The stomach

Question 1

What are the functions of the stomach?

Answer 1

• Stores food temporarily
• Starts digestion of protein and produces chyme
• A little bit of carbohydrate and fat digestion due to salivary amylase and lingual lipase
• Innate defence due to strongly acidic environment

Question 2

What are the names of the different areas of the stomach?

Answer 2

• Fundus (most superior 3rd)
• Body (middle 3rd)
• Antrum (inferior 3rd)
• Cardia is region where oesophagus joins to stomach
• Whole of stomach lies very close to heart

Question 3

What type of cells lines the stomach?

Answer 3

• Simple columnar
• Stratified squamous epithelium lines oesophagus
• Abrupt transition between the two cell types

Question 4

What mechanisms help prevent reflux of stomach contents into the oesophagus?

Answer 4

• Lower oesophageal sphincter
• Muscles of crura of diaphragm
• Right crus of diaphragm loops around lower oesophageal sphincter and closes it when intra-abdominal pressure increases
• Oblique angle of oesophagus as it enters stomach

Question 5

What is the function of the pyloric sphincter?

Answer 5

• Very muscular
• Controls release of chyme into duodenum

Question 6

When do rugae appear?

Answer 6

• When stomach is relaxed
• Allow stomach to expand when food enters
• Keeps pressure constant

Question 7

What is receptive relaxation?

Answer 7

• Peristalsis in oesophagus warns stomach that entry of food is imminent
• Fundus distends and muscle relaxes
• Stomach fills without significant rise in pressure

Question 8

Describe the muscles of the stomach wall

Answer 8

• Circular and longitudinal muscle layer
• Innermost layer of oblique muscle
• Proximal part of stomach has much thinner walls
• Walls increase in thickness towards pyloric sphincter

Question 9

What do the muscles of the stomach wall allow?

Answer 9

• When stomach walls contract, food particles are accelerated towards distal portion of stomach
• Smaller particles are accelerated towards pyloric sphincter
• Larger particles remain in stomach for further digestion
• Allows food particles to be separated by size

Question 10

Describe the surface of the stomach

Answer 10

• Covered with gastric pits (invaginations of epithelium)
• These lead on to gastric glands
• Surface mucous cells secrete mucus to form a protective layer over epithelial cells

Question 11

Which cells are found in the gastric glands?

Answer 11

• Parietal cells - acid producing
• Chief cells - secreting pepsinogen
• G cells - secrete gastrin

Question 12

How is pepsin secreted into the stomach?

Answer 12

• Secreted by chief cells as pepsinogen
• Converted to its active form by acidic conditions of stomach

Question 13

How is the stomach protected?

Answer 13

• Stomach needs to defend itself from HCl and pepsin (avoid auto-digestion)
• Cells are regularly replaced
• Layer of mucus containing bicarbonate ions
• Rich blood supply to stomach removes acid

Question 14

How do parietal cells remove H+ ions?

Answer 14

• Exchange intra-luminal K+ ions for H+ ions
• Via H+/K+ ATPase or proton pump
• H+ removed from cell, K+ brought in

Question 15

How do we move parietal cells from resting phase to active phase?

Answer 15

• Apical surface of parietal cells contains K+ channels but no proton pumps
• Proton pumps are contained in tubulovesicles
• In order to enter active phase, proton pumps must be put in apical membrane
• Tubulovesicles fuse with apical membrane when stimulated
• Brings K+ channels and proton pumps together

Question 16

What are the adaptations of the apical membrane of parietal cells?

Answer 16

• Involutions call canaliculi
• Microvilli
• Increase SA

Question 17

What stimulates acid production?

Answer 17

1. Sensory triggers (smell, taste, site)
   - Cephalic phase of digestion
   - Accounts for 30% of HCl
2. Gastric triggers (stretch, amino acids/small peptides, food buffers acid)
   - Gastric phase of digestion
   - Accounts for 60% of HCl
3. Intestinal triggers (chyme in duodenum, presence of partially digested proteins_
   - Intestinal phase of digestion
   - Accounts for 10% of HCl

Question 18

Which 3 receptors does a parietal cell have?

Answer 18

1. Gastrin receptors
   - 2. Histamine receptors
2. Muscarinic (ACh) receptors

Question 19

How does gastrin stimulate the production of HCl?

Answer 19

• Peptides and amino acids in stomach lumen stimulate G cells to produce gastrin
• Gastrin enters bloodstream
• Comes back and binds to CCK receptor
• CCK and gastrin have very similar structure
• HCl production stimulated

Question 20

How does the vagal stimulation stimulate HCl production?

Answer 20

• ACh binds to muscarinic receptor on G cell causing release of gastrin
• Release of ACh occurs due to sensory triggers
• Gastrin releasing peptide produced due to stretch
• Binds to receptor on G cells - gastrin released

Question 21

How does histamine stimulate HCl production?

Answer 21

• Vagal stimulation /gastrin binding to CCK receptor causes entero-chromaffin like cell (ECL cell) to produce histamine
• Histamine binds with H2 receptor
• Stimulates acid production

Question 22

How is HCl secretion inhibited?

Answer 22

• Emptying of stomach means no stretch stimulation
• No release of gastrin releasing peptide
• D cell detects drop in pH as acid in stomach builds
• Produces somatostatin
• Somatostatin binds to G cell via receptor to inhibit gastrin production

Question 23

How is HCl produced in the parietal cell?

Answer 23

• Water and CO2 form carbonic acid
• Carbonic acid dissociates into H+ and HCO3-
• Due to action of carbonic anhydrase
• Cl- moves into cell via anion antiport protein on basolateral membrane
• Moves into lumen of stomach via diffusion through Cl- channel
• H+ and Cl- combine to form HCl

Question 24

What is the alkaline tide?

Answer 24

• Anion antiport protein on basolateral membrane of parietal cell brings Cl- into cell in exchange for HCO3-
  = HCO3- enters venous blod
• Raises pH