Gastrointestinal Tract 3 - Stomach

Question 1

How does the stomach do chemical breakdown

Answer 1

• Secretes pepsinogen, Converted to pepsin (protein digesting enzyme)

Secretes HCl for

• Dissolving food
• Partially digesting macromolecules in food
• Sterilization of food

Question 2

Stomach controls

Answer 2

• rate food enters SI

- intrinsic factor secretion for B12

Question 3

Is there absorption that occurs across the stomach

Answer 3

Very little

Question 4

Stomach Components

Answer 4

Fundus and body:

• Thin layer of smooth muscle
• secrete: mucous, pepsinogen, HCl

Anthrum:

• thicker smooth muscle layer
• secretes: mucous pepsinogen, gastrin

Question 5

Major secretions of the stomach

Answer 5

• mucous: prevent self digestion
• HCl: hydrolysis of proteins
• pepsinogen: digests proteins

Question 6

Minor secretions of the stomach

Answer 6

• Intrinsic factor-for B12 absorption
• Gastrin: (endocrine) stimulates HCl production and stomach motility
• Histamine: (paracrine) stimulate HCl
• somatostin: (paracrine) inhibits HCl

Question 7

Generalized gastric glands

Answer 7

• mucous cells
• parietal cells
• chief cells
• enteroendrocrine cells
• enterochromaffin-like cells (ECL) cells
• D cells

Question 8

Chief cells

Answer 8

• Gastric glands in all regions
• Secrete pepsinogen
• Inactive precursor to pepsin (zymogen)
• Pepsinogen cleaved by acid to pepsin, which accelerates protein digestion.

Question 9

Enteroendocrine cell

Answer 9

• Gastric glands in antrum
• AKA G cells
• Secretes gastrin (hormone)
• Stimulates HCl production by parietal cell - Stimulates GI motility

Question 10

D cells

Answer 10

• Gastric glands in all regions (more in antrum)

- Secrete somatostatin: Negative regulator of HCl secretion

Question 11

Enterochromaffin-like (ECL) cells

Answer 11

• Gastric glands in all regions (more in antrum)
• Secrete histamine
• stimulates HCl release

Question 12

Parietal cell

Answer 12

• Found in gastric glands contained in the fundus/body regions
• AKA, oxyntic cell
• Secretes HCl and intrinsic factor
• Canaliculi increase the surface area of the cells and maximize secretion into the stomach lumen

Question 13

Parietal cell during secretion

Answer 13

1) Acid secretion requires energy-lots of mitochondria

2) Actively secreting cell has better defined “canaliculus”

Question 14

Acidification of the Stomach Lumen

Answer 14

• Stomach secretes ~ 2L of 0.1 M H+Cl- /day
• Lumen pH 1 versus cytosol pH 7
• Multiple transporters/channels involved to acidify stomach while maintaining neural pH in cell

Question 15

Acidification in the stomach lime through the parietal cells

Answer 15

1. H + /K+ ATPase
   - Luminal membrane.
   - Pumps H+ into lumen in exchange for K+ into cell.
   
   • ACTIVE transport (ions moved up concentration gradient)
   • electro neutral
2. Carbonic anhydrase (CA)
   - Catalyzes the formation of H2CO3 from H2O and CO2
   - H2CO3 dissociates into H+ (for secretion into lumen) and HCO3-
3. Cl-/HCO3- exchanger (secondary active transport)
   - Excess HO- is effluxed from the cell as HCO3- in exchange for Cl-.
   - Critical step (inconjunction with CA) for maintenance of neutral cellular pH
4. K+ channels
   - K+ recycled back into stomach lumen, via diffusion through channel, = loss of + charge.
5. Cl- channels
   - Cl-leaks back into stomach lumen via diffusion through channel = compensates for loss of + charge through K+ channels

Question 16

4 chemical messengers regulate the insertion of the H+/K+ ATPase into the plasma membrane of the parietal cell

Answer 16

• Gastrin (gastric hormone)
• Acetylcholine (neurotransmitter)
• Histamine (paracrine): increases gastrin and ACh
• Somatostatin (paracrine): Inhibits release of HCl, gastrin and histamine

Question 17

Pepsinogen secretion and activation

Answer 17

Pepsinogen secreted by chief cells (inactive precursor)

• Stimulated by ENS
• Parallels release of HCl

Cleaved and activated to pepsin by acidic pH in stomach lumen

Question 18

What is the advantage of inactive precursor secretion ??

