Session 5 - Function Of The Stomach

Question 1

What are the basic functions of the stomach?

Answer 1

Receive food - short term storage facility
Disrupt food - vigorous contractions of smooth muscle
Continue digestion - mainly of proteins
Disinfect

Question 2

What cells are found in gastric pits?

Answer 2

Mucous cells
Parietal cells
Chief cells
G cells

Question 3

Describe the action of smooth muscle in the upper and lower stomach.

Answer 3

Upper stomach:

• sustained contractions
• creates basal tone

Lower stomach:

• strong peristalsis mixes stomach contents
• coordinated movements (contractions every 20 seconds or so, proximal to distal)

Question 4

The shape of the stomach is such that it is larger proximally than it is distally. What is the purpose of this?

Answer 4

Accelerates contents
Separates contents (lumps left behind)
Only liquid chyme ejected into duodenum

Question 5

What mechanism allows food to be stored in the stomach without raising intra-gastric pressure.

Answer 5

Receptive relaxation:

• vagally mediated relaxation of stomach
• food can enter stomach without increasing pressure too much
• prevents reflux of stomach contents during swallowing
• rugae allow distension

Question 6

How do the acidic conditions of the stomach help in digestion of proteins?

Answer 6

Acid denatures proteins
Activates proteases
Pepsinogen —> Pepsin

Question 7

What products are secreted into the stomach lumen?

Answer 7

HCl
Intrinsic factor
Mucus/HCO3-
Pepsinogen (pepsin)

Question 8

What do parietal cells secrete?

Answer 8

HCl and intrinsic factor

Question 9

What do G cells secrete?

Answer 9

Gastrin

Question 10

What do enterochromaffin like (ECL) cells secrete?

Answer 10

Histamine

Question 11

What do chief cells secrete?

Answer 11

Pepsinogen

Question 12

What do D cells secrete?

Answer 12

Somatostatin

Question 13

Describe the general cell distribution across the stomach.

Answer 13

Cardia:
- predominantly mucus cells

Fundus/body:

• mucus cells
• parietal cells (HCl)
• chief cells (pepsinogen)

Pylorus:

• G cells (gastrin)
• D cells (somatostatin)

Question 14

What things stimulate parietal cells to secrete HCl?

Answer 14

Gastrin (hormone released by G cells)
Histamine (paracrine signalling molecule, released by ECL cells)
ACh (neurotransmitter released by the vagus nerve)

Question 15

What receptor does gastrin bind to?

Answer 15

CCK receptor

Question 16

What stimulates G cells to release gastrin?

Answer 16

Peptides/amino acids in the stomach

Vagal stimulation - acetylcholine and gastrin-releasing peptide (GRP)

Question 17

How does somatostatin inhibit acid secretion?

Answer 17

Somatostatin acts on G cells, inhibiting their release of gastrin. This therefore inhibits HCl release as there is less gastrin to act on parietal cells.

Question 18

How is HCl production by parietal cells inhibited?

Answer 18

When food leaves the stomach pH drops (food acts as a buffer).
Low pH activates D cells, these release somatostatin which inhibits G cells and ECL cells (less gastrin and histamine).
Reduced stomach distension due to less food causes reduced vagal activity.

Question 19

What are the phases of digestion?

Answer 19

Cephalic
Gastric
Intestinal

Question 20

What happens in the cephalic stage of digestion?

Answer 20

During the cephalic stage of digestion, gastric secretion increases before food even enters the stomach. It results from smelling, tasting, chewing and swallowing causing parasympathetic stimulation to parietal cells and G cell via the vagus nerve. This ‘anticipation of food’ also causes a slight increase in gastric motility.

Question 21

What happens in the gastric phase of digestion?

Answer 21

Distension of the stomach stimulates the vagus nerve
- this stimulates parietal cells and g cells
Presence of amino acids and small peptides
- stimulates G cells
Food acts as a buffer in the stomach
- removes inhibition on G cells

Enteric nervous system and gastrin cause strong smooth muscle contractions to occur

Question 22

What happens in the intestinal stage of digestion?

Answer 22

Chyme initially stimulates gastrin secretion.
Soon overtaken by the inhibition of G cells
- presence of lipids activates the enterogastric reflex —> reduces vagal stimulation
- chyme stimulates CCK and secretin

Question 23

In what ways does the stomach defend itself from digestion?

Answer 23

Mucus/HCO3- release
Released by 
- surface mucus cells
- neck cells in gastric glands
Forms thick alkaline viscous layer that adheres to the epithelium, therefore the epithelial surface is kept at a higher pH. 

High turnover of epithelial cells
- helps keep epithelia intact

Prostaglandins
- maintain mucosal blood flow, supplying the epithelium with nutrients

Question 24

How do non-steroidal anti inflammatory drugs (NSAIDs) such as aspirin increase the risk of peptic ulcers?

Answer 24

NSAIDs inhibit prostaglandin release, reducing blood flow to gastric epithelium meaning fewer nutrients reach epithelial cells and there is a reduced turnover.

Question 25

What things can breach the stomachs defences?

Answer 25

Alcohol
- dissolves the mucus layer
Helicobacter pylori
- causes chronic active gastritis
NSAIDs
- inhibit prostaglandin