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Intro to Clinical Biochemistry > GI tract > Flashcards

GI tract Flashcards

1
Q

Function of the GI tract?

A

Ingestion, digestion, absorption and elimination

All parts occur within or assisted by specific accessory organs

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2
Q

What regulates the GI tract and what do they regulate?

A

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

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3
Q

Give a detailed idea of the phases of digestion

A

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
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4
Q

Name the 3 phases of digestion and absorption, with their function in the pathway.

A

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

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5
Q

Describe the differences between peristalsis and segmentation.

A

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

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6
Q

Function of the stomach to digestion.

A

Mechanical breakdown
Release of HCl and intrinsic factors from pariental cells
Release of pespinogen from chief cells
Gastrin release from G cells

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7
Q

Give detailed response to how gastric phase works.

A
  • 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
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8
Q

What is gastrin?

A

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)

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9
Q

Name 3 parts making up the small intestine

A

Duodenum, jejunum and Ileum

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10
Q

What occurs in SI during digestion?

A

Mainly to reabsorb small nutrients

  • chyme enters the duodenum causing the release of cholescystokinin (CCK) by the duodenum
  • Secretin is also released by duodenum
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11
Q

What does CCK do?

A

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

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12
Q

What does secretin do?

A

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

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13
Q

Intestinal phase

A
  • 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
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14
Q

Function of bile.

A

Bile salt emulsifaction and absorption of fat at an alkaline pH

Bile salts produced by liver but stored in gallbladder

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15
Q

Pancreatic function in digestion and absorption.

A
  • 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
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16
Q

Composition of pancreatic juice.

A

Bicarbonate, electrolytes, enzymes (proteases, lipase and amylase)

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17
Q

How are carbohydrates digested and absorbed?

A

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)

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18
Q

Protein digestion

A

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

19
Q

Digestion of fats

A

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

20
Q

Name and discuss the methods of assessing pancreatic function.

A

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

21
Q

In a clinical setting, is the indirect or direct test better in assessing pancreatic function?

A

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

22
Q

How is the direct pancreatic function test done?

A

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

23
Q

If a patient is experiencing symptoms associated with pancreatic disorders, what tests are likely to be administered?

A

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.

24
Q

Is blood or stool sample more sensitive and specific?

A

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

25
Q

Why can faecal test be inappropriate in pancreatic insufficiencies?

A

Stool sample needs to be solid, but one symptom of pancreatic insufficiency is that the patient will experience diarrhoea

26
Q

Name 3 pancreatic disorders

A

Pancreatitis, pancreatic insufficiency and pancreatic carcinoma

27
Q

Describe pancreatitis with clear distinctions between acute and chronic.

A

Acute

  • Abdo pain, nausea, vomiting
  • Blood sample investigated - amylase, lipase or CRP (non-specific)
  • Can also measure urea & electrolyte, LFT, calcium, blood gases - general homeostasis

Chronic

  • follows acute
  • Common cause - alcohol
  • measured with faecal elastase
28
Q

What can cause pancreatic insufficiency and what does it entail?

A

Long term damage to organ because of chronic pancreatitis, CF, surgery (bypass or pancreatic), Coeliac disease

Effective digestion is lost causing malnutrition, vit defiency, weight loss, fatty stool and diabetes.

29
Q

Test and treatment for pancreatic insufficiency?

A

Faecal enzymes - none/low in stool means insufficiency present

Treatment include; supplementation or insulin

30
Q

Define diarrhoea as a disorders

A

Increased faecal mass per day. Severe cases can cause dehydration.

Lower GI losses

  • Large loss of faeces result in loss of bicarbonates - metabolic acidosis
  • Dehydration- increase of urea
  • Mg2+ and K+ imbalances
  • Usually loss in K is associated with increased bicarbonate, but in diarrhoea both are decreased
31
Q

Types of diarrhoea

A

Osmotic - due ot solution of high osmotic potential. Causing water to be drawn from the body to equilibrate chyme

Secretory - increased secretion or reduced absorption of water and ions

Inflammatory - damage to intestinal mucosa

32
Q

Describe what happens in vomiting

A

Upper GI losses which cause loss of acid (increased bicarbonate), decreased Cl, dehydration resulting in low K

33
Q

How is a upper GI bleed diagnosed?

A

Increased potassium from intracellular release
Increased blood urea from protein metabolism
Raised urea:creatinine ratio

34
Q

Examples of inflammatory bowel disease and how is it diagnosed

A

Crohns

  • can affect any part of GI from mouth to anus
  • most commonly starts in terminal ileum

Ulcerative colitis
- restricted to colon and rectum

Diagnosis can be done by measuring faecal calprotecting to ID who needs colonoscopy. CRP/ESR can give indication of level of inflammation

35
Q

What stimulates CCK secretion?

A

AA, lipids, fats and carbohydrates

36
Q

What stimulates secretin secretion?

A

A fall in pH stimulates secretin secretion from the duodenal S
- Act to rise pH by secreting bicarbonate

37
Q

Why are the pancreatic enzymes released as zymogens?

A

To minimise autodigestion and make sure they only become active in duodenum
- Stimulated either by vagus nerve, secretin, CCK

38
Q

What does the brown colour of faeces tell you?

A

Indicates the presence of stercobilin, which is produced from the breakdown of bilirubin

39
Q

What is the purpose of beta and alpha cells?

A

Found in the pancrease and aid in the glucose homeostasis by secreting either insulin (beta) or glucagone (alpha). Acting to modulate blood glucose

40
Q

How is irritable bowel syndrome diagnosed?

A

By diagnosis of exclusion.

- No detectable organic cause

41
Q

Describe the stool sample and nutrition of people with IBD?

A

Crohns
- May have bloody stool, but malnutrition is more common

Ulcerative colitis

  • Common to have bloody stool
  • Malnutrition is less common
42
Q

What is coeliac disease and how is it diagnosed?

A
  • Gluten sensitive enteropathy
  • Autoimmune disease: upon exposure to gliadin -> immunological response to tissue transglutaminase

Screened by ELISA (TTG) - histology to confirm

43
Q

How is bowel cancer tested for?

A

Use faecal immunochemical tests to detect small amounts of faecal occult blood

Intro to Clinical Biochemistry (12 decks)

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