Gastric secretions

Question 1

Major components of gastric juice

Answer 1

water
organic compounds: intrinsic factor, pepsinogen, lipase, mucus
Ions: Na, H, K, Cl, HCO3

Question 2

Oxyntic/gastric glands

Answer 2

Parietal
Chief (Peptic)
Mucous Neck
Paracrine

Question 3

Products of parietal glands

Answer 3

HCl IF

Question 4

Products of chief/peptic glands

Answer 4

pepsinogen, lipase

Question 5

Products of mucous neck glands

Answer 5

mucin

Question 6

Products of paracrine glands

Answer 6

histamine (ECL), somatostatin (D)

Question 7

Pyloric glands

Answer 7

Mucous
Endocrine
Paracrine

Question 8

Products of mucous glands

Answer 8

mucous/HCO3-

Question 9

Products of endocrine glands (pyloric)

Answer 9

gastrin (G)

Question 10

Products of paracrine glands (pyloric)

Answer 10

somatostatin (D)

Question 11

Major functions of stomach acid

Answer 11

bacteriostatic agent
activate pepsinogen
denature protein
facilitate absorption of vitamin B12, calcium

Question 12

Mechanism of acid secretion by the parietal cell

Answer 12

1. K+ dependent H+ ATPase transports H+ out of the cell in exchange for K+ at the secretory membrane
2. OH- remaining in cytoplasm is converted to HCO3- by carbonic anhydrase
3. HCO3- exchanged for Cl- at basolateral membrane surface, passes into bloodstream. HCO3- makes blood slightly alkaline

Question 13

Tubulovascular system

Answer 13

contain H/K ATPase and glycoprotein intrinsic factor
large amounts of tubulovesicles close to the cell membrane in parietal cells
Stimuli –> vesicles fuse with canaliculus, increase surface area for pumping out acid

Question 14

Conversion of pepsinogen –> pepsin

Answer 14

Pepsinogen secreted by chief cells into gastric lumen
Pepsinogen activated by acid –> pepsin
Pepsin can activate pepsinogen (autocatalysis)

Question 15

Components of the gastric mucosal barrier

Answer 15

mucus

surface active phospholipids

Question 16

Function of the gastric mucosal barrier

Answer 16

mucus: lubricant, acid neutralization, trapping alkaline epithelial cell secretions, prevents back diffusion of H+ across the mucosa

Surface active phospholipids: binds to mucus and repels protons

PGs: inhibits acid secretion, prevents surface epithelial cell exfoliation, increases mucosal blood flow, stimulates mucus and bicarbonate secretion, enhances synthesis of surface-active phospholipids that line the gastric mucosa

Question 17

Gastrin

Answer 17

originates in G cells (pyloric gland)
receptor on parietal cell, stimulate acid secretion
stimulate histamine release

Question 18

Histamine (gastric)

Answer 18

origin: ECL cells, (oxyntic gland), mast cells, neurons

H2 receptor on parietal cells –> acid secretion

Question 19

ACh (gastric)

Answer 19

origin: intrinsic neurons
M3 receptors on parietal cells –> acid secretion
stimulates histamine release

Question 20

Characteristics of H. pylori

Answer 20

High mutation rate
motility
ability to attach to epithelial cells
virulence factors
urease
evasion of immune response

Question 21

Stimulatory phases of gastric secretion

Answer 21

1. Cephalic
2. Gastric
3. Intestinal

Question 22

Cephalic phase - stimulatory

Answer 22

Prior to food being swallowed
Conscious thoughts/stimulation of olfactory, gustatory and mechanoreceptors

1. Sensory input stimulates hypothalamus
2. Parasympathetic motor output via vagus nerve –> stimulates parietal and chief cells (via ACh) and G cells (GRP) due to neurotransmitter release from enteric neuron

Question 23

Gastric phase - stimulatory

Answer 23

1. Food enters the stomach and activates stretch/mechanoreceptor-based reflex, both local and via the vago-vagal loop that stimulates secretion (neuronal component)
2. Food buffers gastric juice –> feeds back to activate parietal cells and the products of protein breakdown (peptones) directly stimulate G cells to produce gastrin (non-neuronal component)

Question 24

Intestinal phase - stimulatory

Answer 24

1. Food reaches proximal part of the small intestine
2. Peptone in the chyme stimulates G cells located in duodenal mucosa to produce more gastrin –> acts on parietal & chief cells in the gastric glands (via gastrin)

Question 25

Cephalic phase - inhibitory

Answer 25

stomach filling (reflex) and feeling of satiety (higher centres)

Question 26

Gastric phase - inhibitory

Answer 26

pH drops in the antrum (food is being ground down into smaller chyme), stimulates “D” cells in the pyloric glands to release somatostatin: acts locally/paracrine to inhibit G cell production of gastrin

Question 27

Intestinal phase - inhibitory

Answer 27

1. Passage of chyme into duodenum stimulates a short neuronal “enterogastric reflex arc” through the sym celiac ganglion
2. Stimulate production of enterogastrones - hormones that feedback to inhibit G cell, parietal and chief cell activity and smooth muscle contraction

Enterogastrones: secretin (stimulated by acid), cholecystokinin/CCK (stimulated by amino acids and fat), GIP and GLP-1 peptides (stimulated by fat and glucose)

Question 28

Prostaglandin function in secretion

Answer 28

Inhibits acid secretion
Prevents surface epithelial cell exfoliation
increases mucosal blood flow
Stimulates mucus and bicarb secretion
Enhances synthesis of surface-active phospholipids