GI tract secretion Flashcards
What is the GI system generally co-ordinated by?
The integration of neural and hormonal signals
What can the signals be genearlly in the GI system?
- Physical (stretch receptors)
- Chemical (sight and presence of food)
- Additionally, there is considerable input from higher centres
What are the three phases the signals in the GI tract can be split into?
- Cephalic
- Gastric
- Intestinal
What is the cephalic phase ?
HEaring or smelling food, control is largely parasympathetic/vagal stimulation
What is the gastric phase controlled by?
- Parasympathetic
- Feedback control from small intestine
What can act as a feedback control mechanism?
- Secretin
- CCK
- GIP
- Distention
What substance can act as a feed-forward mechanism from gastric to intestinal phase?
Gastrin
What can change in the mouth in the cephalic phase in response to food/thinking of food?
Increased salivary secretion
What can change in the stomach in the cephalic phase?
- Increased:
- Secretion in stomach
- HCL secretion
- Mucus secretion
- Pepsin secretion
- Gastrin secretion
- Blood flow to stomach
What is the gastric phase initiated by?
Food entering the stomach causing distention and release of gastrin - espec caffeine, alcohol, calcium
What stimuli cause the G cells to produce gastrin?
- Parasympathetic pathways
- Decreased acidity in stomach due to buffering of food
- Distention of antrum
- Proteins, peptides and amino acids
What substances works to inhibit the action of G cells?
HCl
What does gastrin target?
- Increases gastric motility
- Parietal cells
- Trophic maintenance of GI epithelium
What is the intestinal phase initiated by?
Presence of food in the duodenum
What happens if the pH is >3 in the duodenum?
Duodenal peptides / amino acids cause release of gastrin
What happens if the pH is <2 in the duodenum?
Gaastric inhibition and intestinal stimulation
What substances do the duodenal fats and breakdown products cause the release of
- GIP
- CCK
What substance is released in response to acid entering the intestine?
Secretin
What is secretion?
The addition of substances (fluids, enzymes, ions etc.) into the lumen of the GI tract
What places secrete substances into the GI system?
- Salivary glands
- Cells of the gastric mucosa (gastric secretion)
- Exocrine cells of the pancreas (pancreatic secretion)
- Liver (bile)
What factors decrease secretion of saliva?
- Sleep
- Dehydration
- Atropine
How much saliva is secreted per day roughly?
1 litre
What are the functions of saliva?
- Initial digestion of starches and lipids (very little)
- Dilution and buffering of ingested foods
- Protection of teeth and gums
- Lubrication of ingested foods with mucous
What cells does the sublingual gland mostly contain?
Mucous cells
What cells does the submandibular gland contain?
Mixed glands containing serous and mucous cells
What cells does the parotid gland gland contain?
Serous cells secreting an aqueous fluid composed of water, ions and enzymes
What are the 3 stages of salivary secretion?
- Acini cells secrete primary secretion (isotonic)
- Myoepithelial cells stimulated by neural input to eject saliva
- Duct cells - secondary modification
What is found in the primary secretion?
- Na+, Cl-, K+, HCO3-
- Amylase and mucin production
What happens in secondary modification?
- Duct cells
- Reabsorb Na+ and Cl- and add K+
- HCO3- conc is altered depending on flow rate
- High flow rate, saliva has increased HCO3-
- Low flow rate, more HCO3- is extracted
What stimulates the salivary centre in the medulla?
- Simple - Chemo/presssure receptors in mouth , impulses sent via afferent nerves
- Acquired (conditioned), thinking about food, impulses sent from cerebral cortex triggers salivary centre
What kind of saliva is produced as a result of parasympathetic stimulation and how is it produced?
- IP3, Ca2+ produced in acinar or ductal cells
- Large volume of watery enzyme rich saliva produced
What kind of saliva is produced as a result of sympathetic stimulation and how?
- cAMP production increased
- Small volume of thick mucousy saliva produced
What is the main body of the oesophagus lined with and why?
- Simple mucous glands
- To protect against mechanical damage
What glands at the gastric end of the oesophagus protect it against chemical damage?
- Compound mucous glands
What are parietal cells?
Located in the body of the stomach and produce HCl and intrinsic factor
What are cheif cells?
Located in the body of the stomach and produce pepsinogen
Where are G cells located?
The antrum
Where are mucous cells located?
Antrum
How does HCl help with protein digestion?
Starts to denature them and activates pepsinogen to form pepsin
What can be absorbed as a result of the action of intrinsic factor?
Vitamin B12 (absorbed in ileum)
What does mucus do?
Protection and lubrication
What cells make up the oxyntic (gastric) gland “gastric pit”?
- Surface epithelium
- Mucous neck cells
- Oxyntic (or parietal) cells
- Peptic (or chief) cells
What cells secrete mucous?
Mucous neck cells
What cells secrete HCl and intrinsic factor?
Oxyntic (or parietal) cells
What cells secrete pepsinogen?
Peptic (or chief) cells
How is HCl made?
- Secreted seperately in parietal cells
- H+ is secreted into the lumen via the H+ K+ ATPase
- Cl- follows by diffusion through an apical channel
What happens at the basolateral membrane?
- HCO-3 is exchanged for Cl- via the chloride - bicarbonate exchanger (alkaline tide)
- Eventually HCO-3 is secreted back into the GI tract in pancreatic secretions
What is the ‘alkaline tide’?
- During digestion HCO-3 is exchanged for Cl- via the chloride bicarbonate exchanger
- Results in increased HCO-3 in blood therefore increased pH
What percentage of secretion is in the cephalic phase?
30%
- Smell, taste, chewing, swallowing, conditioned reflex in anticipated food
What stimulates secretion in the cephalic phase?
