Oesophagus and stomach Flashcards
1
Q
What are the components of the GI system?
A
- parotid, sublingual and sub mandibular gland (salivary glands)
- oesophagus
- stomach
- liver
- gall bladder
- pancreas
- transverse colon, descending colon, ascending colon, sigmoid colon (colon)
- duodenum, ileum, jejunum (small intestine)
- appendix
- rectum
- anus
2
Q
What are the main layers of the gut wall?
A
- mucosa (epithelium, lamina propria: loose connective tissue, musclaris muscosae: thin layer of muscle)
- submucosa (connective tissue and nerve plexus)
- musclaris (smooth muscle and nerve plexus)
- serosa/adventia (connective tissue, epithelium)
3
Q
Which side is the mucosa towards?
A
the lumen
4
Q
At which vertebral level does the oesophagus start and finish?
A
Starts at C5 and ends at T10
5
Q
What are some structures that are located near the oesophagus?
A
trachea, aorta and diaphragm
6
Q
Is the trachea and pericardium anterior or posterior to the oesophagus?
A
anterior
7
Q
Is the descending aorta and thoracic duct anterior or posterior to the oesophagus?
A
posterior
8
Q
At which vertebral levels is the thoracic duct posterior?
A
Between T7 and T4
9
Q
What type of epithelium is in the oesophagus and why?
A
Non-keratinizing stratified squamous
- Withstands ‘wear & tear’ in extremes of temperatures and textures.
- Lubricates with mucus glands.
10
Q
What is the function of the oesophagus?
A
conduit for food, drink and swallowed secretions from the pharynx to the stomach
11
Q
What are the two main places where muscles are needed in the oesophagus?
A
- upper oesophageal sphincter
- lower oesophageal sphincter
Usually they are kept closed
12
Q
What is the composition of the upper oesophageal sphincter?
A
It is made of skeletal muscle and is under involuntary control
13
Q
What is the composition of the lower oesophageal sphincter?
A
It is made of smooth and skeletal muscle and is activated by the swallowing centre of the brain
14
Q
What is the purpose of the sphincter?
A
The thorax has a negative pressure while the gut is slightly above atmospheric pressure so the sphincters act to restrict the escape of substances from the stomach as it has a tendency to rise.
15
Q
How does the muscle change from the pharynx to the diaphragm?
A
from skeletal muscle to smooth
in between there is a mixture
16
Q
How is muscle organised to allow peristalsis to occur?
A
- Longitudinal and circular layers of muscle exist
- Circular muscle contracts above the food while it relaxes below the food
which enables peristalsis - Gravity has no effect on peristalsis
17
Q
The gastro-oesophageal junction
A
- Reflux is prevented by the diaphragm.
- There is an epithelial transition at the Z-line from stratified squamous to simple columnar in the stomach
- Stomach epithelium is bright red compared to pink in oesophagus
- The columnar cells are better adapted to secretion
- There are gastric folds called rugae in the stomach to allow for large changes in volume of the stomach
18
Q
What is the function of the stomach?
A
To break down food into smaller particles, hold food and release at controlled steady rates into the duodenum and kill pathogens
19
Q
What are the main regions of the stomach and roughly describe where they are?
A
- body (middle)
- fundus (top)
- pyloric antrum (second to end)
- pyloric canal (end)
- cardia (opening)
20
Q
What do the different regions of the stomach do?
A
- body and fundus: mucus, HCl and pepsinogen
- pyloric antrum: gastrin
- cardia and pyloric (canal and antrum) regions: mucus
21
Q
How much acid is made daily?
What is the pH at the lumen and epithelial surface?
A
- Epithelial surface = 6-7 pH
- Lumen = 1-2 pH.
- Approx. 2L acid per day.
22
Q
Mucous cells
A
- Mucous cells are high in number and secrete a bicarbonate-rich mucous which helps to protect the stomach lining
- Surface mucous cells and neck mucous cells
- The presence of mucous keeps the pH next to the lining much closer to 7
- The mucus lining also helps to protect the stomach lining from active lipase and proteases, which may interfere with the lipid bilayer and its membranous transporters
23
Q
Parietal Cells
A
- Acid-secreting cells of the stomach
- Quiescent state until activated
- Resting parietal cell: lots of mitochondria, cytoplasmic tubovesicles (contain H+/K+ ATPase pumps), internal canaliculi (extend to the apical surface)
- Secreting parietal cell:
tubovesicles fuse with the membrane and microvilli project into the canaliculi. - Tubovesicles in the cytoplasm fuse with the small invaginations on the apical surface to make complicated canalicular surface, with a large surface area for acid secretion
- Parietal cells also secrete intrinsic factor, a glycoprotein essential for the absorption of vitamin B12
- Deficiency in this substance will lead to pernicious anaemia
24
Q
What are the functions of HCl?
A
1) to kill ingested pathogens
2) activate protease zymogens
3) alter protein structure to help digestion
25
Chief cells
- Produce a protease zymogen (pepsinogen) and a lipase (gastric lipase)
- Pepsinogen is activated to pepsin in the presence of HCl in the gastric lumen; it is secreted as a precursor to prevent it auto digesting the chief cells
- Pepsin breaks dietary proteins into smaller peptide chains
- Gastric lipase is an enzyme that digests fats by removing a fatty acid from a triglyceride molecule
26
G cells
- Enteroendocrine cells found at the bottom of the gastric pits
- Release the hormone gastrin into the bloodstream in response to vagus nerve stimulation, the presence of peptides in the stomach, and stomach distension
27
How does the parietal cell secrete acid?
