3- Stomarch

Question 1

3 functions of the stomarch

Answer 1

Digestion of macronutrients: this can be chemical (acid and enzymes) and mechanical (mixing and churning)
Storage reservoir for food: until downstream organs are ready to receive the stomach contents
Immunological protection: Strong acid helps to destroy ingested pathogens

Question 2

Where is gastric acid secreted from

Answer 2

fundus and body in high volume, up to 2 L per day.

Question 3

What feature of the stomach helps with mechanical digestion

Answer 3

extra oblique layer of smooth muscle

Question 4

How is stomach different went empty and full

Answer 4

In the empty state, the stomach is contracted and its mucosa and submucosa are thrown up into folds called rugae. Following consumption of food and fluids, as the volume of the stomach increases, the rugae are stretched and become flat.

Question 5

How are the stomach and oesophagus separated and how are they structured differently

Answer 5

The z-line is a visible threshold between epithelia of the oesophagus (stratified squamous cells) and the stomach (simple columnar cells). The oesophageal epithelium is light pink in colour, and functional as a wear and tear lining. The stomach lining is a bright red and more resistant to low pH.

The mucous gel lining of the stomach provides considerable protection against the corrosive acid.

Question 6

What is the function of mucous cells

Answer 6

Mucous cells are high in number and secrete a bicarbonate-rich mucous which helps to protect the stomach lining. The acidic environment in the lumen of the stomach is very low (pH 2-3), however 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

Question 7

What are parietal cells

Answer 7

These are the acid-secreting cells of the stomach. They exist in a quiescent (sleeping) state until activated. Then the high number of 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.

Question 8

What are 3 functions of stomach acid

Answer 8

The strong HCl has a number of useful functions: 1) to kill ingested pathogens; 2) activate protease zymogens; 3) alter protein structure to help digestion

Question 9

What does intrinsic factor do

Answer 9

Parietal cells also secrete intrinsic factor, a glycoprotein essential for the absorption of vitamin B12. Deficiency in this substance will lead to pernicious anaemia.

Question 10

What do chief cells do

Answer 10

These 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 then breaks dietary proteins into smaller peptide chains. Gastric lipase is an enzyme that digests fats by removing a fatty acid from a triglyceride molecule.

Question 11

What do g cells do

Answer 11

These are enteroendocrine cells found at the bottom of the gastric pits. G-cells release the hormone gastrin into the bloodstream in response to vagus nerve stimulation, the presence of peptides in the stomach, and stomach distension.

Question 12

What does gastrin do

Answer 12

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.

Question 13

What are Enterochromaffin-like cells (ECL)

Answer 13

These cells are 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.

Question 14

What are d cells

Answer 14

These enteroendocrine cells secrete somatostatin, which has a generally 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.

Question 15

What are gastric stem cells

Answer 15

These pluripotent cells are capable of differentiating into all of the different cells of the stomach, under the influence of different factors. The development pathway varies for different parts of the stomach

Question 16

How do parietal cells make Hcl

Answer 16

CO2 diffuses from the blood into the parietal cell and, in the presence of carbonic anhydrase, is combined with water to form carbonic acid, which dissociates into bicarbonate and a proton.
The bicarbonate is exchanged with a chloride ion in the interstitial space. This causes chloride ions to move into the stomach lumen via chloride channels.
To pump the protons into the lumen the cell needs to move potassium from the interstitial space to the lumen. This is achieved via a sodium potassium exchanger in the basolateral membrane, and chloride channels in the apical membrane. Now, potassium can be pumped into the cell in exchange for protons. The potassium then re-enters the lumen through the potassium channels and the secreted proton combines with chloride to form hydrochloric acid.

Question 17

What are the 3 stomach activity phases

Answer 17

Cephalic
Gastric
Intestinal

Question 18

What happens in the cephalic phase

Answer 18

Afferents: Sight, smell, taste and thought of food.

Efferents: Vagus nerve (neurons secreting ACh) stimulate secretion from mucous cells, chief cells, parietal cells and G-cells via the submucosal plexus. Gatsrin (from G-cells) and histamine (from ECL cells) also stimulate parietal cell secretion.

Effects: Small secretion for a few minutes

Question 19

What happens in the gastric phase

Answer 19

Afferents: Distension of stomach and chemoreception of nutrients and a reduced pH.

Efferents: Vagus nerve (neurons secreting ACh) stimulate secretion from mucous cells, chief cells, parietal cells and G-cells via the submucosal plexus and increase motility (mixing waves) via the myenteric plexus. Gatsrin (from G-cells) and histamine (from ECL cells) also stimulate parietal cell secretion.

Effect: 3-4 hours of gastric activity (secretion of acid, enzymes and hormones) and mechanical digestion.

Question 20

What happens in the intestinal phase

Answer 20

Afferents: Duodenal stretch and chemodetection of reduced pH and duodenal distension

Efferents: I-cells secrete cholecystokinin (CCK) and S-cells secrete secretin into the blood. This decreases parietal cell secretion and inhibits gastric motility and emptying. 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.