Lecture 11 - Physiology of the stomach Flashcards
Where in the stomach are the walls thickest and why?
Antrum, to allow for more powerful contractions (segmentation) to mix food
Sphincters of the stomach
Lower oesophageal sphincter (at the top of stomach), pyloric sphincter (doorway between stomach and duodenum)
What is chyme?
food mixed with gastric juice
Cells in gastric gland
goblet/neck cells, peptic/chief cells, oxyntic/parietal cells, epithelial cells
zymogens
inactive precursors of enzymes
What do chief cells secrete?
pepsinogen (zymogen), which is activated by HCL
What do parietal cells secrete?
secrete HCL, needed for pepsinogen to become active
Where in the stomach are there less parietal cells and why?
Antrum, food churning more important here.
G cells
secrete gastrin (G for Gastrin)
Enterochromaffin (mast-like) cells
secrete histamine
D cells
secrete somatostatin
Neck cells
secrete mucus to liquify contents
Formation of HCL
-CO2 diffuses into Enterochromaffin (mast-like) cells
-CO2 combines with water via carbonic anhydrase enzyme forming carbonic acid
-Proton combines with hydroxide ion forming water and dissociates again and there is a build up of hydrogen ions
-H+ transported out of cell and into stomach lumen via proton pump
-Chloride and potassium ions diffuse out of cell via facilitated diffusion through proton pump
-
Activation of Pepsin
-pepsinOGEN secreted by chief cells
-HCL (parietal cell) causes pepsinogen to become pepsin
-pepsin is an enzyme which catalyses the breakdown of protein into peptides (amino acids)
What prevents the stomach from digesting itself?
-mucosal barrier prevents acid from leaking in and causing ulcers, mucus has a higher pH therefore provides localised neutralisation.
-Prostaglandin increases thickness of mucus and stimulates alkaline production and inhibit acid secretion
How is the cephalic phase stimulated
Stimulated by sight, smell, taste and thought of food or a decrease in blood glucose
What happens in the Cephalic phase?
-nerve impulses to the medulla oblongata cause parasympathetic neurones via vagus nerves to increase HCL and pepsin in stomach
- gastrin secretion also promoted in the antrum of stomach which stimulates further HCL and pepsinogen secretion
How is the gastric phase stimulated?
by stomach distension due to the presence of food.
What is triggered by stomach distension in gastric phase
triggers parasympathetic reflex leading to further HCL, pepsin and gastrin secretion.
How is the intestinal phase stimulted?
by digested proteins/fats in the duodenum
What happens in the intestinal phase of digestion?
-the presence of fat or low pH in duodenum inhibits gastric secretion.
-Chyme entering the duodenum containing fat or enough HCl to lower the pH to below 2
will trigger inhibition of gastric secretion via three main routes:
Neuronal impulses are sent to the medulla to decrease parasympathetic stimulation
of gastric glands.
(ii) Local reflexes in the gut wall lead to decreased secretion.
(iii) Release of three local hormones (secretin, gastric inhibitory peptide and
cholecystokinin) travel via the bloodstream to the gastric glands and inhibit secretion.
secretion decreases
3 phases of digestion
(in order)
cephalic phase, gastric phase and intestinal phase
Neuronal inhibition of HCL secretion
- partially digested protein and presence of fat in duodenum, combined with lower pH from gastric acid inhibits secretion by “enterogastric reflex”
- Reflex mediated by medulla oblongata
leads to parasympathetic stimulation
Hormonal Inhibition of HCL secretion
- Secretin and Cholecystokinin released from duodenum
- GIP (gastric inhibitory peptide)
- Gastrone
- Glucagon
- VIP (vasoactive intestinal peptide)
- Somatostatin (paracrine agent) released in response to increase [HCL]
prostoglandin
increases thickness of mucus and stimulates alkaline production and inhibit acid secretion
