Session 4 - The Stomach

Question 1

Describe the functions of the stomach

Answer 1

• Stores food
• Disinfects food
• Breaks food down into chyme
○ Chemical disruption (acid and enzymes)

Physical disruption (motility)

Question 2

What is the structural component of the stomach through which acid is secreted?

Answer 2

Gastric pits, indentations in the stomach mucosa

Question 3

What are the five cell types found in the gastric pits?

Answer 3

• Neck cells
  
  • Parietal cells
  • Chief cells
  • G-cells
  • Smooth muscle cells

Question 4

Give five secretions of the gastric pits

Answer 4

• Hydrochloric acid
  
  • Proteolytic enzymes
  • Mucus
  • HCO3-

Gastrin

Question 5

What is the purpose of HCL and from where is it secreted?

Answer 5

• Keeps luminal pH <2

Parietal cells

Question 6

What is the purpose of proteolytic enzymes and from where are they secreted?

Answer 6

• Non-specifically break down protein into peptides

* Chief cells

Question 7

What is the purpose of mucus and from where is it secreted?

Answer 7

• Sticky (not easily removed from stomach lining)
  
  • Basic, due to amine groups on proteins

Neck cells (surface cells)

Question 8

What is the purpose of HCO3- and from where is it secreted?

Answer 8

• Secreted by surface cells into the mucus
  
  • Provides a buffer for H+ ions

Neck cells (surface cells)

Question 9

What is the purpose of gastrin and from where is it secreted?

Answer 9

• Binds to surface receptor on parietal cell, stimulating acid and intrinsic factor

Question 10

What is a key issue when considering the difficulties surrounding the secretion of stomach acid?

Answer 10

• Most body fluids are slightly alkaline, so to secrete H+ ions they must be created in large quantities

Question 11

How are H+ ions produced in gastric pits?

Answer 11

• Formed in the mitochondria of parietal cells by splitting water into H+ and OH-

Question 12

What does the splitting of water into H+ and OH- do?

Answer 12

• OH- ions removed by combining with CO2 from metabolism to form HCO3-, which is exported to blood

Question 13

What is the functions of canaliculus?

Answer 13

• Invaginations in cell walls into which protons are pumped by proton pumps
  
  • Avoids accumulation of protons within the cell

Question 14

What is useful about the proton pumps which line the canaliculi?

Answer 14

If inhibited they will reduce the amount of acid in the stomach

Question 15

What three hormones stimulate parietal cells to release acid?

Answer 15

• Acetycholine
  
  • Gastrin
  • Histamine

Question 16

Where is acetycholine released from and why?

Answer 16

• Postganglionic parasympathetic neurones

* Released as a result of distension of the stomach as food arrives and CNS stimulation after food stimuli sensed

Question 17

What is Gastrin released from in the stomach and why?

Answer 17

• G-cells

* The presence of peptides and Ach stimulates its release

Question 18

Outline the structure of gastrin

Answer 18

• 17 amino acid

* Binds to surface receptors on parietal cells

Question 19

What inhibits gastin release from the stomach?

Answer 19

• Low pH

Question 20

Where is histamine released from?

Answer 20

• Mast cells

* Diffuses locally to bind H2 surface receptors on parietal cells

Question 21

How does histamine secretion stimulate acid secretion?

Answer 21

• Acid secretion stimulated by c-amp

Question 22

What stimulates mast cells to release histamine?

Answer 22

Gastrin and Ach

Question 23

What are the three phase of gastric secretion?

Answer 23

• Cephalic phase
  
  • Gastric Phase
  • Intestinal phase

Question 24

Outline what occurs in the cephalic phase

Answer 24

• The sight, small and act of swallowing food activates the parasympathetic nervous system
  
  • Ach released, which directly stimulates parietal cells to release acid, and sitmulates release of histamine

Question 25

Outline what occurs in the gastric phase

Answer 25

* Food reaches stomach, causing distension, the release of Ach and thus parietal cells to release acid * Arrival of food buffers small amount of stomach acid, causing luminal pH to rise * Disinhibits Gastrin (increase in acid) * Acid and enzymes then act on proteins to produce peptides, further stimulating gastrin

Question 26

Outline what happens in the intestinal phase

Answer 26

* Chyme leaves stomach, stimulating release of cholecystokinin and gastric inhibitory polypeptide from the intestines * These antagonise gastrin (decrease stomach acid) * Small amount of acid in stomach no longer buffered by food, low pH inhibits gastrin (decreased acid)

Question 27

Outline two ways in which acid secretion can be reduced?

Answer 27

* Inhibition of histamine at H2 receptors | * Proton pump inhibitors

Question 28

What does inhibitions of histamine at H2 receptors do?

Answer 28

• Removes the amplification of gastrin/Ach signal

Question 29

What do proton pump inhibitors do?

Answer 29

Prevent H+ ions being pumped into parietal cell canaliculi

Question 30

Why does the stomach need defences?

Answer 30

* Because the luminal pH is usually below 2 | * Without any protection this would dissolve mucosa

Question 31

Where is mucus secreted from in the stomach?

Answer 31

• Neck cells

Question 32

What two properties of mucus make it good for defense?

Answer 32

• Sticky so not easily removed from the stomach lining Basic, due to amine groups on the proteins

Question 33

What does the mucus form which prevents acid reaching the stomach wall?

Answer 33

* An 'unstirred layer' that ions cannot move through easily | * H+ ions slowly diffuse in and react with the basic groups on mucus and HCO3- by surface epithelial cells

Question 34

What is the pH of the surface cells?

Answer 34

6

Question 35

What causes the secretion of mucus and HCO3- from neck and surface cells?

Answer 35

• Prostaglandins, which are promoted by factors that stimulate acid secretion

Question 36

Give three factors which break down the stomach's defences

Answer 36

* Alcohol * H pylori * NSADIs

Question 37

What does alcohol do to the stomachs defences?

Answer 37

• Dissolves the mucus, allowing the acid to attack the stomach

Question 38

What does H pylori do to stomach's defences?

Answer 38

• Surface cells become infected, inhibiting mucus/HCO3- production

Question 39

What do NSAIDs do?

Answer 39

• Inhibit prostaglandin, reducing defences ASPIRIN

Question 40

What occurs if the stomachs defences are breached?

Answer 40

Peptic ulcers

Question 41

What is receptive relaxation?

Answer 41

* As food travels down the oesophagus, a neural reflex carried out by the vagus nerve triggers the relaxation of muscle in the stomach's wall, so pressure does not increase * This prevents reflux and allows us to consume large meals7

Question 42

What causes rhythmic contractions of the stomach?

Answer 42

* Has longitudinal and circular muscle driven by a pacemaker in the cardiac regions * Pacemaker fires 3 times a minute, causing regular, accelerating peristaltic contractions from the Cardia -> Pylorus * This, combined with the stomach's funnel shape mixes the contents of the ttoamch and moves liquid chyme into the pyloric regions

Question 43

What keeps larger lumps of food in the stomach in the fundus of the stomach?

Answer 43

* Accelerating peristaltic wave overtakes larger lumps, driving them back into fundus * Chyme decanted into pyloric region

Question 44

Outline gastric emptying

Answer 44

• Three peristaltic waves -> Three ejectd squirts of chyme occur a minute

Question 45

What is squirt volume affected by?

Answer 45

• The rate of acceleration of peristaltic waves and hormones from the instestine

Question 46

What is gastric emptying slowed by?

Answer 46

* Fat * Low pH * Hypertonicity in the duodenum