- List the maim pancreatic enzymes and explain how they are activated
How is GI function coordinated?
Where can signals be from?
Where is there also input from?
What are the 3 phases signals can be split into?
- GI function is co-ordinated by the integration of neural and hormonal signals
- Signals can be physical (stretch receptors) or chemical (sight and presence of food)
- In addition, there is considerable input from higher centres
- 3 phases signals can be split into:
1) Cephalic
2) Gastric
3) Intestinal
Important regulatory events in the cephalic phase.
What is the cephalic phase initiated by?
What is the input from higher centres?
What are the changes in the mouth during the cephalic phase (in picture)?
What are changes in the stomach?
What are other changes?
- Important regulatory events in the cephalic phase
- The cephalic phase is initiated by taste and smell of food
- High integrating centres stimulate vagal activation, which has a parasympathetic branch that acts as the most important regulator
- This regulator leads to changes in the mouth, changes in the stomach and other changes (in photo)
Important regulatory events in the gastric phase.
What is the gastric phase initiated by?
What does this cause?
What 3 things especially cause this?
What are the 4 inputs to G cells of the stomach for gastrin release?
What are 3 targets for gastrin?
What serves as negative feedback for gastrin release from G cells in the stomach?
- Important regulatory events in the gastric phase
- The gastric phase is Initiated by food entering the stomach causing distension
- This results in the release of gastrin from G cells (especially caffeine, alcohol, and calcium)
- 4 inputs to G cells of the stomach for gastrin release:
1) Parasympathetic pathways
2) Decreased acidity in stomach due to buffering of food
3) Distension of antrum
4) Proteins, peptides and amino acids - 3 targets for gastrin:
1) Gastric motility
2) Parietal cells
3) Trophic maintenance of GI epithelium (increased repair from damage sustained eating food) - HCl serves as negative feedback for G cells (low pH inhibits gastrin release)
Important regulatory events in the intestinal phase.
How is the intestinal phase initiated?
How does pH affect this phase?
What causes the release of HIP and CCK?
What causes the release of secretin?
- Important regulatory events in the intestinal phase
- Initiated by presence of food in the duodenum
- If pH > 3, duodenal peptides / amino acids cause release of gastrin
- If pH < 2 gastric inhibition and intestinal stimulation
- Duodenal fats and breakdown products cause release of GIP (Gastric inhibitory polypeptide) and CCK (Cholecystokinin)
- Acid entering intestine causes secretin release
What is secretion?
What 4 structures produce secretion?
- Secretion is the addition of substances (fluids, enzymes, ions etc) into the lumen of the gastrointestinal tract
- 4 structures produce secretion:
1) Produced by salivary glands (saliva)
2) Cells of the gastric mucosa (gastric secretion)
3) Exocrine cells of the pancreas (pancreatic secretion)
4) Liver (bile)
What is a secretagogues?
What determines the selectivity of a secretagogues?
What can secretagogues be?
- A secretagogues is a substance that promotes the secretion of another substance (e.g., hormones, neurohormones, chemical neurotransmitters, enzymes etc) from a cell
- The selectivity or specificity of a secretagogue is determined by the structure of the secretagogue and that of its receptors on or within the cell
- Secretagogues may be endogenous biological agents, or exogenous (natural or synthetic) compounds
(in picture).
What are 5 secretagogue hormone and neurotransmitters?
What are 5 secretagogue bacteria enterotoxins?
What are each of their second messengers?
(in picture)
What are the End effects (in the gut) of 3 second messengers of secretagogues?
What are 4 characteristics of saliva?
What 2 factors increase secretion of saliva?
What are 3 factors that decrease secretion of saliva?
What is a hypotonic solution?
How is net movement of water affected in hypotonic solutions in the body?
Why can decreased seliva secretion lead to rotting teeth?
