7.1 Gastric Physiology Flashcards
Give some functions of the stomach
-storage
-defence against infection
-mechanical digestion, where muscles in the stomach break food down to chunks
-chemical digestion, where muscles in the stomach break food down to molecular level
-gastric motility mechanisms
-feedbacks such as intrinsic factors that aid with absorption
Describe the different stomach regions and their respective luminal secretions
At the top of the stomach, there is the lower esophical sphincter and cardia. This area has luminal secretions of mucus and bicarbonate
Next is the fundus and body of the stomach. This area has luminal secretions from gastric glands (such as H+, intrinsic factor, mucus, bicarbonaye, pepsinogens and lipase)
Then the antrum and pylorus of the stomach. This area has luminal secretions from pyloric glands such as mucus and bicarbonate.
Describe the anatomy of glands in the luminal layer of the stomach
There are glastric glands deep down that are lined with different types of cells, such as enteroendocrine cells, parietal cells, chief cells and mucous neck cells. These gastric glands then lead to gastric pits. The secretions then make their way up through the gastric pit and into the epithelium. The epithelial surface is smooth
Describe enteroendocrine
Enteroendocrine cells are located at the bottom of glands and can be found throughout the GI tract. There are many types of of enteroendocrine cells producing a range of hormones (e.g gastrin, somatostatin, histamine and ghrelin) resulting in a range of different effects on the GI system.
Describe gastric juice and it’s composition
Gastric juice aids digestion and protects the body from microorganisms. Gastric juice is composed of inorganic and organic components. Gastric juice production levels. can be controlled, its composition depends on the secretory rates of cells such as chief cells and parietal cells (primarily) and enetroendocrine and mucous neck cells. In gastric juices at lower secretory levels, Na+ and Cl- levels are high and K+ and H+ are low. At higher secretory levels, H+ and Cl- levels are high and Na+ and K+ are low. Rate of gastric juice secretion varies between individuals
Describe chief cells
Chief cells are found near the bottom of gastric glands. They have 2 primary functions. They secrete gastric lipase to split short chain triglycerides into fatty acids and monoglycerides. They also secrete pepsinogen which is then converted to pepsin by hydrochloric acid, this pepsin breaks peptide bonds. Chief cells can be stimulated from many different sources.
Describe how the structure of parietal cells can change
When parietal cells are stimulated, their tubulovesicles merge with the cell membrane to form intracellular canliculi. These contain H+ pumps.
What are the functions of parietal cells
They secrete intrinsic factor which is a glycoprotein that is required for the absorption of vitamin B12 in the ileum.
They also secrete gastric acid which is made of hydrochloric acid. Hydrochloric acid has a very low pH that can only be found in stomach. It kills microbes, denatures proteins and activates enzymes.
Describe the mechanism of hydrochloric acid secretion from parietal cells.
-A Na/K -ATPase on the basolateral membrane brings Na out of the cell and K into the cell to establish concentration gradients
-The K+ then exits the cell via a K channel in the apical membrane
-A K/H+ -ATPase antiporter in the apical membrane then pumps K back into the cell and moves H+ out of the cell, creating a large H+ concentration in the lumen.
-In order to facilitate all this H+ being pumped into the lumen, carbonic anhydrase converts water and CO2 into H+ and bicarbonate.
-These bicarbonate ions are then moved out of the cell by a channel on the basolateral membrane in exchange for Cl- entering the cell
-The Cl- build up in the cell before exiting out a Cl- channel in the apical membrane
-So now there is lots of Cl- and H+ that have been released into the lumen, forming hydrochloric acid
-The final secretion contains water, hydrochloric acid and a bit of KCl and NaCl
What is the post prandial alkaline tide
Follow eating, the parietal cells are stimulated. Hence in the secretion of hydrochloric acid, lots of bicarbonate ions are moved out of the cells. These bicarbonate ions then move into the blood, causing an increase in the pH of the blood.
Describe some ways that the secretion of hydrochloric acid can be regulated
-the vagus nerve uses acetylcholine as it’s neurotransmitter to target M3 receptors on parietal cells, this can stimulate hydrochloric acid release
-G cells (a type of enteroendocrine cell) produce gastrin which binds to CCK2 receptors on parietal cells, stimulating release
-ECL cells (a type of enteroendocrine cell) produce histamine which binds to H2 receptors on parietal cells, stimulating release
-prostaglandin cells can inhibit ECL cells, hence reducing release
Describe some ways drugs can influence hydrochloric acid secretion
-arachidonic acid is what producesprostaglandin, NSAID drugs inhibit arachidonic acid, thus promoting HCl
-H2 receptor antagonists (e.g cimetidine or ranitidine) prevent histamine from stimulating HCl
-PPIs block H+ channels on apical membrane, reducing HCl
Describe mucous secreting cells.
They secret mucus which forms a protective barrier and has alkaline properties. It helps protect the epithelia from the acidic gastric juices. It is secreted from superficial epithelia and mucous neck cells
What are another mechanisms to protect epithelial cells from acidic secretions.
Tight junctions between epithelial cells also help act as protective barrier against acidic secretions
Stomach secretions can be inhibited to protect the stomach. Somatostatin produced from D cells (a type of enteroendocrine cell) can stop G cells (another type of enteroendocrine cell) from producing gastrin
