Pancreas and GI Tract Physiology Flashcards
Decribe the GI Tract
- Muscle that extends from the mouth to the anus (~10 m)
- Mainly smooth muscle under involuntary control
- Striated muscle under voluntary control at upper oesophagus and external anal sphincter
- Main organs of the GI Tract are Stomach, Small intestine, Large intestine, Pancreas, Gall bladder
- Accessory organs are Salivary glands, Liver, Gall bladder, Pancreas
What are the functions of the GI Tract?
- Ingestion: Mouth, Oesophagus)
- Digestion (Mechanical/Chemical): Mouth, Stomach, Small intestine, Large intestine
- Absorption: Small intestine, Large intestine
How is GI Tract function regulated?
Parasympathetic nervous system
- Stimulation of vagal nerve, release of acetylcholine (neurotransmitter)
Intrinsic nervous system
- Stretch receptors located in oesophagus, stomach, intestine
Hormones
- Released as a result of the above act on nearby (paracrine) or far away tissues (endocrine)
What are the phases of digestion and absorption?
- Neurogenic phase
- Gastric phase
- Intestinal phase
Describe the Neurogenic Phase of Digestion and Absorption
- Initiated via intake of food into the mouth (sight, smell, taste)
- Stimulation of cerebral cortex and appetite centres in amygdala and hypothalamus
- Stimulates the parasympathetic nervous system, the vagal nerve & acetylcholine release to prepare stomach for arrival of food.
Describe the Gastric Phase of Digestion and Absorption?
- Initiated by distension of the stomach which stimulates contraction of stomach
- Increased Gastrin secretion from G cells via vagal stimulation and stretch receptors stimulates parietal cells to produce gastric acid
- Gastrin also increases release of pepsinogen from Chief cells which is activated to pepsin in the acidic environment and begins to degrade protein
- Release of intrinsic factor from parietal cells to bind vitamin B12 preventing degradation and facilitating
Describe the intestinal phase of Digestion and absorption?
- Intestinal phase of digestion and absorption begins when chyme (mixture of partially digested food) enters duodenum
- Fats, amino acids, and carbohydrates stimulate cholecystokinin (CCK) release from duodenal I cells (chemoreceptors). This stimulates pancreatic acinar cells to secrete digestive enzymes, and the gall bladder to expel stored bile
- A fall in pH (pH<4.5) stimulates duodenal S cells to secret secretin. This stimulates pancreatic duct cells to secrete HCO3-to neutralise pH, and also inhibits gastrin secretion
How does food travel in the Gastrointestinal system?
- Symmetrical contraction and relaxation of smooth muscles that propagates in a wave down a tube
- Behind food contract and infornt of the food relaxes
What are the functions of the stomach?
- Mechanical breakdown of food
- Release of gastric acid (HCl) from parietal cells
- Release of intrinsic factor from parietal cells (B12 absorption)
- Release of pepsinogen from chief cells
- Release of gastrin from G cells
What are stomach cell types?
- Mucous neck cell - Mucus and Bicarbonate secretion
- Parietal Cells - Gastric acid and Intrinsic factor secretion
- Enterochromaffin-like cell - Histamine secretion
- Chief Cells - Pepsinogen and Gastric lipase secretion
- D-Cells - Somatostatin secretion (inhibits acid)
- G-Cells - Gastric Secretion (stimulates acid)
How is Gastrin Regulated?
Released from G cells in antrum of stomach, duodenum and pancreas
Gastrin release is stimulated by:
- Neurogenic control
- Stretch receptors in stomach
- Presence of partially digested proteins in stomach
Gastrin release is inhibited by:
- Presence of acid in stomach
- Somatostatin
What effect that Gastrin have in the Gut?
Gastrin stimulates secretion of:
- Gastric acid
- Pancreatic juice
- Pepsinogen
- Somatostatin
What is the structure of the intestines?
Small intestine (95% absorption of small nutrients)
- Duodenum
- Jejunum
- Ileum
Large intestine (absorption of fluid)
What is the function of the pancreas?
- Endocrine – secretes insulin, glucagon, somatostatin
- Exocrine – secretes pancreatic juice
Responsible for most digestion that takes place
What is the function of the gallbladder?
Stores Bile produced by the Liver
What makes up Pancreatic Juice?
- Fluid
- Bicarbonate – to neutralise gastric acid
- Electrolytes
- Enzymes
- Proteases – trypsin, chymotrypsin, elastase
- Lipase, cholesterol esterase
- Amylase
What is the digestion and absorption of Carbohydrates?
Mostly in the form of starch
- Amylase in saliva helps to digest starch but inhibited by stomach pH
- Amylase released from pancreas in duodenum to further digestion
- Disaccharides are digested by enzymes at brush border membrane
Leads to absorption of monosaccharides: Glucose, Fructose, Galactose
How are proteins digested and absorbed?