- 4 characteristics of saliva:
1) High HCO3
2) High K+
3) Hypotonic
4) D-amylase and lingual lipase - 2 factors increase secretion of saliva:
1) Parasympathetic (primarily)
2) Sympathetic (some) - 3 factors that decrease secretion of saliva:
1) Sleep
2) Dehydration
3) Atropine - A hypotonic tonic solution is any external solution that has a low solute concentration and high water concentration compared to body fluids. (more dilute than plasma)
- In hypotonic solutions, there is a net movement of water from the solution into the body
- Saliva secretion is rich in Bicarbonate ions which neutralise acid
- When there is a lack of saliva secretion, there will not be bicarbonate ions to neutralise acid from food, leading to rotting teeth
How much saliva do we produce a day?
What are 4 functions of saliva?
- We produce about 1 litre of saliva a day
- 4 functions of saliva:
1) Initial digestion of starches and lipids (very little)
2) Dilution and buffering of ingested foods
3) Protection of teeth and gums
4) Lubrication of ingested foods with mucous (mucin)
What are the 3 salivary glands?
Where are they found?
What kind of cells do they contain?
- 3 salivary glands:
1) Sublingual
* Deep in floor of mouth, underneath tongue
* Mostly mucous cells
2) Submandibular
* Under lower edge of mandible
* Mixed glands containing serous and mucous cells
3) Parotid
* Below ear and over the masseter (jaw muscle)
* Serous cells secreting an aqueous fluid composed of water, ions and enzymes
What are the 3 stages of salivation from salivary glands?
Is saliva in the mouth isotonic or hypotonic?
- 3 stages of salivation from salivary glands:
1) Acini cells secrete 1° (primary) secretion (isotonic solution – same concentration of solutes as plasma)
* Na+, Cl-, K+, HCO3
* Amylase & mucin production
2) Myoepithelial cells stimulated by neural input to eject saliva
3) Duct cells - 2° (secondary) modification
* Reabsorb Na+ & Cl- and add K+ -
* HCO3- concentration is altered depending on flow rate
* High flow rate, saliva has ↑ HCO3-
* Low flow rate, more HCO3- is extracted
- In mouth saliva is hypotonic - more dilute than plasma
Describe the flow chart for simple and acquired salivation reflexes (in picture)
Describe the flow chart for sympathetic and parasympathetic regulation of salivary secretion (in picture)
What does the oesophagus secrete?
What is the main body of the oesophagus lined with?
What does the gastric end of the oesophagus contain?
What is the purpose of these structures?
- The oesophagus Only secretes mucous for lubrication/protection (no real digestive function)
- The main body of the oesophagus is lined with simple mucous glands which protects against mechanical damage
- The gastric end (especially) has compound mucous glands which protects against chemical damage
What are the 5 different cell types in the stomach?
Where are they each found in the stomach?
What do they each secrete?
- 5 different cell types in the stomach:
1) Parietal cells
* Found in the body of the stomach
* Secretes HCl and intrinsic factor
2) Chief cells
* Found in the body of the stomach
* Secretes pepsinogen
3) G cells
* Found in the antrum of the stomach
* Secretes gastrin to the circulation
4) Mucous cells
* Found in the antrum of the stomach
* Secretes mucus
5) Delta (D) cells
* Found in the antrum and pylorus of the stomach
* Secretes somatostatin
What are 4 factors that increase gastric secretions?
What are 8 factors that decrease gastric secretions?
- 4 factors that increase gastric secretions:
1) Gastrin
2) Acetylcholine
3) Histamine
4) Parasympathetic - 8 factors that decrease gastric secretions
1) Chyme in duodenum
2) H+ in stomach
3) Secretin
4) GIP
5) Somatostatin
6) Atropine
7) Cimetidine
8) Omeprazole
How is surface area in the stomach increased?
What are the 4 cells types found in gastric (oxyntic) glands?
What does each secrete?
- The surface area in the stomach is increase by invaginations called gastric (oxyntic) glands (aka gastric pits)
- 4 cells types found in gastric (oxyntic) glands and what each secretes:
1) Surface epithelium
2) Mucous neck cells
* Secrete mucus
3) Oxyntic (parietal) cells
* Secrete HCl and intrinsic factor
4) Peptic (chief) cells
* Secrete pepsinogen
HCl secretion – apical surface.
