Lecture 11 - Physiology of the stomach

Question 1

Where in the stomach are the walls thickest and why?

Answer 1

Antrum, to allow for more powerful contractions (segmentation) to mix food

Question 2

Sphincters of the stomach

Answer 2

Lower oesophageal sphincter (at the top of stomach), pyloric sphincter (doorway between stomach and duodenum)

Question 3

What is chyme?

Answer 3

food mixed with gastric juice

Question 4

Cells in gastric gland

Answer 4

goblet/neck cells, peptic/chief cells, oxyntic/parietal cells, epithelial cells

Question 5

zymogens

Answer 5

inactive precursors of enzymes

Question 6

What do chief cells secrete?

Answer 6

pepsinogen (zymogen), which is activated by HCL

Question 7

What do parietal cells secrete?

Answer 7

secrete HCL, needed for pepsinogen to become active

Question 8

Where in the stomach are there less parietal cells and why?

Answer 8

Antrum, food churning more important here.

Question 9

G cells

Answer 9

secrete gastrin (G for Gastrin)

Question 10

Enterochromaffin (mast-like) cells

Answer 10

secrete histamine

Question 11

D cells

Answer 11

secrete somatostatin

Question 12

Neck cells

Answer 12

secrete mucus to liquify contents

Question 13

Formation of HCL

Answer 13

-CO2 diffuses into Enterochromaffin (mast-like) cells
-CO2 combines with water via carbonic anhydrase enzyme forming carbonic acid
-Proton combines with hydroxide ion forming water and dissociates again and there is a build up of hydrogen ions
-H+ transported out of cell and into stomach lumen via proton pump
-Chloride and potassium ions diffuse out of cell via facilitated diffusion through proton pump
-

Question 14

Activation of Pepsin

Answer 14

-pepsinOGEN secreted by chief cells
-HCL (parietal cell) causes pepsinogen to become pepsin
-pepsin is an enzyme which catalyses the breakdown of protein into peptides (amino acids)

Question 15

What prevents the stomach from digesting itself?

Answer 15

-mucosal barrier prevents acid from leaking in and causing ulcers, mucus has a higher pH therefore provides localised neutralisation.
-Prostaglandin increases thickness of mucus and stimulates alkaline production and inhibit acid secretion

Question 16

How is the cephalic phase stimulated

Answer 16

Stimulated by sight, smell, taste and thought of food or a decrease in blood glucose

Question 17

What happens in the Cephalic phase?

Answer 17

-nerve impulses to the medulla oblongata cause parasympathetic neurones via vagus nerves to increase HCL and pepsin in stomach
- gastrin secretion also promoted in the antrum of stomach which stimulates further HCL and pepsinogen secretion

Question 18

How is the gastric phase stimulated?

Answer 18

by stomach distension due to the presence of food.

Question 19

What is triggered by stomach distension in gastric phase

Answer 19

triggers parasympathetic reflex leading to further HCL, pepsin and gastrin secretion.

Question 20

How is the intestinal phase stimulted?

Answer 20

by digested proteins/fats in the duodenum

Question 21

What happens in the intestinal phase of digestion?

Answer 21

-the presence of fat or low pH in duodenum inhibits gastric secretion.
-Chyme entering the duodenum containing fat or enough HCl to lower the pH to below 2
will trigger inhibition of gastric secretion via three main routes:
Neuronal impulses are sent to the medulla to decrease parasympathetic stimulation
of gastric glands.
(ii) Local reflexes in the gut wall lead to decreased secretion.
(iii) Release of three local hormones (secretin, gastric inhibitory peptide and
cholecystokinin) travel via the bloodstream to the gastric glands and inhibit secretion.

secretion decreases

Question 22

3 phases of digestion

(in order)

Answer 22

cephalic phase, gastric phase and intestinal phase

Question 23

Neuronal inhibition of HCL secretion

Answer 23

• partially digested protein and presence of fat in duodenum, combined with lower pH from gastric acid inhibits secretion by “enterogastric reflex”
• Reflex mediated by medulla oblongata
  leads to  parasympathetic stimulation

Question 24

Hormonal Inhibition of HCL secretion

Answer 24

• Secretin and Cholecystokinin released from duodenum
• GIP (gastric inhibitory peptide)
• Gastrone
• Glucagon
• VIP (vasoactive intestinal peptide)
• Somatostatin (paracrine agent) released in response to increase [HCL]

Question 25

prostoglandin

Answer 25

increases thickness of mucus and stimulates alkaline production and inhibit acid secretion