- Acid in the stomach helps to denature proteins and activate pepsin from pepsinogen
- Trypsin, chymotrypsin, elastase and other proteases are secreted by the pancreas into duodenum
- Eventual digestion to di and tripeptides and disingle amino acids which are absorbed
- Small peptides and amino acids are absorbed by the brush border membrane
How are Fats disgested and absorbed?
- Triglycerides, cholesterol, fat-soluble vitamins are non-water soluble fats so need bile acids to emulsify
- This allows lipase and cholesterol esterase released from the pancreas to digest the fats
- Fats can then diffuse across the phospholipid membrane of the brush border enterocyte cells
- These are made into chylomicrons and enter the lymph system and then liver
What are some signs and symptoms of malabsorption?
- Diarrhoea
- Steatorrhoea (fatty stools –foul odour, pale)
- Malnutrition
- Abdominal pain
- Abdominal distension
Which tests are used for confirmation of malabsorption?
- U&E - Electrolyte Balance
- Calcium, Albumin - Vitamin D deficiency
- Phosphate, Mg2+
- Vitamins and trace elements
- CRP, ESR - Inflammation
- FBC - Anaemia
- Iron, Ferritin - Iron deficiency
- Prothrombin time/INR - Vitamin K deficiency
- Vitamin B12/Folate - Haematinic deficiency
Describe presentation and cause of Gastric Ulcers?
- Gastric and duodenal ulcers present with epigastric pain at mealtimes, bloating, nausea and vomiting, haematemesis
- Helicobactor pylori is the main cause of peptic ulcer disease
- Rarely (~0.5% cases) ulcers may be due to Zollinger-Ellison syndrome due to a gastrin secreting neuroendocrine tumour. Likely refractory to common treatments
What are the investigations used for the causes of Gastric Ulcers?
- H.pylori stool antigen test
- Fasting gastrin is useful in diagnosis of Zollinger-Ellison
What are some investigations for intestinal function?
- Anti-TTG antibodies - Coeliac screen
- Hydrogen breath test - Disaccharidase deficiency, Lactose intolerance
- Markers of intestinal inflammation - CRP/ESR (non-specific), Faecal calprotectin specific IBD
- Endoscopy
- Biopsy
What is Faecal Calprotectin?
- Calcium-binding protein present in high concentration in neutrophils
- When neutrophils are localised to intestine due to inflammation, faecal calprotectin is secreted into the lumen and excreted in faeces
- Used to differentiate function disease (IBS) from organic disease (IBD) but also raised in: GI malignancy, infection, gastric ulcers, esophagitis, diverticulitis, use of NSAIDs
- Good ‘rule out test’ . Used to stratify referrals/ colonoscopies
Describe Pancreatic Function tests
Invasive (direct) function tests
- Specific
- Sensitive
- BUT unpleasant for patient, time consuming, specialist skills and equipment required
- Rarely used in routine practice
Non-invasive (indirect) function tests
- Not as specific for pancreatic function
- Not as sensitive – may not pick up pancreatic insufficiency until severe
- BUT quick and easy to carry out, convenient for the patient and cheap
- Used in routine practice
How is Direct Pancreatic Function testing conducted?
Stimulate pancreatic secretion of enzymes, bicarbonate, fluid and collect directly from pancreas using an endoscope
Can stimulate with:
- CCK, Secretin or CCK AND Secretin (Gold Standard)
- Lundh test meal
Measure volume, [bicarbonate], enzyme activity. This can pick up mild insufficiency
How is Indirect Pancreatic Function testing conducted?
Measurement of pancreatic enzymes in:
- Stool - faecal elastase (mostly useful test for chronic pancreatic insufficiency)
- Blood - amylase, lipase (only used for acute pancreatitis)
Convenient – random faeces sample
Not sensitive at picking up mild pancreatic insufficiency
Measured by immunoassay
How is the Hydrogen Breath test conducted?
Hydrogen breath test (Disaccharidase deficiency,Intestinal bacterial overgrowth)
- Sugars that are not digested and absorbed into the enterocyte travel to the large intestine
- Bacteria ferments with the sugars and produce methane and/or hydrogen which increases in breath
- Fast overnight
Intestinal Bacterial Overgrowth - Give glucose/ lactulose (not absorbed by intestine)
Dissacharide Deficiency - Give fructose, lactose for disaccharidase deficiency
What are non-biochemical tests for Pancreatic Function?
- CT scan - Computed Tomography
- ERCP - Endoscopic Retrograde Cholangiopancreatography
- MRCP - Magnetic Resonance Cholangiopancreatography