How often is HCl secreted?
Describe the process of HCl being secreted from the apical membrane of gastric parietal cells
- HCl secretion – apical surface
- HCl is secreted in stages so parietal cells aren’t denatured
- From the apical membrane of gastric parietal cells, H+ is secreted into the lumen via the H+-K+ ATPase and Cl- follows by diffusion through an apical channel
- The H+ and Cl- come together to form HCl
HCl secretion – basolateral surface.
How do exchanges on the basolateral surface of gastric parietal cells allow for the formation of HCl?
- HCl secretion – basolateral surface
- Carbonic anhydrase quickly forms carbonic acid from CO2 and H2O, which dissociates into H+ and HCO3- ions
- The HCO3- is exchanged for Cl-
- H+ and Cl- can later come together to form HCl
- These exchanges on the basolateral surfaces of gastric parietal cells provide the necessary components for HCl secretion
When is HCl secretion inhibited?
When will this occur?
What are the direct and indirect pathways somatostatin uses to inhibit HCl secretion?
- HCl secretion is inhibited when HCl is no longer needed to convert pepsinogen to pepsin
- This occurs after the chyme moves into the small intestine and the H+ buffering capacity of the food is no longer a factor
- Somatostatin HCl inhibitors pathways:
- Direct pathway – binds to receptors on parietal cells (and inhibits adenylate cyclase via Gi protein so inhibits HCl release)
- Indirect pathway – inhibits histamine release from stomach and gastrin release from G cells so HCl inhibited
What is pepsinogen secreted by?
What local reflexes can trigger pepsinogen secretion?
When will pepisongen be activated?
- Secreted by chief and mucous cells in the oxyntic glands in response to vagal stimulation
- H+ triggers local reflexes which stimulate chief cells to secrete pepsinogen
- Pepsiongen is a proenzyme which is cleaved and activated to pepsin when it comes into contact with HCl
How much pancreas secretions are released per day?
What is it composed of?
What is the function of these components?
What are 3 factors that increase pancreatic secretions?
- Approximately 1 litre of exocrine pancreas secretions are released into the duodenum per day
- These secretions are composed of an aqueous solution containing enzymes and high HCO3
- The function of the HCO3 is to neutralise stomach H+
- Enzymes digest carbohydrates (amylases – breakdowns starch), proteins (broken down by proteases) and lipids (broken down by lipases)
- 3 factors that increase pancreatic secretions:
1) Secretin
2) Cholecystokinin (CCK) (potentiates secretin)
3) Parasympathetic supply
What does pancreatic secretion consist of?
Where is the aqueous component of pancreatic secretion released from?
What is pancreatic fluid?
What does it contain?
How is the composition of the fluid modified?
Where are enzymes from pancreatic secretions released from?
Which pancreatic enzymes are released active and inactive?
- Pancreatic secretion consists of an aqueous component and enzymes
- Aqueous component of secretion released from centroacinar cells and ductal cells
- Pancreatic fluid is an isotonic fluid containing Na+, K+ Cl- and HCO3
- Modification of the (ion) composition of the pancreatic fluid by the ductal cells results in a fluid secretion rich in HCO3
- Enzymes in pancreatic secretions are released from acinar cells
- Pancreatic amylase and lipases are secreted as active enzymes
- Pancreatic proteases are secreted in an inactive form and activated in the duodenum
Describe the activation of the 3 pancreatic proteolytic enzymes flowchart (in picture)
How are pancreatic secretions regulated in the three phases of digestion?
- How pancreatic secretions are regulated in the three phases of digestion:
1) Cephalic phase
* Mediated by the vagus nerve (mainly enzymatic secretion)
2) Gastric phase
* Initiated by distension of the stomach and mediated by the vagus nerve (mainly enzymatic secretion)
3) Intestinal phase
* Regulated by acinar and ductal cells
* Accounts for 80% of pancreatic secretion and both enzymatic and aqueous secretions are stimulated
Pancreatic response in intestinal phase 1 - acinar cells.
