GI S4 (Done)

Question 1

What are the general functions of the stomach?

Answer 1

Store food

Disinfect food

Break down ingested food into Chyme via chemical and physical disruption

Question 2

What is the major challenge regarding the stomach’s function?

Answer 2

Designed to break down biological material

However it itself is made of biological material

Needs self protection

Question 3

List the major secretions of the stomach

Answer 3

Attack:

HCl

Pepsin (proteolytic)

Defense:

Mucus

HCO3-

Question 4

What are the cellular sources of secretions in the stomach?

Answer 4

Cells all found in Gastric Pits

**Chief cells - **Enzymes

Parietal/oxyntic cells - Acid

Neck cells - Mucus

**Endocrine cells - **Gastrin

Question 5

Describe the mechanism of production of acid by Oxyntic cells

Answer 5

Made from slightly alkaline plasma

1mol of acid produced for every 1 mol of alkali

Acid is secreted into stomach, alkali into blood

Water is split to form H+ and OH-

OH- + CO2 = HCO3-

H+ is produced in mitochondria

Question 6

Describe the export of H+ int the stomach

Answer 6

Oxyntic cells have lots of mitochondria, H+ production rate high

H+ cannot accumulate in cells

Oxyntic cells have canaliculi (invaginations of cell membrane into cell)

Canaliculi lined with Proton pumps

Expel H+ against large conc gradient (Requires lots of energy)

Question 7

What are the 3 main factors involved in control of H+ secretion in the stomach?

Answer 7

Gastrin

Histamine

Ach

Question 8

What is gastrin and how does it exert control over acid secretion from oxyntic cells?

Answer 8

Polypeptide Hormone (17 AA) secreted by G-cells in stomach

Binds to surface receptor of oxyntic cells

Stimulates acid and intrinsic factor secretion via secondary messenger pathway

Question 9

What cells release histamine and what is it’s function regarding control of H+ secretion?

Answer 9

Released from Mast cells

Binds to H2 receptors on Oxyntic cell surface

Stimulates acid secretion via cAMP pathway

Question 10

Where is Ach released from and how does it affect control of H+ secretion in the stomach?

Answer 10

Released from post-ganglionic parasympathetic neurones

Acts on muscarinic receptors on Oxyntic cell surface

Stimulates acid secretion via secondary messenger pathway

Question 11

What are the factors affecting gastrin secretion?

Answer 11

Stimulated by:

Peptides

Ach from intrinsic neurones (due to distention or acting directly on oxyntic cells)

Inhibited by:

Low pH in stomach (‘feedback control’)

Question 12

What is the clincal relevance of H2 receptors action on oxyntic cells?

Answer 12

H2 receptors are uncommon therfore antagonists are fairly specific (only reduce acid secretion)

H2 receptors are also a good target as Gastrin and Ach both stimulate histamine release

Therefore histamine acts as an amplifier of their action

Question 13

What are the 3 phases of control regarding the stomach immediately after ingestion of food?

Answer 13

Cephalic phase

Gastric phase

Intestinal phase

Question 14

Describe the cephalic phase

Answer 14

Detection of ingested food

Autonomic stimulation (Ach stimulates Oxyntic cells directly and via histamine)

Question 15

Describe the gastric phase of control

Answer 15

Food reaches the stomach

Stomach acid buffere by food so pH rises

This disinhibits gastrin

Stomach distends:

Stimulates intrinsic nerves which release Ach

Initial digestion releases peptides:

Stimulation of gastrin release

Histamine release stimulated by gastrin and Ach

Question 16

Descirbe the intestinal phase of stomach control

Answer 16

Stomach begins to empty

Chyme stimulates release from intestines of hormones which antagonise gastrin

Accumulation of acid in stomach inhibits gastrin release

Stomach returns to the low pH of its ‘between meals’ state

• • Can aggravate ulcers*
• • Night pain*

Question 17

Describe the defenses of the stomach against it’s own acid/enzymes

Answer 17

Mucus covering of the stomach is sticky and basic

Forms an unstirred layer that** **H+ ions can only diffuse into slowly

H+ in the unstirred layer reacts with basic groups on mucus and HCO3- secreted from the surface epithelial cells

pH at surface of mucosa well above 6

Question 18

What stimulates production of stomach defenses?

Answer 18

Mucus and HCO3- production stimulated by prostaglandins

Also promoted by most factors that promote acid secretion

Defenses match the attach

Question 19

What are some of the causes of breach of the unstirred layer/stomach defense and what is the primary concequence?

Answer 19

Causes:

Alcohol dissolves mucus

H. Pylori

NSAIDS inhibit prostaglandins therefore reduce defenses

Conequence:

Peptic Ulcer

Question 20

What drugs do we use to reduce acid secretion?

Answer 20

H2 receptor antagonists - E.g. Cimetidine

Proton pump inhibitors - E.g. Omeprazole

Question 21

How do we treat peptic ulcers?

Answer 21

Reduction fo acid secretion or tretment of H. Pylori infection with antibiotics

Question 22

What are the 3 main functions of stomach motility?

Answer 22

Relaxation to accomodate food

Contraction rhythymically to break down large food particles

Deliver chyme slowly to duodenum

Question 23

Describe control of stomach relaxation

Answer 23

Receptive relaxation

Neural reflex triggered by swallowing

Efferent pathway via vagus

Tension in resting stomach wall actively relaxed

Pressure in the stomach therefore doesnt rise as it fills to limit reflux

Due to all this, we can consume large meals (barring damage to vagus nerve)

Question 24

Describe the rhythmic contractions of the stomach

Answer 24

Pacemaker region in cardiac region of stomach

Longitudinal and cicrular muscle contraction 3 times a minute

Wave of peristalsis spreads towards antrum

Wave of peristalsis drives contents ahead of it

Accelerates as it travels

Eventually overtakes larger particles which are propelled back towards fundus

Question 25

Describe the delivery of chyme to the duodenum

Answer 25

Rhythmic stomach contraction (3x a min) leads to:

Small squirt of chyme ejected into the duodenum

Then wave of peristalsis reaches pylorus and closes it

Question 26

What are the foctors affecting rate of delivery of food from the stomach to duodenum?

Answer 26

Volume of each squirt determined by:

Rate of acceleration of each peristaltic wave

Affected by hormones from intestine

Gastric emptying slowed by:

Fat in Duodenum

Low pH in Duodenum

Hypertonicity in Duodenum

Rate of stomach emptying matched to rate of digestion