GI S4 (Done) Flashcards
In this deck, please keep in mind that Oxyntic cells are just Parietal cells. I prefer oxyntic cell as this is how I was taught at A level.
What are the general functions of the stomach?
Store food
Disinfect food
Break down ingested food into Chyme via chemical and physical disruption
What is the major challenge regarding the stomach’s function?
Designed to break down biological material
However it itself is made of biological material
Needs self protection
List the major secretions of the stomach
Attack:
HCl
Pepsin (proteolytic)
Defense:
Mucus
HCO3-
What are the cellular sources of secretions in the stomach?
Cells all found in Gastric Pits
**Chief cells - **Enzymes
Parietal/oxyntic cells - Acid
Neck cells - Mucus
**Endocrine cells - **Gastrin
Describe the mechanism of production of acid by Oxyntic cells
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
Describe the export of H+ int the stomach
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)
What are the 3 main factors involved in control of H+ secretion in the stomach?
Gastrin
Histamine
Ach
What is gastrin and how does it exert control over acid secretion from oxyntic cells?
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
What cells release histamine and what is it’s function regarding control of H+ secretion?
Released from Mast cells
Binds to H2 receptors on Oxyntic cell surface
Stimulates acid secretion via cAMP pathway
Where is Ach released from and how does it affect control of H+ secretion in the stomach?
Released from post-ganglionic parasympathetic neurones
Acts on muscarinic receptors on Oxyntic cell surface
Stimulates acid secretion via secondary messenger pathway
What are the factors affecting gastrin secretion?
Stimulated by:
Peptides
Ach from intrinsic neurones (due to distention or acting directly on oxyntic cells)
Inhibited by:
Low pH in stomach (‘feedback control’)
What is the clincal relevance of H2 receptors action on oxyntic cells?
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
What are the 3 phases of control regarding the stomach immediately after ingestion of food?
Cephalic phase
Gastric phase
Intestinal phase
Describe the cephalic phase
Detection of ingested food
Autonomic stimulation (Ach stimulates Oxyntic cells directly and via histamine)
Describe the gastric phase of control
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
Descirbe the intestinal phase of stomach control
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*
Describe the defenses of the stomach against it’s own acid/enzymes
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
What stimulates production of stomach defenses?
Mucus and HCO3- production stimulated by prostaglandins
Also promoted by most factors that promote acid secretion
Defenses match the attach
What are some of the causes of breach of the unstirred layer/stomach defense and what is the primary concequence?
Causes:
Alcohol dissolves mucus
H. Pylori
NSAIDS inhibit prostaglandins therefore reduce defenses
Conequence:
Peptic Ulcer
What drugs do we use to reduce acid secretion?
H2 receptor antagonists - E.g. Cimetidine
Proton pump inhibitors - E.g. Omeprazole
How do we treat peptic ulcers?
Reduction fo acid secretion or tretment of H. Pylori infection with antibiotics
What are the 3 main functions of stomach motility?
Relaxation to accomodate food
Contraction rhythymically to break down large food particles
Deliver chyme slowly to duodenum
Describe control of stomach relaxation
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)
Describe the rhythmic contractions of the stomach
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
Describe the delivery of chyme to the duodenum
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
What are the foctors affecting rate of delivery of food from the stomach to duodenum?
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
