Regulation and Disorders of Gastric Secretion Flashcards
Describe the stomach (anatomically), and what each part of it secretes.
It is made up of a fundus, body, antrum, and pylorus.
FUNDUS: has glands that secrete pepsinogen, mucus, and acid
CARDIAC-AREA: mucus-secreting
PYLORIC-AREA: mucus-secreting
BODY: contains chief cells (pepsinogen-secreting) and parietal cells (intrinsic factor-secreting)
ANTRUM: secretes mucus, pepsinogen and gastrin
[the cardiac area of the stomach is where the contents of the oesophagus enter the stomach]
How is gastric acid made in the stomach lumen?
HCO3- is exchanged for Cl- in the blood, decreasing the acidity of the venous blood from the stomach compared to the blood serving it.
The excess Cl- diffuses into the stomach through chloride channels as the H+ is pumped into the stomach lumen (K+/H+ ATPase pumps H+ out of the stomach lumen).
The net effect is the net flow of H+ and Cl- out of the parietal cells and into the stomach lumen.
List some gastric secretions and their functions.
MUCUS: alkaline, thick and sticky; increased HCO3-; forms a water-insoluble gel on epithelial surfaces, protecting against H+ secretion
RENNIN: curdles milk into casein clots (particularly in children)
LIPASE: converts triglycerides into fatty acids and glycerol
INTRINSIC FACTOR: aids in the absorption of Vit B12, preventing pernicious anaemia
HCL: kills bacteria; acid denaturation of digested food; activates pepsinogen (for protein digestion)
What are the two different ways in which we can control HCl secretion?
DIRECTLY: In the direct pathway, ACh, gastrin and histamine stimulate the parietal cell directly, triggering the secretion of H+ into the lumen.
INDIRECTLY: In the indirect pathway, ACh and gastrin also stimulate the ECL cells, resulting in the secretion of histamine. Histamine then acts on the parietal cell.
What are the three phases of digestion?
1) Cephalic
2) Gastric
3) Intestinal
Describe acid secretion regulation during the cephalic phase.
During the cephalic phase, smell, sight, taste, chewing, etc. stimulate ACh release.
The ACh stimulates histamine release from ECL cells. The ACh also acts directly on the parietal cells, stimulating HCl secretion.
Acid secretion decreases as the acidity of the lumen increase.
Explain why.
Acid can be damaging, so we need a way to regulate and curtail (decrease) HCl release. We do this by HCL-stimulating somatostatin-releasing cells (D cells).
Somatostatin inhibits ECL and G-cells to curtail the hypersecretion of acid.
Describe acid secretion regulation during the gastric phase.
During the gastric phase, increased distention of the stomach increases peptide concentration, which increases the acidity of the stomach.
The combination of H+ and proteins decreases the [H+]. By acting as a buffer, the proteins remove the inhibitory powers of HCl on gastric secretion and, hence, acid secretion.
This then increases gastrin-mediated acid secretion.
Describe acid secretion regulation during the intestinal phase.
During the intestinal phase, there is a balance of the secretory activity of the stomach and the digestive and absorptive capacities of the small intestine.
The high acidity of the duodenal contents reflexly inhibits acid secretion (since the increased acidity would inhibit the activity of digestive enzymes, bicarbonate and bile salts).
The distention of the duodenum, a hypertonic solution (indicating that the food has already been digested), amino acids, fatty acids, and monosaccharides all inhibit acid secretion.
Thus, the inhibition of acid secretion in the small intestine depends on:
- the composition of the cyme
- the volume of the chyme
How is acid secretion inhibited during the intestinal phase?
Short (within the ENS) and long (vagal) neuronal reflexes and hormones (enterogastrones) inhibit acid secretion by the parietal cells or gastrin secretion by the G cells, which is inhibited by somatostatin.
Increased sympathetic discharge is inhibitory.
Decreased parasympathetic discharge is stimulatory.
List some HCl secretion stimulants or factors that increase HCl secretion.
- histamine
- acetylcholine
- gastrin
- caffeine
- alcohol
- NSAIDs
- nicotine
- H. Pylori
- Zollinger-Ellison Syndrome
- hyperparathyroidism
- stress
[HCl] can reach 150 mM. What does this depend on?
- rate of secretion
- amount of buffering provided by the resting juice
- composition of the ingested food
- gastric motility
- rate of gastric emptying
- amount of diffusion back into the mucosa
In what ways is HCl essential for life?
- defence
- protein digestion: activates pepsinogen to pepsin
- stimulates flow of bile and pancreatic juice
A lack of HCl causes the failure of protein digestion (achlorhydria or hypochlorhydria = when the production of gastric acid in the stomach is low or absent).
What stimulates the secretion of pepsinogen?
There are parallels between acid secretion and pepsinogen secretion. The stimulators/inhibitors of acid secretion during the cephalic and intestinal phases exert the same effect on pepsinogen secretion.
Pepsin is secreted by chief cells in the form of pepsinogen (zymogen). It becomes activated if [H+] is high; the shape of the enzyme is altered by the high acidity which exposes its active site.
It is inactivated upon entry of food in the small intestine (HCO3- and peptides neutralise the H+).
What is the point of pepsin secretion?
It initiates the digestion of proteins by degrading food proteins into peptides. However, it is not required for food digestion.
There are other enzymes that can digest proteins, such as trypsin, chymotrypsin and elastases.
