GI tract Flashcards
(43 cards)
Function of the GI tract?
Ingestion, digestion, absorption and elimination
All parts occur within or assisted by specific accessory organs
What regulates the GI tract and what do they regulate?
The parasympathetic nervous system (rest and digest - involuntary) - stimulated by the vagus nerve
Enteric/interic nervous system: set of neurones controlling the GI tract - stimulated by stretch receptors located in oesophagus, stomach and intestine
Hormones: can be paracrine or endocrine
Give a detailed idea of the phases of digestion
Neurogenic phase: Stimulated by
- Sight, smell and taste of good
- Stimulates the parasympathetic nervous system
Gastric phase: Stimulated by
- distension of stomach activates vagovagal reflex (means that when food enters stomach the vagovagal relfex goes from stomach to brain and back resulting in active contraction of smooth muscles)
- Increased gastrin secretion by G-cells via vagal stimulation and stretch receptor stimulates pariental cell to produce gastric acid.
- Gastrin also releases pepsinogen from chief cells which in an acidic environment becomes pepsinin to degrade protein
- Intrinsic factor released by pariental cell to bind vitamin B12 to protect it from degradation
Intestinal phase: stimulated by
- peptides and FA in duodenum (stimulates CCK secretion)
- pH<4.5 - stimulates secretin secretion
Name the 3 phases of digestion and absorption, with their function in the pathway.
Neurogenic phase: stimulate the parasympathetic nervous system
Gastric phase: To stimulate the secretion of HCl by releasing gastrin, histamine and acetylcholine
Intestinal phase: respond to incoming chyme and moderate gastric activity via hormones and nervous reflexes
Describe the differences between peristalsis and segmentation.
Peristalsis uses the symmetrical contraction and relaxation of muscle propegating a wave down a tube, whilst segmentation is when parts of the SI which contract and relax indepenenly. Essentially wave vs random contraction. Segmentation is better for churning
Function of the stomach to digestion.
Mechanical breakdown
Release of HCl and intrinsic factors from pariental cells
Release of pespinogen from chief cells
Gastrin release from G cells
Give detailed response to how gastric phase works.
- Initiated by distension of stomach - vagovagal reflex
- Increase in gastrin secretion from G cells via vagal stimulation and stretch receptors stimulates pariental cells to produce gastric acid - AA and small peptides directly activate G cells to release gastrin
- Gastrin increase release of pepsinogen from chief cells which is activated to pepsin in the acidic environment and begins to degrade protein
- Release of intrinsic factors from pariental cells to bind vitamin B12 preventing degradation and facilitating absorption
- Activated ENS release ACh stimulating pariental cells - ACh secreted by parasympathetic nerve fibres
- Gastrin, ACh and histamine stimulate pariental cells to release HCl
What is gastrin?
Peptide hormone released by G cells to secrete HCl . G cells are stimulated by neurogenic control, stretch receptors, presence fo partially digested proteins in stomach
Gastrin can be inhibited by: the presence of acid in stomach or somatostatin
Gastrin stimulates the secretion of: gastric acid, histamine, pancreatic juice, pepsinogen and somatostatin (negative feedback)
Name 3 parts making up the small intestine
Duodenum, jejunum and Ileum
What occurs in SI during digestion?
Mainly to reabsorb small nutrients
- chyme enters the duodenum causing the release of cholescystokinin (CCK) by the duodenum
- Secretin is also released by duodenum
What does CCK do?
Peptide hormone of 33 amino acids
Stimulates gallbladder to contract and release stored bile into the intestine as well as pancreatic juice secretion into the intestine
What does secretin do?
Peptide hormone
It is released from the duodenum when chyme has entered. It induces the release of bicarbonate rich juices from the pancreas to change the acidity of the chyme more basic - better for pancreatic enzymes
Intestinal phase
- Begins with chyme entering duodenum
- Fats, AA, carbohydrates stimulate CCK release from duodenal I cells.
- CCK stimulate pancreatic acinar cells to secrete digestive enzymes, and contract the gall bladder to expel stored bile
- fall in pH stimulates duodenal S cells to secrete secretin to stimulate pancreatic duct cells to neutralise pH by HCO3 release and inhibition of gastrin secretion
Function of bile.
Bile salt emulsifaction and absorption of fat at an alkaline pH
Bile salts produced by liver but stored in gallbladder
Pancreatic function in digestion and absorption.
- 99% of it made up of glandular epithelial cells which produce enzymes - enzymes secreted break down sugar, fats and starch
- Enzymes become active in duodenum, eg. zymogens which minimise autodigestion
- secretion regulated by impulses from the vagus nerve, secretin and CCK
Composition of pancreatic juice.
Bicarbonate, electrolytes, enzymes (proteases, lipase and amylase)
How are carbohydrates digested and absorbed?
Amylase in the saliva - inhibited by stomach pH
Amylase released from pancreas in duodenum - polysaccharides broken down
Disaccharidases at brush border membrane breakdown the small saccharides to allow the absorption of monosaccharides (glucose, fructose and galactose)
Protein digestion
Proteins are denatured in the stomach acid, which activates the gastric pepsinogen to pepsin.
Trypsin, chymotrypsin, elastase and other proteases secreted by the pancreas into duodenum
Smaller peptides absorbed into the blood
Digestion of fats
Triglycerides, cholesterol and fat-soluble vitamins - need bile salts to be emulsified
Once emulsified the pancreatic enzymes can help break down e.g. lipase and cholesterol esterase
Fats can then diffuse across bilayerof the brush border enterocyte cells
Made into chylomicrons and enter the lymph system and then liver
Name and discuss the methods of assessing pancreatic function.
Multiple different ones exist.
Imaging using a CT scan is available but this does not reveal enough about the function, rather size, shape and damage.
Endoscopic retrograde CP - very invasive to patients, but extremely specific - precise imaging
Magnetic resonance cholangiopancreatography - expensive
Biochemical testing - best option to test functionality because there is a non-invasive method and a functional test which is invasive. Non-invasive involves examining urine and blood tests, whilst invasive requires stimulating the pancreas to use its function
In a clinical setting, is the indirect or direct test better in assessing pancreatic function?
There is no ideal test.
However, the invasive test do have an increased specificity and sensitivity compared to the non-invasive. This is however, at the expense of the test being unpleasant to the patient, time consuming, require specialist.
To conclude in the clinical setting the non-invasive tests are more appropriate and this is what happens in current practice
How is the direct pancreatic function test done?
Involve the stimulation of the pancreatic secretion, of enzymes, bicarbonate, fluid. These will be collected using an endoscope.
- To measure volume, conc of bicarbonate, enzymatic activity
Stimulation occurs with Lundh meal test CCK, secretin or CCK and secretin (gold standard)
Appropriate in mild insufficiency
If a patient is experiencing symptoms associated with pancreatic disorders, what tests are likely to be administered?
Indirect function tests using blood and stool samples.
Stool - measure the faecal elastase to test for chronic pancreatic insufficiency
Blood - amylase or lipase measured to test for acute pancreatitis -lipase is more sensitive
Blood is appropriate for acute pancreatitis because here some pancreatic cells die and release their enzymes into the blood.
Is blood or stool sample more sensitive and specific?
Measuring for faecal elastase in stool sample is the most specific. Typically the elastase should be excreted into the faeces due to its resistance to digestion. Measured using immunoassays.
Not sensitive at picking up mild pancreatic insufficiciency
