Upper Digestive Tract Secretion

Question 1

How much saliva is produced each day?

Answer 1

1-1.5L

Question 2

What does decreased function of the salivary glands lead to?

Answer 2

• dry mouth
• dental caries
• infections of buccal mucosa

Question 3

What are amylases?

Answer 3

enzymes in the saliva that are active between pH 4-11

Question 4

How does the parasympathetic NS mediate salivary secretion?

Answer 4

by the release of ACh which binds to muscarinic receptors which cause watery saliva to be secreted

Question 5

How does the sympathetic NS mediate salivary secretion?

Answer 5

by the release of noradrenaline that binds to alpha and beta adrenergic receptors which cause thicker saliva to be secreted

Question 6

What are the 7 organic constituents of saliva?

Answer 6

• mucin
• alpha-amylase
• RNAse and DNAase
• lingual lipase
• lactoferrin
• secretory IgA
• lysozyme

Question 7

What is HCl produced by?

Answer 7

parietal cells

Question 8

What does HCl do?

Answer 8

• kill most ingested
  bacteria
• cleave pepsinogens to pepsins

Question 9

What are pepsinogens produced by?

Answer 9

chief cells in the stomach

Question 10

What are intrinsic factors?

Answer 10

proteins that help the intestines absorb vitamin B12, which is the only gastric secretion required for life

Question 11

What is gastrin released by?

Answer 11

G cells in the stomach and duodenum

Question 12

What happens to gastrin?

Answer 12

it is secreted straight into the blood and does not enter the lumen

Question 13

What does mucus do?

Answer 13

protect gastric mucosa from mechanical and chemical destruction

Question 14

What is mucus?

Answer 14

a thin layer of fluid that covers the luminal surface of the airway

Question 15

What is the composition of gastric juice?

Answer 15

99.5% water and 0.5% solids

Question 16

What are the inorganic substances of gastric juice?

Answer 16

• HCl
• electrolytes
• bicarbonate
• sulphate

Question 17

Where is rennin found and what does it do?

Answer 17

in infants to help with milk protein digestion

Question 18

How does the rate of gastric secretion affect the ionic composition of gastric juice?

Answer 18

• higher rate = higher [H+]
• lower rate = lower [H+]

Question 19

What can prolonged vomiting lead to?

Answer 19

hypokalaemia since [K+] is higher in gastric juice than plasma

Question 20

What is the basal and max rate of gastric acid secretion?

Answer 20

• basal = 1-5 mM/h
• max = 6-40 mM/h

Question 21

What condition will lead to a higher gastric rate than normal?

Answer 21

duodenal ulcer

Question 22

What conditions will lead to a lower gastric rate than normal?

Answer 22

gastric ulcer and pernicious anaemia

Question 23

What are the 3 stimulants of gastric acid secretion?

Answer 23

ACh, gastrin and histamine

Question 24

What are the functions of gastrin?

Answer 24

• stimulate HCl secretion
• increase gastric and intestinal motility
• increase pancreatic secretion
• help with proper growth of GI mucosa

Question 25

What is the pH at which pepsins have the highest activity?

Answer 25

anything less than 3

Question 26

What do pepsins do?

Answer 26

digest 10-20% of total dietary protein

Question 27

When are pepsins irreversibly inactivated?

Answer 27

at duodenum pH

Question 28

What is intrinsic factor secreted by?

Answer 28

parietal cells in the fundus

Question 29

Where is the IF-B12 complex completely absorbed?

Answer 29

at the terminal ileum

Question 30

How is intrinsic factor stimulated?

Answer 30

by the same stimuli that evoke HCl secretion (ACh)

Question 31

What are mucins?

Answer 31

the main component of gastric mucus; large molecules with CHO side chains that are insoluble in acid pH and can be destroyed by pepsin

Question 32

What is the function of mucins?

Answer 32

to coat and lubricate the mucosal surface

Question 33

How are mucins regulated?

Answer 33

by the same mechanism that enhances HCl secretion (ACh) and mechanically stimulated by food

Question 34

What are the paracrine effects of prostaglandins?

Answer 34

- directly inhibit parietal cell secretion - mediate mucosal defence

Question 35

What are the 3 components of mucosal defence?

Answer 35

- stimulation of mucus, phospholipid and HCO3- secretion - enhancement of mucosal blood flow - stimulation of mucosal cell turnover

Question 36

What does prostaglandin deficiency predispose to?

Answer 36

gastric mucosal injury

Question 37

What does the gastric mucosal barrier do?

Answer 37

protect against abrasion, HCl and pepsin

Question 38

What is the gastric mucosal barrier?

Answer 38

a thick mucus layer (1mm) made up of mucins and HCO3-

Question 39

What does the unstirred layer of the gastric mucosal barrier do?

Answer 39

slow the inward diffusion of H+ and outward diffusion of HCO3-

Question 40

What is the pH of the gastric mucosal barrier at the cell surface and gastric juice in the lumen respectively?

Answer 40

- cell surface = 7 - lumen = 2

Question 41

What does the gastric mucosal barrier depend on?

Answer 41

- Ca2+ and cholinergic agonists to stimulate HCO3- secretion - adrenergic agonists to decrease HCO3- secretion - aspirin and NSAIDs to inhibit mucous and HCO3- secretion

Question 42

What does the rate of repair of the gastric mucosal barrier depend on?

Answer 42

the extent of the injury; can be between 48 hours to 3-5 months

Question 43

Give examples of ulcer therapy

Answer 43

- neutralising HCl (antacids) - prevent acid release (omeprazole or cimetidine) - helicobacter pylori bacillus (antibiotic) - prostaglandin agonist (misoprostol)

Question 44

What does carbohydrate gastric digestion depend on?

Answer 44

the action of salivary alpha-amylase (halts at low pH)

Question 45

Why is fat gastric digestion minimal?

Answer 45

high acidity inhibits fats from forming emulsions

Question 46

What are the only substances absorbed during gastric absorption?

Answer 46

highly lipid-soluble substances e.g. ethanol

Question 47

What is gastritis?

Answer 47

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

Question 48

What factors cause gastritis?

Answer 48

- overeating - smoking - fatty foods, coffee, alcohol - lying down immediately after meal - tight clothing