1. Na+/K+ ATPase pumps sodium out and potassium into the gastric lumen
2. HCO3-/Cl- pump, pumps bicarbonate out and chloride into the gastric lumen
3. ATP from the mitochondria provides energy for the cell to pump H+ into the gastric lumen (large concentration gradient).
4. Potassium then passes back into the cell.
5. H+ + Cl- = HCl.
28
What are the two types of motility in the stomach?
1. peristalsis
| 2. segmentation (mixing)
29
What would carbonic anhydride inhibitors do?
- They convert carbon dioxide and water to bicarbonate and protons
- It would increase the pH as less acid would be made
30
How is pepsinogen activated?
Pepsinogen is produces by chief cells and is activated by the HCl secreted by the parietal cells.
31
Gastrin production and what does it do?
- Gastrin is produced in the Pyloric Antrum by g-cells
- Acts as a local peptide hormone
- Stimulates histamine release from chromaffin cells in the lamina propria
- Gastrin travels through the blood to receptor cells in the stomach where it stimulates gastric secretion and motility
- Stimulation of smooth muscles by gastrin leads to stronger contractions of the stomach and the opening of the pyloric sphincter to move food into the duodenum
- Gastrin also binds to receptors on cells in the pancreas and gallbladder where it increases the secretion of pancreatic juice and bile
32
Enterochromaffin like cells
A type of neuroendocrine cell found deep in the gastric glands, usually in the vicinity of parietal cells. They secrete histamine which stimulates the secretion of acid from the parietal cells.
33
D cells
- Enteroendocrine cells secrete somatostatin, which has an inhibitory effect on gastrointestinal function
- Within the gastric gland somatostatin inhibits ECL production of histamine and parietal cell activity, both of which inhibit the secretion of hydrochloric acid
34
Gastric stem cells
Pluripotent cells that are capable of differentiating into all of the different cells of the stomach.
35
Cephalic phase
Stimulation to secrete gastric acid through thought of food, taste, smell, food in stomach.
1. Medulla to Vagus nerve:
- Stimulates parietal cells to secrete acid
- Stimulates ECL cells to release histamine
- Releases gastrin releasing peptide on G-cells
- Inhibits somatostatin release from D-cells.
CENTRALLY MEDIATED
Effect: Small secretion for a few minutes
36
Gastric phase
Mechanical stimulus
1. Acid secretion stimulated by distension of stomach and amino-acid presence in food
2. Vagus nerve stimulates secretion from mucous cells, chief cells, parietal cells and G-cells via the submucosal plexus and increase motility (mixing waves) via the myenteric plexus.
3. Gastrin (from G-cells) and histamine (from ECL cells) also stimulate parietal cell secretion
Same as during cephalic phase but more powerful
CENTRAL AND ENTERIC (local)
Effect: 3-4 hours of gastric activity (secretion of acid, enzymes and hormones) and mechanical digestion
37
Intestinal phase
Chyme entering small intestine
1. Acid secretion stimulated by small intestine distention
2. Much stimulation here is inhibitory and acts to raise pH of chyme so duodenal enzymes can function well
3. 3 hormones released if pH of chyme is too low; gastric inhibitory peptide, cholecystokinin, secretin
4. I-cells secrete cholecystokinin (CCK) and S-cells secrete secretin into the blood. This decreases parietal cell secretion and inhibits gastric motility and emptying
5. Stretch receptors input into the enteric nervous system, which reduces activation of the stomach
Effect: Gastric emptying slows down to allow downstream organs to deal with current contents
38
What does ranitidine do?
- Histamine release from ECL cells acts on H2 receptors on parietal cells, stimulating HCl release
- These H2 receptors are inhibited by ranitidine drugs
39
What does omeprazole do?
A proton pump inhibitor – inhibits H+ secretion by the H+ pumps
40
What is segmentation?
- 80% of stomach contractions
- Relatively weak contractions that have the purpose to mix ingested food with stomach secretions to form chyme
- The more fluid part of the chyme is pushed toward the pyloric sphincter
- The more solid part is pushed back toward the body of the stomach
- Stretching activates enteric nervous system
41
What is peristalsis?
- 20% of stomach contractions
- Purpose is to move food
- ANS involved
- Waves of peristalsis begin as gentle muscular contractions
- As the contractile waves near the end of the stomach, they become stronger and more forceful
- The muscle before the food contracts while the muscle after it relaxes
- Mainly circular muscle involved but also longitudinal
- As the peristaltic wave passes through the partly opened pyloric sphincter, it causes the chyme to move through it in a back and forth fashion
- This breaks up the larger materials left in the chyme
42
What initiates swallowing?
The swallowing centre in the medulla which sends signals to cause the upper oesophageal sphincter to relax. The upper is usually voluntary and further down the GI tract becomes involuntary and eventually by enteric nervous system
