Pancreas And GI Tract Physiology Flashcards
What is 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:
- Stomach
- Pancreas
- Gall bladder
- Small intestine
- Large intestine
What are the functions of the GI tract?
Ingestion Propulsion Digestion (Mechanical/Chemical) Absorption Defecation
What are the actions performed by the GI?
- Ingestion:
- Mouth : Mastication, involves amylase (breaks down starch)
- Oesophagus: Transport, oesophagi contracts after food passes to stop
regurgitation - Propulsion:
- Peristalsis: Symmetrical contraction and relaxation of muscles that
propagates in a wave down a tube, via the smooth muscle tissue - Digestion:
- Breakdown of large food molecules into small, water soluble
components, via the the following GI components:- Mouth- chemical and mechanical (main digestive processes start)
- Stomach
- Pancreas
- Gall bladder
- Small intestine (absorption)
- Large intestine
- Absorption:
- Small intestine: (5-6 m long, but main absorption occurs in 1st meter)
95% absorption of small nutrients due to large surface area achieved
by vili and micro-vili. Nutrients are then transported by hepatic
portal vein to liver.
- Large intestine: absorption of fluid (water and electrolytes). Starts to
produce waste. Contains lots of helpful bacteria (eg cellulose and fibre
that we can’t personally digest - this produces vitamins such as vitamin
K that we can then absorb) - Defecation:
- Expulsion of waste products from rectum, following successful
reabsorption of everything we can use. Leaving unbreakdown-able
products such as fibre. Under voluntary muscle control (anus).
How are the functions of the GI tract regulated?
Parasympathetic nervous system (no control at all - automatic unconscious movement)
Stimulation of vagal nerve, release of acetylcholine (neurotransmitter) that stimulates processes in GI tract, eg propulsion.
Intrinsic nervous system
Stretch receptors located in oesophagus, stomach, intestine, each are stimulated and regulate function
Hormones
Released as a result of the above act on nearby (paracrine) or far away tissues (endocrine)
Lots of different ones
What are the phases of digestion and absorption?
Neurogenic phase – Stimulated by sight, smell, taste food
Stimulates the parasympathetic nervous system, the vagal nerve & acetylcholine release, starts the production of things such as HCl secretion in the stomach. To allow digestion to occur.
Gastric phase – Stimulated by food already eaten:
Distension of stomach (mechanoreceptors – acetylcholine)
HCl release from direct stimulation of parietal cells by the vagus nerve
Chemoreceptors are activated by food breakdown, producing amino acids/ peptides in stomach
Causes release of gastrin
Intestinal phase – Stimulated by:
Peptides and Fatty acids (food) in duodenum – (chemoreceptors) stimulate hormone release, such as CCK secretion
pH <4.5 – stimulates Secretin secretion
What are the main functions of the stomach?
Mechanical breakdown of food (i.e. it is always churning). Chemical is the species produced to cause digestion:
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 is gastrin?
Gastrin release is stimulated by:
Neurogenic control – smelling food – then released by vagal nerve and Acetylcholine
Then stretch receptors in stomach following food intake
Actions of Gastrin:
- Stimulation of gastric acid release
- Stimulation of secretion of pancreatic juice
- Stimulation of secretion of pepsinogen
- Stimulates secretion of somatostatin (then inhibits gastrin’s actions -
feedback
What are the main functions of the small intestine?
Small intestine
- Duodenum
- Jejunum
- Ileum
Food is now ‘Chyme’ – mixture of partially digested food enters the duodenum
Pyloric sphincter allows only small amounts of chyme to pass into the duodenum at a time
Cholecystokinin (CCK) released from the duodenum, stimulated by the presence of HCl, amino acids and fatty acids
Secretin released from the duodenum, in response to HCl in the duodenum. Inhibits HCl production in the stomach and encourages bicarbonate release in pancreas. Also stimulates liver to produce bile and gall bladder to release this
What are the main functions of the gall bladder?
Bile is full of bile salts (from hepatocytes), which are recycled and sent back to liver. Liver also solubilises fats and neutralises HCl by producing bicarbonate.
