Regulation of the GI tract - secretion Flashcards
Digestion depends on extensive well regulated secretion of digestive juices. What aids this?
- epithelial cells within the walls of the stomach/intestine
- discrete large glands situated outside the tract - salivary gland, liver and pancreas
In order to aid enzymes present in the tract what do digestive juices have?
- digestive juices have a composition and pH that is well adapted to the characteristic of the enzymes in that part of the tract
What makes up digestive secretions?
- water (98%)
- ions
- mucus
- enzymes
(Wallace must ingest edam)
What are the 4 functions of saliva?
- lubrication
- antibacterial effect
- enzymic digestion
- pH regulation
How does saliva lubrication aid digestion?
- contains mucin which facilitates chewing and swallowing
What are the antibacterial substances in saliva?
- lysozyme and antibodies present in saliva
How does saliva enzymic digestion aid in digestion?
- amylase breaks down starch
How does saliva help pH regulation?
what is Ruminants pH?
- bicarbonate neutralises acid produced by bacteria
- pH = 8.2
Saliva secretion is under control by what?
- saliva secretion is entirely under neural control (automimic - parasympathetic)
Epithelial cells in the stomach produce what?
- Epithelial cells produce a mucus which is viscous and high in bicarbonate
What are the 4 types of cells found in the glandular part of the stomach?
- mucin producing cells
- parietal cells
- chief producing cells
- endocrine cells
(my people can eat)
What are the functions of mucin producing cells, parietal cells and chief cells?
- mucin producing cells produce alkaline mucus
- parietal cells produce hydrochloric acid
- chief cells produce pepsinogen (inactive zymogen for pepsin)
What are the 3 types of endocrine cells found in the glandular region of the stomach? what layer are they found in?
- G-cells which produce gastrin
- ECL-cells which produce histamine
- D-cells which produce somatostatin
- all of these cells found within the epithelial layer
What is the purpose of intrinsic factors within the stomach?
- Intrinsic factors help to ensure absorption of B12 in the stomach
How does somatostatin impact G cells?
- somatostatin has a negative impact on G-cells
- the more somatostatin produced, the fewer G cells there are
What are the 3 phases of gastric secretion?
- cephalic phase
- gastric phase
- intestinal phase
Describe the cephalic phase
- normally elicited by sight, smell, and taste of food.
- entirely mediated by the vagus nerve
Describe the gastric phase
- elicited by the presence of food in the stomach
- mediated via neural reflexes and hormones
Describe the intestinal phase
- initial stimulatory effect, followed by an inhibitory effect
- mediated via neural reflexes and hormones
During the intestinal phase of gastric secretions there is initially a stimulatory effect followed by an inhibitory effect why is this?
- to ensure the secretions are not continuous
The pancreas has both exocrine and endocrine functions what are these functions?
- exocrine - products are pancreatic juices
- endocrine - products are insulin, glucagon, somatostatin
Pancreatic juices have an aqueous components and an enzymic component - what do these components do?
- aqueous component - alkaline due to a high concentration of bicarbonate stimulated by secretin
- enzymic component - digestion for all major classes of foodstuffs
Proteases digest proteins what are the three proteases produced by the pancreas? give their inactive forms and active form
- trypsinogen > trypsin
- chymotrypsinogen > chymotripsin
- procarboxypeptidase > carboxypeptidase
What is the function of alpha amylase?
- to digest starch
Lipases digest fats, what are the 3 lipases produced by the pancreas?
- triacylglycerol hydrolase
- cholesterol ester hydrolase
- phospholipidase A2
What are the key points about the cephalic phase of pancreatic secretions?
- cephalic phase increases in association with feeding
- its mediated by the vagal nerves that run into the glands
- affected by gastrin secretion in the stomach
What are the key points about the gastric phase of the pancreatic secretions?
- increases in association with feeding
- mediated by the vagal nerves to the glands and gastrin secretion in the stomach
when does the intestinal phase of pancreatic secretion begin?
- when chyme enters the duodenum
How is the intestinal phase of pancreatic secretion mediated?
- mediated by secretin and CCK release
What is the function of secretin?
- inhibits gastrin release from G cells
- inhibits response of parietal cells to secretagogues
- stimulates pancreas to produce bicarbonate rich fluid
- increase production of bile by the liver
What is the function of CCK?
- inhibits secretions from parietal cells
- stimulates pancreas to increase enzyme secretion
The liver receives what type of blood?
- both arterial and venous blood
What is the role of the liver in digestion?
- it has hepatocytes that produce bile which are important in fat digestion
- it handles nutrients absorbed from the intestine and regulates their release into the blood
- ( other functions not directly associated with processing nutrients)
Bile which is produced by hepatocytes is excreted into what?
- bile canaliculi (common bile duct)
where is bile stored when there is no digestion occurring?
- the gall bladder
When digestion starts the gall bladder slowly contracts, sphincter of Oddi relaxes - how does this control the release of bile?
- when the sphincter is relaxed bile is released into the small intestine
- when its contracted there is no bile released
What does bile contain?
- 95% water
- bile acids
- cholesterol
- phospholipids
- bile pigments (Bilirubin)
How are primary bile acids made?
- synthesised from cholesterol in the hepatocyte
How are secondary bile acids formed?
- when primary bile acids undergo dihydroxylation by bacteria in the small intestine
How are bile salts formed?
- primary + secondary bile acids conjugate with an amino acids (taurine/glycine)
What is the function of bile salts?
- bile salts are more water soluble than bile acids and act as detergents to emulsify lipids
- they form spherical micelles which are important in digestion and absorption of fats
How do bile acids recirculate back to the liver?
- by enterohepatic circulation
How is bile and bicarbonate production increased?
- acidic duodenal contents stimulate secretin which in addition to increasing bicarbonate from the pancreas and production of bile
What happens when fat is detected on CCK producing cells?
- stimulates CCK
- opens the sphincter of Oddi
- more bile is released into the small intestine
Is neural stimulation important in bile production?
- NO
