Gastrointestinal Secretions Flashcards
How much water leaves the gut in 24 hours?
~150ml
Where is water absorbed in the gut?
Small intestine
Large intestine
What is the cephalic phase of digestion? (3)
Thinking about food/presence of food in mouth promotes salivary and gastric secretions
Chewing breaks food down into smaller particles
Amylase in saliva initiates starch digestion
What controls the cephalic phase of digestion?
CNS
What is the gastric phase of digestion? (3)
Stomach secretes in response to presence of food in stomach
Gastric motility for further mechanical breakdown
Protein digestion starts
What controls the gastric phase of digestion?
CNS
ENS
Hormones
What is the intestinal phase of digestion? (3)
Food entering small intestine gradually causes release of hormones that inhibit gastric secretion and motility
Hormones provoke release of biliary and pancreatic secretions into duodenum
Digestion and absorption
What controls the intestinal phase of digestion?
Mostly ENS (some CNS)
Hormones
Which cells in salivary glands produce saliva?
Acinar cells
Describe the process by which the primary salivary secretion is produced. (6)
BL Na/K-ATPase moves Na out of cell
BL NKCC2 allows 2Cl to enter cell; Cl also enters in exchange for HCO3
BL K channel
Cl leaves cell via luminal channels
Negative charge in lumen attracts Na paracellularly
Water follows by osmosis and aquaporins
What is the tonicity of the primary salivary secretion in relation to plasma?
Isotonic
When is the primary salivary secretion modified?
As it passes along the duct
How is the primary salivary secretion modified?
Some Na and Cl reabsorbed (tight junctions prevent paracellular movement back) which also results in water reabsorption
K and HCO3 added
Describe the secretion produced after the primary salivary secretion has been modified.
HCO3 rich, hypotonic secretion
How does high flow rate affect the tonicity of saliva (compared to plasma) and why?
Modification is less effective
Less hypotonic
What are the other components of saliva (other than Na, Cl, water, K, HCO3)?
Calcium and phosphate
Large molecule components (mucins, lysozyme, amylase)
IgA
How are large molecule components transported into saliva?
Produced by acinar cells and released by exocytosis
What cells produce the IgA in saliva?
Nearby plasma cells of salivary gland
What are the functions of saliva? (4)
Lubrication
Amylase initiates starch digestion
Antimicrobial activities
pH and ions protect teeth from demineralisation
What is the relative amount of amylase and mucus in saliva produced by the parotid gland?
Amylase > mucus
What is the relative amount of amylase and mucus in saliva produced by the submandibular gland?
Amylase < mucus
What is the relative amount of amylase and mucus in saliva produced by the sublingual gland?
Mucus and no amylase
Which salivary gland has the highest flow rate during basal conditions?
Submandibular
Which salivary gland has the highest flow rate during stimulation (eg. by eating)?
Parotid
How does ANS control generally affect saliva?
Parasympathetic - increase formation of fluid and electrolyte components
Sympathetic - increase release of large molecule components
Describe the glands in the oesophagus.
Widespread minor glands which only produce mucus
What do gastric chief/peptic cells produce?
Pepsinogen
What do gastric parietal cells produce?
HCl and intrinsic factor
What is another name for a gastric parietal cell?
Oxyntic cell
What do the gastric enterocytes produce?
Mucus
What are the main gastric secretions? (6)
Pepsinogen
HCl
Intrinsic factor
Mucus
Gastric lipase
Water
What does intrinsic factor bind?
Vitamin B12/cobalamin
Describe the mechanism of gastric acid secretion. (4)
Main drive is H/K-ATPase
Carbonic acid dissociates into HCO3 (into blood exchanged for Cl) and H+ (into lumen via ATPase)
K and Cl balance maintained by luminal channels
Water follows net ion movement into lumen
How much more acidic is the stomach lumen than the blood?
3 times
How does gastric acid secretion increase in response to stimulation?
Resting = proton pumps confined to intracellular tubulovesicles
Stimulated = tubulovesicles rearrange and fuse with canaliculi (continuous with luminal membrane)
Describe how stomach distension/presence of food in mouth affects gastric acid secretion.
Triggers CNS/ENS = ACh released which acts on:
- M3 receptors on parietal cells
- Activates enterochromaffin-like cells = histamine = H2 receptors on parietal cells
Protein and amino acids in stomach cause gastrin release from G cells which travel via blood to bind CCKB receptors on parietal cells
Increased gastric acid secretion
What is the effect of ACh on D cells?
Binds muscarinic receptor to release somatostatin
What negative feedback system prevents oversecretion of stomach acid in response to ACh?