Chyme from stomach comes in to the duodenum with HCl stomach acid.
Cholesistekinin is released in order to promotes bile production, contraction of gall bladder and promote the release of digestive enzymes in to the pancreas. Finally it stimulates opening of the sphincter of Oddy. Bile and pancreatic juices are both released into the duodenum.
As the acid travels further through the duodenum it promotes the release of secretin, which causes bicarbonate to be produced in the pancreas to neutralise the acid.
The small molecules that remain of the ‘food’ can continue through the duodenum resulting in somatostatin production, which has an inhibitory effect on cholesistekinin and secretin, thus having a feedback effect to end the digestive process.
What are the main functions of the pancreas?
Endocrine and Exocrine functions:
Endocrine – secretes insulin and glucagon (not part of digestive function) and somatostatin
Exocrine – secretes pancreatic juice, which contains:
- Fluid
- Bicarbonate – to neutralise gastric acid
- Electrolytes
- Enzymes (vast majority required for digestion) such as:
- Proteases – trypsin, chymotrypsin, elastase
- Lipase, cholesterol esterase (breakdown lipids)
- Amylase (breaks down carbohydrates)
Responsible for most digestion that takes place
How are carbohydrates ingested and absorbed?
Mostly in the form of starch
Amylase in saliva
Starts in the mouth but is then Inhibited by stomach pH
But not all was broken down so Amylase is also released from pancreas into the duodenum
Disaccharidases on the brush border membrane of the small intestine break down disaccharides to monosaccharides, allowing you to absorb them
Absorption of monosaccharides and diffusion into stomach by facilitated diffusion, into the blood stream. Such as:
- Glucose
- Fructose
- Galactose
How are proteins ingested and absorbed?
Pepsinogen in stomach activated to pepsin
Trypsin, chymotrypsin, elastase and other proteases secreted by the pancreas into duodenum - break down large proteins into small, these are then broken down into amino acids by aminopeptidases and carboxipeptidases (from the pancreas)
Small peptides and amino acids can also be absorbed by the brush border membrane in the small intestine, small peptides by endocytosis and AA by active transport.
How are fats ingested and absorbed?
Fats are insoluble
Triglycerides, cholesterol, fat-soluble vitamins
Non-water soluble so need bile acids to emulsify fats (broken down into smaller droplets which can travel through the digestive system, driven by chemical disruption in the stomach and bile salts (emulsifiers).
Remaining small particles are acted upon by enzymes, lipase and cholesterol esterase, released from the pancreas. Break down proglycerides into monoglycerides and fatty acids.
Form micelles (have insoluble particles in the middle) - go to brush border membrane to be absorbed
Fats can then diffuse across the phospholipid membrane of the brush border enterocyte cells, they than reform their triglycerides
Made into chylomicrons and enter the lymph system and then liver
What happens if there is dysfunction in the GI tract?
Dysfunction of GI Tract
- Digestion
- Absorption
Leads to malabsorption
Signs and symptoms of malabsorption that can be recognised in biochemistry lab:
- Diarrhoea
- Steatorrhoea (fatty stools –foul odour, pale)
- Malnutrition (particularly seen in children)
- Abdominal pain (common in primary care)
- Abdominal distension
Symptoms related with a specific disorder.
Can affect the: - Gastric - Intestinal - Pancreatic => Can lead to severe electrolyte disorders, must monitor Us and Es!
What are common investigations of GI function?
Someone presenting with stomach pain; 2 tests
H.pylori – in peptic ulcer disease (feaces tests, specifically for someone who gets a lot of heart burn)
- H.pylori stool antigen test
- Abdominal pain, distention
- Bleeding of GI tract – malena (bloody stool due to bleeding in upper GI
tract) , haematemesis (blood in vomit) - Requires antibiotic treatment.
Fasting Gastrin (gastrin is unstable, must be down as an in-patient – in Zollinger-Ellison Syndrome (much rarer - causes gastronomas producing lots of gastric in pancreas and duodenum) Peptic ulcers, diarrhoea, abdominal pain