Answer 18

Pepsin is active only at low pH- irreversibly inactivated when it enters small intestine

Question 19

Phases of gastric secretion

Answer 19

1) Cephalic phase:
- Anticipatory, excitatory, mainly via vagus

2) Gastric phase:
- Major phase, excitatory, mainly via gastrin

3) intestinal phase:
- Mainly inhibitory, due to presence of acid, fat, digestion products and hypertonic solutions in duodenum

Question 20

Acetylcholine, gastrin, and histamine all directly

Answer 20

INCREASE acid secretion by parietal cell

Question 21

Somatostatin inhibits

Answer 21

acid secretion by parietal cell

Question 22

Acetylcholine also INCREASES acid secretion by the parietal cell by:

Answer 22

• Stimulating the release of gastrin from G-cells (increasing stimulant)
• Stimulating the release of histamine from enterochromaffin like cells (ECL) (increasing stimulant)
• Inhibiting somatostatin release from D-cells (inhibiting inhibitor)

Question 23

Gastrin also INCREASES acid secretion by the

Answer 23

parietal cell by stimulating histamine

release (increasing stimulant)

Question 24

Once acid secretion is at a high rate

Answer 24

• Parasympathetic input will be reduced (cephalic phase)
• Negative feedback occurs for gastrin production (acid inhibits release)
• Somatostatin release increases

Question 25

Why doe somatostanin increase once acid segregation is at a high rate?

Answer 25

- Reduced parasympathetic inhibition of D-cell - Direct stimulation of somatostatin release by acid Somatostatin then: - Inhibits acid secretion from parietal cell - Inhibits histamine release from ECL cell - Inhibits gastrin release from G cell

Question 26

Gastric motility

Answer 26

- Consumption of meal —> smooth muscle relaxation —> stomach can increase to 1.5 L without —> pressure - Relaxation mediated by PS nerves to ENS - Food stimulates peristaltic waves - Closure of pyloric sphincter

Question 27

When Food in the stomach stimulates peristaltic waves:

Answer 27

- Weak contractions in body of the stomach - Powerful contraction in antrum - Mixes luminal contents and causes the closure of the pyloric sphincter

Question 28

For gastric motility, Closure of pyloric sphincter results in:

Answer 28

- Small amount of stomach contents released to duodenum - Most antral contents forced backward towards the body of stomach - Mixing of contents with enzymes and acid

Question 29

Does the stomach have pacemaker cells, and if so where?

Answer 29

YES, in smooth muscle layer.

Question 30

Does the Basic electrical rhythm cause significant contractions?

Answer 30

NO

Question 31

What determines the strength of contraction in the stomach ?

Answer 31

Excitatory hormones and NT which further depolarize the slow waves.

Question 32

Strength of contraction (and therefore degree of mixing) decided by

Answer 32

stimulatory input (to the antral smooth muscle cells).

Question 33

Vomiting

Answer 33

- Nausea, salivation, breath held in mid-inspiration - Glottis closes off trachea - Lower esophageal sphincter and esophagus - Diaphragm and abdominal muscles contract - Reverse peristalsis moves upper intestinal contents into stomach - Stomach contents move up through esophagus and out through mouth (soft palate is raised)

Question 34

Vomiting-beneficial and negative consequences | BENEFITS

Answer 34

BENEFITS: - Removal of harmful substances prior to absorption - Bacteria, toxins - Nausea and feeling bad should prevent individual from consuming noxious substance again NEGATIVE: - Dehydration - Loss of salts (electrolyte imbalance) - Metabolic alkalosis due to loss of H+ - Acid erosion of tooth enamel

Question 35

Ulcers are

Answer 35

damaged/eroded area of the GIT mucosa, usually in acidic regions (e.g., esophagus, stomach or most commonly the duodenum)

Question 36

Ulcers are caused by

Answer 36

- Imbalance between “aggressive” factors (acid, pepsin) and “protective” factors (mucus, HCO3-). - Helicobacter pylori infection major cause (chronic inflammation and erosion) - NSAIDs (e.g., aspirin, ibuprofen) which decrease prostaglandin production - smoking - excessive alcohol - “stress” (unlikely) - gastrinomas (rare)

Question 37

Treatment for ulcers:

Answer 37

- Antibiotics - H+/K+ ATPase inhibitor - Histamine (H2) antagonist - Prostaglandin type drugs

Question 38

Gastric bypass surgery

Answer 38

Removal / tieing off of the stomach, food goes straight to the small intestine, meaning u don’t NEED a stomach.