The stomach Flashcards
1
Q
What are the functions of the stomach?
A
- Stores food temporarily
- Starts digestion of protein and produces chyme
- A little bit of carbohydrate and fat digestion due to salivary amylase and lingual lipase
- Innate defence due to strongly acidic environment
2
Q
What are the names of the different areas of the stomach?
A
- Fundus (most superior 3rd)
- Body (middle 3rd)
- Antrum (inferior 3rd)
- Cardia is region where oesophagus joins to stomach
- Whole of stomach lies very close to heart
3
Q
What type of cells lines the stomach?
A
- Simple columnar
- Stratified squamous epithelium lines oesophagus
- Abrupt transition between the two cell types
4
Q
What mechanisms help prevent reflux of stomach contents into the oesophagus?
A
- Lower oesophageal sphincter
- Muscles of crura of diaphragm
- Right crus of diaphragm loops around lower oesophageal sphincter and closes it when intra-abdominal pressure increases
- Oblique angle of oesophagus as it enters stomach
5
Q
What is the function of the pyloric sphincter?
A
- Very muscular
- Controls release of chyme into duodenum
6
Q
When do rugae appear?
A
- When stomach is relaxed
- Allow stomach to expand when food enters
- Keeps pressure constant
7
Q
What is receptive relaxation?
A
- Peristalsis in oesophagus warns stomach that entry of food is imminent
- Fundus distends and muscle relaxes
- Stomach fills without significant rise in pressure
8
Q
Describe the muscles of the stomach wall
A
- Circular and longitudinal muscle layer
- Innermost layer of oblique muscle
- Proximal part of stomach has much thinner walls
- Walls increase in thickness towards pyloric sphincter
9
Q
What do the muscles of the stomach wall allow?
A
- When stomach walls contract, food particles are accelerated towards distal portion of stomach
- Smaller particles are accelerated towards pyloric sphincter
- Larger particles remain in stomach for further digestion
- Allows food particles to be separated by size
10
Q
Describe the surface of the stomach
A
- Covered with gastric pits (invaginations of epithelium)
- These lead on to gastric glands
- Surface mucous cells secrete mucus to form a protective layer over epithelial cells
11
Q
Which cells are found in the gastric glands?
A
- Parietal cells - acid producing
- Chief cells - secreting pepsinogen
- G cells - secrete gastrin
12
Q
How is pepsin secreted into the stomach?
A
- Secreted by chief cells as pepsinogen
- Converted to its active form by acidic conditions of stomach
13
Q
How is the stomach protected?
A
- Stomach needs to defend itself from HCl and pepsin (avoid auto-digestion)
- Cells are regularly replaced
- Layer of mucus containing bicarbonate ions
- Rich blood supply to stomach removes acid
14
Q
How do parietal cells remove H+ ions?
A
- Exchange intra-luminal K+ ions for H+ ions
- Via H+/K+ ATPase or proton pump
- H+ removed from cell, K+ brought in
15
Q
How do we move parietal cells from resting phase to active phase?
A
- Apical surface of parietal cells contains K+ channels but no proton pumps
- Proton pumps are contained in tubulovesicles
- In order to enter active phase, proton pumps must be put in apical membrane
- Tubulovesicles fuse with apical membrane when stimulated
- Brings K+ channels and proton pumps together
16
Q
What are the adaptations of the apical membrane of parietal cells?
A
- Involutions call canaliculi
- Microvilli
- Increase SA
17
Q
What stimulates acid production?
A
- Sensory triggers (smell, taste, site)
- Cephalic phase of digestion
- Accounts for 30% of HCl - Gastric triggers (stretch, amino acids/small peptides, food buffers acid)
- Gastric phase of digestion
- Accounts for 60% of HCl - Intestinal triggers (chyme in duodenum, presence of partially digested proteins_
- Intestinal phase of digestion
- Accounts for 10% of HCl
18
Q
Which 3 receptors does a parietal cell have?
A
- Gastrin receptors
- 2. Histamine receptors - Muscarinic (ACh) receptors
19
Q
How does gastrin stimulate the production of HCl?
A
- Peptides and amino acids in stomach lumen stimulate G cells to produce gastrin
- Gastrin enters bloodstream
- Comes back and binds to CCK receptor
- CCK and gastrin have very similar structure
- HCl production stimulated
20
Q
How does the vagal stimulation stimulate HCl production?
A
- ACh binds to muscarinic receptor on G cell causing release of gastrin
- Release of ACh occurs due to sensory triggers
- Gastrin releasing peptide produced due to stretch
- Binds to receptor on G cells - gastrin released
21
Q
How does histamine stimulate HCl production?
A
- Vagal stimulation /gastrin binding to CCK receptor causes entero-chromaffin like cell (ECL cell) to produce histamine
- Histamine binds with H2 receptor
- Stimulates acid production
22
Q
How is HCl secretion inhibited?
A
- Emptying of stomach means no stretch stimulation
- No release of gastrin releasing peptide
- D cell detects drop in pH as acid in stomach builds
- Produces somatostatin
- Somatostatin binds to G cell via receptor to inhibit gastrin production
23
Q
How is HCl produced in the parietal cell?
A
- Water and CO2 form carbonic acid
- Carbonic acid dissociates into H+ and HCO3-
- Due to action of carbonic anhydrase
- Cl- moves into cell via anion antiport protein on basolateral membrane
- Moves into lumen of stomach via diffusion through Cl- channel
- H+ and Cl- combine to form HCl
24
Q
What is the alkaline tide?
A
- Anion antiport protein on basolateral membrane of parietal cell brings Cl- into cell in exchange for HCO3-
= HCO3- enters venous blod - Raises pH