ACh binds muscarinic receptors on D cells to release somatostatin
SST:
- Inhibits ECL cells
- Inhibit G cells
- Directly inhibit parietal cells
Which endocrine factors affect the stomach and where are they released from? (5)
Gastrin from pyloric antrum
Cholecystokinin from duodenal wall
Secretin from duodenal wall
GIP from small intestine
GLP-1 from ileum/colon
What is another name for cholecystokinin?
Pancreozymin
How does gastrin affect the stomach?
Induces gastric secretions
Increases motility
What affects the release of gastrin (non-hormonal)?
Stimulated by proteins, coffee, alcohol…
Inibited by low gastric pH
How does cholecystokinin affect the stomach?
Depresses gastric motility and secretion
What stimulates the release of cholecystokinin from the duodenal wall?
Fats
How does secretin affect the stomach?
Inhibits gastric secretion
What stimulates the release of secretin from the duodenal wall?
Acid
How do GIP and GLP-1 affect the stomach?
Inhibit gastric motility and secretion
What stimulates the release of GIP and GLP-1?
Fat and chyme in lumen
What is secreted to achieve efficient absorption in the small intestine? (4)
Mucus (goblet cells)
Isotonic saline (crypt cells)
Alkaline mucus (Brunner’s glands)
Digestive enzymes and HCO3 rich fluid (surface enterocytes)
Where are the digestive enzymes produced by small intestine enterocytes found?
Embedded in the glycocalyx of their brush border
The relaxation of which sphincter allows secretions to enter the duodenum?
Sphincter of Oddi
What are the two main components of pancreatic exocrine secretions and what produces them?
Alkaline, HCO3-rich fluid by pancreatic duct cells
Digestive enzymes by acinar cells
Why are pancreatic secretions alkaline?
To neutralise acidic chyme entering from stomach
How are the digestive enzymes of pancreatic secretions stored before exocytosis?
Intracellularly as inactive precursor forms in zymogen granules
What hormones affect pancreatic exocrine secretion and where are they released from?
Cholecystokinin
Secretin
Both from duodenal wall
How does cholecystokinin affect the pancreas?
Induces release of enzyme-rich secretions
How does secretin affect the pancreas?
Induces release of HCO3-rich secretions
Describe the mechanism of alkaline fluid secretion by pancreatic duct cells. (6)
Driven by BL Na/K-ATPase
BL Na and HCO3 move into cell in exchange for H
CO2 from blood => carbonic acid in cells (carbonic anhydrase) => HCO3 + H
HCO3 into lumen via anion exchanger (Cl into cell)
Cl leaks into lumen
Negative charge in lumen attracts Na paracellularly and water by osmosis
What disorder can affect the alkaline fluid secretion of pancreatic duct cells and why?
Cystic fibrosis
Defect in Cl channels (CTFR-type)
What does the primary biliary secretion consist of? (4)
Solutes extracted from blood by hepatocytes
Bile salt/acids
Bile pigments
Cholesterol and lecithin
When is the primary biliary secretion modified?
As it passes along bile duct
How is the primary biliary secretion modified?
Water and HCO3 added (similar to pancreatic alkaline secretion)
What stimulates the modification of the primary biliary secretion?
Secretin
What causes the gall bladder to contract and the sphincter of Oddi to relax?
Cholecystokinin
What does GIP do?
Stimulates insulin release (feed-forward)
What does GLP-1 do?
Stimulates insulin release (feed-forward)
Inhibits glucagon release
Promotes satiety
Which hormones promote satiety?
Cholecystokinin
GLP-1
What is ghrelin and what inhibits its release?
Hunger hormone (promotes appetite)
Food digestion
What ions are secreted in the colon?
K+
HCO3-
Describe the secretory/reabsorptive process in the colon. (4)
Na/K-ATPase drives process
Luminal Na channel allows Na to enter cell; Na enters in exchange for H
More Na is absorbed than Cl = negative charge in lumen driving K paracellularly into lumen
Water follows sodium paracellularly and via aquaporins for reabsorption
What can inadequate production of saliva lead to? (4)
Dry mouth
Difficulty swallowing
Enamel damage
Reduced microbiological protection
What can gastric atrophy lead to?
Lack of intrinsic factor
Pernicious anaemia
What can gastritis lead to?
Gastric (and duodenal) ulcers
What can excess gastric acid production lead to? (3)
Duodenal ulcers
Malabsorption
Diarrhoea
What can pancreatitis lead to?
Inadequate enzyme production so malabsorption
How does cystic fibrosis affect digestion?
Decreased Cl conductance so decreased formation of pancreatic alkaline fluid
Decreased delivery of enzymes to duodenum so less digestion
What can insufficient biliary secretions lead to?
(Fat) malabsorption