- Direct stimulation of the parietal cells by the vagus
- Indirect stimulation by parietal cells by gastrin
Vagal gastrin releasing peptide (GRP) stimulates gastrin release from G cells. Gastrin hormone enters the circulation and stimulates parietal cells to release HCl
What percentage of secretion is in the gastric phase?
60%
What does distention cause?
- Direct vagal stimulation of parietal cells
- Indirect stimulation via gastrin
- Local reflexes in the antrum that stimulate gastrin release
Waht percentage of secretions are in the intestinal phase?
10%
When is HCl secretion inhibited?
- When HCl is no longer needed to convert pepsinogen to pepsin
- Occurs after the chyme moves into the SI and the H+ buffering capacity of the food is no longer a factor
What peptide hormone inhibits HCl secretion?
Somatostatin
Explain the 2 ways in which somatostatin can inhibit HCl secretion?
- 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 cells secrete pepsinogen?
Chief and mucous cells in the oxyntic glands in response to vagal stimulation
What triggers chief cells to secrete pepsinogen?
H+
How much fluid does the exocrine pancreas secrete?
~ 1L into duodenum
What does the exocrine solution produced from the pancreas contain?
Aqueous solution containing enzymes and high HCO-3
What is the function of the bicarbonate from the exocrine pancreas?
Neutralises stomach H+
What does the exocrine panceatic secretion contain?
- High HCO-3 (isotonic)
- Pancreatic lipase, amylase and proteases
What factors increase exocrine pancreatic secretion?
- Secretin
- CCK (potentiates secretin)
- Parasympathetic supply
What innervates the pancreas?
- Vagus stimulates secretion
- Sympathetic innervation inhibits secretion
What are some pancreatic enzymes stored in until release?
Condensed zymogen granules
What cells cause a modification of the ion composition which results in a fluid secretion rich in HCO-3?
Ductal cells
What ions make the pancreatic fluid ‘isotonic’?
Na+, K+, Cl- and HCO-3
What reactionn creates the bicarbonate ions which are then secreted into the duodenum?
HCO3- H2CO3 (carbonic anhydrase) CO2 + H2O
What cells produce the enzymes for pancreatic secretion?
Acinar cells
What enzymes are secreted in an inactive form and are activated in the duodenum?
Proteases
What enzyme breaks down starch?
Amylase
What are the 3 types of pancreatic enzymes?
- Pancreatic amylase
- Pancreatic lipase
- Proteolytic
What does pancreatic amylase breakdown?
Polysaccharides to disaccharides
What do pancreatic lipases breakdown?
Triglycerides -> monoglycerides + FA
What do proteolytic enzymes do?
Cleave proteins at different sites -> AA and small peptides
What are the 3 major proteolytic enzymes?
- Trypsinogen
- Chymotrypsinogen
- Procarboxypeptidase
All inactive (zymogens) to prevent self digestion
What enzyme activates trypsinogen into trypsin?
Enteropeptidase (aka) enterokinase
What does trypsin activate?
- Chymotrypsinogen into chymotrypsin
- Procarboxypeptidase into carboxypeptidase
- Positive feedback mechanism which activates more trypsinogen to be produced
The intestinal phase accounts for how much pancreatic secretion?
80%
What cells secrete CCK?
Duodenal I cells
What is CCK produced in response to?
- Presense of AA
- Small peptides
- Fatty acids
in intestinal lumen - Vagal release of ACh potentiates CCK action
What does CCK trigger?
Acinar cells to produce enzymes
What cells release secretin?
S cells of the duodenum
What does Secretin stimulate the secretion of?
Aqueous rich HCO-3
What is secretin released in response to?
- Arrival of acidic chyme in duodenum
- Ach and CCk potentiate secretin action
What does Secretin trigger the releae of?
Ductal cells to produce Na+, K+, Cl- and HCO-3
What is the function of the gall bladder?
- Stores bile
- Concentrates bile
- Ejects bile (CCK triggers release, begins ~30 mins after meal)
How is bile concentrated in the gall bladder?
Epithelial cells lining the gllbladder absorb ions and water iso-osmotically
What cells produce bile?
Hepatocytes, flows to gallbladder through bile ducts
What is bile?
- Essential for digestion and absorption of lipids
- Bile salts emulsify lipids
- Mixture of bile salts, pigments and cholesterol
- Not enzymic
What system recirculates bile salts?
Recirculated via enterohepatic system
What does CCK do to release bile?
- Gall bladder contracion
- Sphincter of Oddi relaxation
What does secretin do to bile?
Triggers bile secretion, espec NaHCO3-
What makes up bile?
- Bile salts (50%)
- Phospholipids (40%)
- Cholesterol (4%)
- Bile pigments (e.g bilirubin) (2%)
- HCO3-
What does secreton stimulate (in relation to bile)?
Production of bile
What are crypts of LIeberkuhn?
- Secretions in SI
What 2 cells covevr crypts and villi in SI?
- Goblet cells - secrete mucus
- Enterocytes - in crypts secrete water and electrolytes (1800ml/day)
- In villi absorb water and electrolytes (along with products of digestion)
What are secretions of the SI largely regulated by?
Distention and tactile or irritative stimuli from chyme
What is present in the alkaline mucus of LI?
- High K+ and bicarbonate
- No digestive enzymes
What are the functions of the alkaline mucus of the colon?
Potection and lubrication
What neutralises alkaline mucus in the colon?
H+ produced by gut bacteria
What triggers the release of alkaline mucus in LI?
Greatest trigger is distention / mechanical stimulation of walls
Increased secretion triggered by Ach and VIP
Decresed secretion triggered by adrenaline and somatostatin
