Session 4 - The Stomach Flashcards Preview

Semester 3 - Gastrointestinal > Session 4 - The Stomach > Flashcards

Flashcards in Session 4 - The Stomach Deck (46):

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-



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?



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


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



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