Intro to Stomach Flashcards
What route do stomach hormones take before they have effect?
Hormones go in to systemic circulation even if locally acting
- Travel by portal vein to liver
- Travel by hepatic vein to IVC
- IVC into heart → travel by aorta to target organ
What are neurocrine hormones of the GI? Give an example
Peptides released by neurones into the GI tract
e.g. Gastrin releasing peptide
What are the 2 broad categories of GI Hormones?
Gastrin and Secretin
Give 2 hormones of the gastrin family, state where they are released and what their function is
1. Gastrin
- released by G cells of stomach antrum
- Increase gastric acid secretion
2. Cholecystokinin (CCK)
- released by I cells of duodenum and jejunum
- Increase pancreatic/ gall bladder secretions
Give 2 hormones of the secretin family, state where they come from and their function
1. Secretin
- S cells of duodenum
- Increase Bicarbonate rich secretions from pancreas/ gall bladder
2. Gastric Inhibitory Peptide
- Cells of duodenum and jejunum
- Stimulated by sugar, aa and fats → increases insulin
What are the basic functions of the stomach?
- Recieve food - short term storage facility
- Distrupt food- vigorous contraction of smooth muscle
- Continue digestion - mainly proteins
- Disinfect- protect against infection
Identify the basic regions of the stomach
What is the benefit of the way that the oesogphagus enters the stomach?
- Oesophagus enters to the side and at an angle to the fundus
- Any increase in stomach pressure will push against the fundus and not the oesophagus
- Helps keep food down and stops reflux
What kind of epithlium lines the stomach?
Stratified Columnar epithelium
What is the function of the smooth muscle in the stomach?
- Mix and Grind contents
- Move contents along through the stomach
Explain how the basic structure of the stomach changes from the upper to lower parts. What is the functional benefit of this?
- The stomach becomes narrower and more muscular as it moves towards the pyloric sphincter
- Narrowing allows liquid elements to move faster towards the pyloric sphincter whilst larger lumps are left behind
How do muscular contractions differ between the upper and lower parts of the stomach?
- Upper Stomach - sustained contractions create basal tone
- Lower Stomach - stronger peristalsis, contractions every 20 seconds mixes stomach contents and moves them through stomach
What are Brunner’s Glands and what is their purpose?
Glands in the duodenum that alkaline fluid
Helps make hyperosmotic, acidic chyme more isotonic
Give the branches of the celiac trunk
Label the main venous drainage of the stomach
How do the acidic conditions of the stomach continue with digestion?
- Unravels (denatures) proteins → gives larger surface area for proteases to work
- Activates proteases
- Disinfect stomach contents
Intrinsic factor, HCl, Pepsinogen, Gastrin and Mucus are all secreted from the stomach. Which cells secrete each?
- Chief cells - secrete pepsinogen
- Parietal cells - secrete HCl and intrinsic factor
- G cells - secrete gastrin
- Mucous cells - secrete mucous
Different areas of the stomach predominantly secrete different substances. State the main locations where different substances are secreted.
- Cardia- mainly mucus
- Fundus/ body - mucus, HCl, Pepsinogen
- Pylorus- Gastrin and somatostatin
Explain how production of HCl is controlled
There are 3 ‘ON’ buttons for HCl secretion
- Gastrin secretion, binds to CCK receptor on Parietal cell to activate H+ pump
- Enterochromaffin like cells secrete Histamine which bind to H2 receptors on Parietal cells
- The Vagus Nerve - postganglionic nerve secretes ACh, which acts on M receptor
Explain the control of Gastrin secretion
G cells in the antrum are stimulated to release Gastrin by:
- Amino acids/ peptides in stomach lumen
- Vagus Nerve releasing ACh, binding to M receptors
- Gastrin Releasing Peptide
How is somatostatin released and what is it’s effect on stomach secretions?
- When food leaves the stomach the pH drops (as food acts as a buffer to acid
- Triggers somatostatin release by D cells
- Somatostatin Inhibits : G cells, ECL cells
- Redcued distension of stomach reduces vagal activity
Explain how HCl is produced by a parietal cell
- H2O split into H+ and OH- ions
- H+ ions pumped into stomach lumen by the H+/ K+ ATPase (pumps 1 H out for 1 K in)
- OH- ion combines with CO2 to form HCO3-
- HCO3- pumped into blood by Anion antiport channel which brings Cl- into the cell
- Cl- pumped into stomach lumen by Cl- channel
What is an alkaline tide?
At the time of eating venous blood becomes slightly more alkaline due to the production on stomach acid which causes HCO3- to be pumped into blood
What are the 3 phases of digestion?
- Cephalic
- Gastric
- Intestinal
Explain what happens in the cephalic stage of digestion
- Smell, taste, chewing and swallowing activate parasympathetic stimulation of the parietal cells by the vagus nerve
- Vagus also activates G cells and GRP released
Explain what happens in the Gastric phase of digestion
- Distension of the stomach stimulates the vagus nerve → stimulates parietal and G cells
- Presence of amino acids from food actives G cells
- Food acts as a stomach acid buffer so stops inhibition of gastrin production
- Enteric NS and Gastrin secretion cause smooth muscle contractions
Explain the intestinal phase of digestion
- Release of chyme into duodenum briefly stimulates gastrin secretion
- Soon after, secretin, CCK and the presence of lipids activating the enterogastric reflex inhibit Gastrin sectrion
- Vagal stimulation is reduced
How does mucus protect the stomach from digesting itself?
- Mucin secreted by foveolar cells adheres to epithelium to prevent physcial damage from food
- HCO3- ions secreted into mucus, buffers against acid
Apart from mucus, how does the stomach protect itself against digestion?
- Rich blood supply - removes and buffers acid that breaches mucus layer
- Prostaglandins promote the blood buffer process
- High epithelial turnover
What kind of things will breach the stomachs defences and how do they do so?
- Alcohol- dissolves the mucus layer
- Heliobacter pylori- causes chronic gastritis
- NSAIDs - inhibit prostaglandins important for maintaining rich blood supply
