Gastric Acid Secretion Regulation

Question 1

Describe the different regions of the stomach

Answer 1

Cardiac and pyloric regions have glands that secrete mucus
Other divisions of the stomach= the body and the pylorus.
Body has epithelial cells with numerous tubular glands. Wall of the glands are lined with parietal cells which secrete HCl and intrinsic factor

Question 2

How is gastric acid made in the stomach lumen?

Answer 2

C02 diffuses in the blood and forms carbonic anhydrase, which dissociates.
HCO3- is exchanged for Cl- in the blood. This decreases acidity of venous blood from the stomach compared to blood serving it
Excess Cl- diffuses out into the stomach’s gastric glands through chloride channels. During this, K+/H+-ATPase pumps H+ into stomach lumen
Effect = net flow of H+ Cl- (HCl) out the parietal cell, into stomach lumen

Question 3

Describe the gastric secretions

Answer 3

Mucus: alkaline, thick and sticky. It ↑HCO3-, forms gel on epithelial surface to protect against H+ secretion
Rennin: curdles milk intocasein
Lipase: TGs → fatty acids+ glycerol

IF (prevents pernicious anaemia): absorbs vit B12
HCl: kills bacteria; acid denaturation of digested food; activates pepsinogen (protein digestion)

Question 4

What are the 3 phases of HCl secretion?

Answer 4

Cephalic phase
Gastric phase
Intestinal phase
HCl secretion is regulated by neuronal pathways and duodenal hormones

Question 5

Describe the cephalic phase

Answer 5

Cephalic phase (meal times- smell, sight, taste, chewing) promotes gastrin and ACh secretion due to vagal outflow from brain to stomach.
ACh and gastrin stimulate histamine release from ECL cells
ACh and gastrin act directly on parietal cells → HCl secretion

Question 6

Describe the gastric phase

Answer 6

Distension of stomach after eating stimulates the neural reflex, releasing ACh–> parietal cells–> HCl

Food ↑ peptide and aa conc. This promotes G cells to secrete gastrin. This acts on ECL cells–>histamine–> parietal cells–>HCl

Food in stomach also buffers acid, which blocks somatostatin secretion

Question 7

Describe the intestinal phase

Answer 7

Intestinal phase: Increased acidity of the duodenum inhibits enzymes, bicarbonate and bile salts
Distension of duodenum, hypertonic solution, aa’s, fatty acids, monosaccharides all inhibit acid secretion

Question 8

How is acid secretion inhibited during the intestinal phase?

Answer 8

Secretin and CCK inhibit acid secretion by the parietal cells or gastrin secretion by the G cells.
Somatostatin (stomach, intestine, delta cells of pancreas, hypothalamus, brainstem, hippocampus) inhibits G, ECL and parietal cells

Question 9

[HCl] can reach ____, depending on:

Answer 9

The [HCl] can reach 150mM, depending on:

Rate of secretion
Amount of buffering provided by the resting juice
Composition of ingested food
Gastric motility
Rate of gastric emptying
Amount of diffusion back into mucosa

Question 10

Describe the secretion of pepsin

Answer 10

Pepsin is stored as inactive pepsinogen in the chief cells of the gastric mucosa.
Inputs to chief cells from nerve plexus stimulate pepsinogen secretion
It is activated and secreted by chief cells if [H+] is high
Its shape is altered by high acidity which exposes its active site
Inactivated upon food entry in the small intestine (HCO3- and peptides neutralise the H+)