Upper Digestive Tract Secretion Flashcards
How much saliva is produced each day?
1-1.5L
What does decreased function of the salivary glands lead to?
- dry mouth
- dental caries
- infections of buccal mucosa
What are amylases?
enzymes in the saliva that are active between pH 4-11
How does the parasympathetic NS mediate salivary secretion?
by the release of ACh which binds to muscarinic receptors which cause watery saliva to be secreted
How does the sympathetic NS mediate salivary secretion?
by the release of noradrenaline that binds to alpha and beta adrenergic receptors which cause thicker saliva to be secreted
What are the 7 organic constituents of saliva?
- mucin
- alpha-amylase
- RNAse and DNAase
- lingual lipase
- lactoferrin
- secretory IgA
- lysozyme
What is HCl produced by?
parietal cells
What does HCl do?
- kill most ingested
bacteria - cleave pepsinogens to pepsins
What are pepsinogens produced by?
chief cells in the stomach
What are intrinsic factors?
proteins that help the intestines absorb vitamin B12, which is the only gastric secretion required for life
What is gastrin released by?
G cells in the stomach and duodenum
What happens to gastrin?
it is secreted straight into the blood and does not enter the lumen
What does mucus do?
protect gastric mucosa from mechanical and chemical destruction
What is mucus?
a thin layer of fluid that covers the luminal surface of the airway
What is the composition of gastric juice?
99.5% water and 0.5% solids
What are the inorganic substances of gastric juice?
- HCl
- electrolytes
- bicarbonate
- sulphate
Where is rennin found and what does it do?
in infants to help with milk protein digestion
How does the rate of gastric secretion affect the ionic composition of gastric juice?
- higher rate = higher [H+]
- lower rate = lower [H+]
What can prolonged vomiting lead to?
hypokalaemia since [K+] is higher in gastric juice than plasma
What is the basal and max rate of gastric acid secretion?
- basal = 1-5 mM/h
- max = 6-40 mM/h
What condition will lead to a higher gastric rate than normal?
duodenal ulcer
What conditions will lead to a lower gastric rate than normal?
gastric ulcer and pernicious anaemia
What are the 3 stimulants of gastric acid secretion?
ACh, gastrin and histamine
What are the functions of gastrin?
- stimulate HCl secretion
- increase gastric and intestinal motility
- increase pancreatic secretion
- help with proper growth of GI mucosa
What is the pH at which pepsins have the highest activity?
anything less than 3
What do pepsins do?
digest 10-20% of total dietary protein
When are pepsins irreversibly inactivated?
at duodenum pH
What is intrinsic factor secreted by?
parietal cells in the fundus
Where is the IF-B12 complex completely absorbed?
at the terminal ileum
How is intrinsic factor stimulated?
by the same stimuli that evoke HCl secretion (ACh)
What are mucins?
the main component of gastric mucus; large molecules with CHO side chains that are insoluble in acid pH and can be destroyed by pepsin
What is the function of mucins?
to coat and lubricate the mucosal surface
How are mucins regulated?
by the same mechanism that enhances HCl secretion (ACh) and mechanically stimulated by food
What are the paracrine effects of prostaglandins?
- directly inhibit parietal cell secretion
- mediate mucosal defence
What are the 3 components of mucosal defence?
- stimulation of mucus, phospholipid and HCO3- secretion
- enhancement of mucosal blood flow
- stimulation of mucosal cell turnover
What does prostaglandin deficiency predispose to?
gastric mucosal injury
What does the gastric mucosal barrier do?
protect against abrasion, HCl and pepsin
What is the gastric mucosal barrier?
a thick mucus layer (1mm) made up of mucins and HCO3-
What does the unstirred layer of the gastric mucosal barrier do?
slow the inward diffusion of H+ and outward diffusion of HCO3-
What is the pH of the gastric mucosal barrier at the cell surface and gastric juice in the lumen respectively?
- cell surface = 7
- lumen = 2
What does the gastric mucosal barrier depend on?
- Ca2+ and cholinergic agonists to stimulate HCO3- secretion
- adrenergic agonists to decrease HCO3- secretion
- aspirin and NSAIDs to inhibit mucous and HCO3- secretion
What does the rate of repair of the gastric mucosal barrier depend on?
the extent of the injury; can be between 48 hours to 3-5 months
Give examples of ulcer therapy
- neutralising HCl (antacids)
- prevent acid release (omeprazole or cimetidine)
- helicobacter pylori bacillus (antibiotic)
- prostaglandin agonist (misoprostol)
What does carbohydrate gastric digestion depend on?
the action of salivary alpha-amylase (halts at low pH)
Why is fat gastric digestion minimal?
high acidity inhibits fats from forming emulsions
What are the only substances absorbed during gastric absorption?
highly lipid-soluble substances e.g. ethanol
What is gastritis?
a painful or burning sensation in the chest due to an increase in gastric acid secretion or a backflush of acidic chyme into the oesophagus
What factors cause gastritis?
- overeating
- smoking
- fatty foods, coffee, alcohol
- lying down immediately after meal
- tight clothing