Describe the 3 steps that trigger acinar cells to produce enzymes in the intestinal phase
- Pancreatic response in intestinal phase 1 - acinar cells
- 3 steps that trigger acinar cells to produce enzymes in the intestinal phase:
1) Duodenal I cells secrete CCK in response to the presence of amino acids, small peptides and fatty acids in the intestinal lumen
2) Vagal release of ACh potentiates CCK action
3) Triggers acinar cells to produce enzymes
Pancreatic response in intestinal phase 2 - ductal cells.
Describe the 4 steps that trigger acinar cells to produce Na+, K+ Cl- and HCO3 in the intestinal phase
- Pancreatic response in intestinal phase 2 - ductal cells
- 4 steps that trigger acinar cells to produce Na+, K+ Cl- and HCO3 in the intestinal phase:
1) Secretin released by the S cells of the duodenum is the major stimulus for aqueous rich HCO3 secretion
2) Secretin release is triggered by the arrival of acidic chyme in the duodenum
3) ACh and CCK potentiate secretin action
4) Triggers ductal cells to produce Na+, K+ Cl- and HCO3
What are 3 functions of the gallbladder?
How do each of these occur?
- 3 functions of the gallbladder:
1) Stores bile
* Bile is continuously produced by the hepatocytes and flows to the gallbladder through the bile ducts
2) Concentrates bile
* Epithelial cells lining the gallbladder absorb ions and water iso-osmotically
3) Ejects bile
* Begins ~30 minutes after a meal.
* The major stimulus for ejection is the release of cholecystokinin from the I cells in the duodenum and jejunum
How is bile secretion regulated, in between meals, during the cephalic phase (just before a meal), and during/after a meal?
- How bile secretion is regulated:
1) In between meals
* Bile salts recirculated to liver via the enterohepatic system, therefore more bile produced
* Bile stored / concentrated in gall bladder
2) During the cephalic phase (just before a meal)
* Neural (parasympathetic) stimulation via vagus nerve to ↑ bile flow
3) During/after a meal
* Chyme in duodenum stimulates release of CCK & secretin
* CCK triggers release of stored bile viagall bladder contraction and Sphincter of Oddi relaxation, allowing bile to enter into the duodenum
* Secretin triggers bile secretion, especially NaHCO3- (sodium bicarbonate)
What are 5 biliary secretions?
What % of bile do they make up?
What are 3 stimuli for bile secretion/production?
- 5 biliary secretions:
1) Bile salts (50%)
2) Bile pigments (e.g., bilirubin) (2%)
3) Cholesterol (4%)
4) Phospholipids (40%)
5) HCO3- - 3 stimuli for bile secretion/production:
1) Secretin (bile production)
2) CCK (gallbladder contraction: relaxation of the sphincter of Oddi)
3) Some central and enteric nerve input
What are the locations of secretion of the small intestine?
Where are they located?
What 2 types of cells cover crypts and villi?
What do they each secrete?
What 2 events does the mechanism of secretion include?
What is the process of secretion in the small intestine largely regulated by?
- The locations of secretion in the small intestine are Crypts of Lieberkühn
- Crypts of Lieberkühn are located over the surface of the SI and lie between intestinal villi
- 2 types of cells cover crypts and villi:
1) Goblet cells
* Secrete mucus
2) Enterocytes
* In crypts secrete water and electrolytes (~1800ml/day)
* In villi absorb water and electrolytes (along with products of digestion)
- 2 Events the mechanism of secretion includes (exact mechanism not known):
1) Involves active secretion of Cl- and HCO3 - into the crypts at apical membrane.
2) Electrical gradient causes Na+ to be drawn through and together, the osmotically active particles draw water though by osmosis
- Process of secretion in the small intestine largely regulated by distension and tactile or irritative stimuli from chyme
