Session 4 - The Stomach Flashcards
Describe the functions of the stomach
• Stores food
• Disinfects food
• Breaks food down into chyme
○ Chemical disruption (acid and enzymes)
Physical disruption (motility)
What is the structural component of the stomach through which acid is secreted?
Gastric pits, indentations in the stomach mucosa
What are the five cell types found in the gastric pits?
- Neck cells
- Parietal cells
- Chief cells
- G-cells
- Smooth muscle cells
Give five secretions of the gastric pits
- Hydrochloric acid
- Proteolytic enzymes
- Mucus
- HCO3-
Gastrin
What is the purpose of HCL and from where is it secreted?
• Keeps luminal pH <2
Parietal cells
What is the purpose of proteolytic enzymes and from where are they secreted?
- Non-specifically break down protein into peptides
* Chief cells
What is the purpose of mucus and from where is it secreted?
- Sticky (not easily removed from stomach lining)
- Basic, due to amine groups on proteins
Neck cells (surface cells)
What is the purpose of HCO3- and from where is it secreted?
- Secreted by surface cells into the mucus
- Provides a buffer for H+ ions
Neck cells (surface cells)
What is the purpose of gastrin and from where is it secreted?
• Binds to surface receptor on parietal cell, stimulating acid and intrinsic factor
What is a key issue when considering the difficulties surrounding the secretion of stomach acid?
• Most body fluids are slightly alkaline, so to secrete H+ ions they must be created in large quantities
How are H+ ions produced in gastric pits?
• Formed in the mitochondria of parietal cells by splitting water into H+ and OH-
What does the splitting of water into H+ and OH- do?
• OH- ions removed by combining with CO2 from metabolism to form HCO3-, which is exported to blood
What is the functions of canaliculus?
- Invaginations in cell walls into which protons are pumped by proton pumps
- Avoids accumulation of protons within the cell
What is useful about the proton pumps which line the canaliculi?
If inhibited they will reduce the amount of acid in the stomach
What three hormones stimulate parietal cells to release acid?
- Acetycholine
- Gastrin
- Histamine
Where is acetycholine released from and why?
- Postganglionic parasympathetic neurones
* Released as a result of distension of the stomach as food arrives and CNS stimulation after food stimuli sensed
What is Gastrin released from in the stomach and why?
- G-cells
* The presence of peptides and Ach stimulates its release
Outline the structure of gastrin
- 17 amino acid
* Binds to surface receptors on parietal cells
What inhibits gastin release from the stomach?
• Low pH
Where is histamine released from?
- Mast cells
* Diffuses locally to bind H2 surface receptors on parietal cells
How does histamine secretion stimulate acid secretion?
• Acid secretion stimulated by c-amp
What stimulates mast cells to release histamine?
Gastrin and Ach
What are the three phase of gastric secretion?
- Cephalic phase
- Gastric Phase
- Intestinal phase
Outline what occurs in the cephalic phase
- 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
Outline what occurs in the gastric phase
- 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
Outline what happens in the intestinal phase
- 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)
Outline two ways in which acid secretion can be reduced?
- Inhibition of histamine at H2 receptors
* Proton pump inhibitors
What does inhibitions of histamine at H2 receptors do?
• Removes the amplification of gastrin/Ach signal
What do proton pump inhibitors do?
Prevent H+ ions being pumped into parietal cell canaliculi
Why does the stomach need defences?
- Because the luminal pH is usually below 2
* Without any protection this would dissolve mucosa
Where is mucus secreted from in the stomach?
• Neck cells
What two properties of mucus make it good for defense?
• Sticky so not easily removed from the stomach lining
Basic, due to amine groups on the proteins
What does the mucus form which prevents acid reaching the stomach wall?
- 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
What is the pH of the surface cells?
6
What causes the secretion of mucus and HCO3- from neck and surface cells?
• Prostaglandins, which are promoted by factors that stimulate acid secretion
Give three factors which break down the stomach’s defences
- Alcohol
- H pylori
- NSADIs
What does alcohol do to the stomachs defences?
• Dissolves the mucus, allowing the acid to attack the stomach
What does H pylori do to stomach’s defences?
• Surface cells become infected, inhibiting mucus/HCO3- production
What do NSAIDs do?
• Inhibit prostaglandin, reducing defences
ASPIRIN
What occurs if the stomachs defences are breached?
Peptic ulcers
What is receptive relaxation?
- 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
What causes rhythmic contractions of the stomach?
- 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
What keeps larger lumps of food in the stomach in the fundus of the stomach?
- Accelerating peristaltic wave overtakes larger lumps, driving them back into fundus
- Chyme decanted into pyloric region
Outline gastric emptying
• Three peristaltic waves -> Three ejectd squirts of chyme occur a minute
What is squirt volume affected by?
• The rate of acceleration of peristaltic waves and hormones from the instestine
What is gastric emptying slowed by?
- Fat
- Low pH
- Hypertonicity in the duodenum
