Gastric Juice

Question 1

What is a zymogen/proenzyme?

Answer 1

Question 2

What is autocatalysis?

Answer 2

Catalysis of a reaction by one of the reactions own products

Question 3

What is a paracrine hormone?

Answer 3

Question 4

What is an endocrine hormone?

Answer 4

travels in the blood to affect remote organs

Question 5

What are the main functions of the stomach?

Answer 5

Question 6

How is protein digested in the stomach?

Answer 6

Each end of the stomach is closed by the cardiac and pyloric sphincter - this allows creation of a microenvironment favourable of protein digestion
- pepsin (protease made from pepsinogen) digests proteins
- HCl creates optimum pH for protein digestion

Question 7

What problems, if unprotected, would pepsin cause in the stomach?

Answer 7

Protease so would break down chief cells

Question 8

What problems, if unprotected, would HCl cause in the stomach?

Answer 8

would dissolve parietal cells
attack any unprotected surface cells of the mucosa causing a peptic ulcer

Question 9

How do chief cells protect themselves from pepsin?

Answer 9

They store pepsin as an inactive proenzyme called pepsinogen (zygomen)
Pepsinogen is released into stomach lumen where pepsinogen is converted to pepsin
Negative feedback loops prevent constant release of gastric juices

Question 10

How does the stomach protect itself from HCl

Answer 10

Parietal cells make H+ and Cl- ions which are released into lumen to form HCl
Mucosal mucous cells secrete mucus containing HCO3- which creates a layer protecting the stomach from pepsin and HCl
Negative feedback loops prevent constant release of gastric juices

Question 11

Where are stomach glands found?

Answer 11

Gastric pits

Question 12

What are the different stomach glands and what do they secrete?

Answer 12

Parietal cells - H+ and Cl-
Chief cells - pepsinogen
G cells - gastrin
ECL cells - histamine
D cells (in surface mucosa) - somatostatin

Question 13

What are the stages of gastric secretion?

Answer 13

Cephalic phase
Gastric phase
Intestinal phase

Question 14

Describe the cephalic phase of gastric secretion

Answer 14

‘prepares’ stomach for food
parasympathetic fibres send signals via vagus nerves onto submucosal plexuses of stomach
- parietal cells release H+ and Cl-
- chief cells release pepsinogen
- G cells release gastrin (enters circulation to further increase H+, Cl- and pepsinogen release in a positive feedback mechanism)

Question 15

Describe the gastric phase of gastric secretion

Answer 15

-food arrives in stomach -> distension
- increased pH of gastric contents
- stretch and chemo receptors in mucosa
- mixing wave of muscular contractions
- takes hours

Question 16

Describe the intestinal phase of gastric secretion

Answer 16

• chyme exits stomach into SI (duodenum) reducing stomach distension
• gut-stomach (enterogastric) reflex reduces gastrin and contractions
• lipids/carbs stimualte intestinal release of enzymes for digestion and inhibitors of gastric secretions

Question 17

what stimulates the medulla oblongata in the cephalic phase of gastric secretion?

Answer 17

taste/smell of food
tactile sensation of food in mouth
chewing
thoughts of food

Question 18

How is pepsinogen activated in the stomach?

Answer 18

acidic environment from HCl converts pepsinogen to pepsin
Pepsin can then unfold more pepsinogens into pepsin (autocatalysis)

Question 19

Describe the hormonal control of the gastric secretion

Answer 19

Gastrin - released from G cells into blood in response to peptides in stomach and vagal stimulation
Histamine - secreted from ECL (enterochromaffin-like cells) in response to gastrin - this locally stimulates parietal cells to release H+ and Cl- (paracrine activity)

Question 20

Describe the parasympathetic NS effect on the stomach

Answer 20

Vagal stimulation:
- proximal stomach - suppresses contractions to allow dilation and food storage
- distal stomach - intense peristalsis
Vagal acetylcholine from parasympathetic impulses stimulates:
- parietal cells to release H+ and Cl-
- Chief cell to make and release pepsinogen
- G cells to make and release gastrin

Question 21

Describe the negative feedback loops that prevent the stomach from continually secreting

Answer 21

• a drop in pH below 4.5 -> secretin released from duodenum enteroendocrine cells
• secretin inhibits parietal and chief cells
• secretin stimulates buffer release from the pancreas
• less stretch of the stomach reduces local stimulation
• if the stomach pH falls below 2, D cells produce somatostatin which inhibits gastrin production
• enterogastric reflex reduces gastrin secretion and stomach contractions when food enters duodenum
• lipids and carbs in duodenum secrete CGK and gastric inhibitory peptide (GIP) which reduce secretions and slow contractions