Gastrointestinal System Flashcards
What are the 3 main functions of the GI system?
- Digestion of food
- Absorption of nutrients + drugs
- Elimination
Where does ADME happen in the GI system?
Absorption - GI tract
Distribution - Circulatory System
Metabolism - Liver
Excretion - Kidneys
What are the 4 main organs of the GI system?
- Mouth
- Esophagus
- Stomach
- Small Intestine
What are the 4 accessory organs of the GI system?
- Salivary Glands
- Liver
- Gall Bladder
- Pancreas
What does the mouth do?
Breaks up food particles.
Saliva assists.
What does the esophagus do?
Transport of food to the stomach.
Entry to stomach via sphincter.
What does the Stomach do?
Secretion of gastric juices for CHEMICAL digestion.
MECHANICAL break up of food.
Mixing of food and gastric juices.
What is peristalsis?
The contraction + relaxation of muscles in a ‘wave’ formation down a tube, pushing food along.
MECHANICAL digestion
What are the gastric juices, what are they controlled by and what do they contain?
HCl
Controlled by the vagus nerve + hormone gastrin.
- Contains digestive enzymes
What is pepsin and what does it, combined with HCl, initiate?
Initiates protein digestion.
CHEMICAL digestion
What does the mucous coating do?
Lubricates + protects epithelial surface against pepsin
What organ forms chyme + moves food to the SI?
The stomach
Chyme - digestive liquid
Moving food to SI - delays gastric emptying, affecting rate of absorption
What does gastric emptying in the stomach depend on?
Depends on meal:
- Vol. of meal
- Kcal content
- Fat content
- Protein content
- liquid/solid state
Depends on the person:
- Age
- Sex
- Particular disease states
What are the 5 roles of the small intestine?
- Completes digestion of nutrients in chyme
- MAJOR site of absorption of different nutrients
- MAJOR site of absorption of orally administered drugs
- Site of first-pass metabolism of drugs (via CYP3A4)
- Movement of food residues to the large intestine
What are the 4 reasons for the SI being a MAJOR site of absorption for different nutrients?
- Large SA
- High perfusion (blood supply)
- Exposure to enzymes + solubilisers
- Receives secretion from liver + pancreas
What 3 components is the SI made up of?
Duodenum
Jejunum
Ileum
Why is the SA in the SI so large?
- Villi = LOTS of highly convoluted, circular folds
- Mucosa = epithelium + connective tissue with blood + lymphatic vessels
- Microvilli = brush border
Why is a large SA important for drug absorption?
The dissolved drug is absorbed across the gut wall mainly via passive diffusion.
Other routes:
- Paracellular = between cells of epithelial layer
- Transport-facilitated = active transport
Erythrocytes in gut lining containing metabolic enzymes (intestinal first-pass).
Blood perfusing intestine goes into liver via HPV (hepatic portal vein).
After liver, drug goes into systemic circulation
What is coeliac disease?
Chronic autoimmune disorder of the small intestine.
- Inflammation process triggered by consumption of gluten-containing food.
- Causes damage to villi in small intestinal epithelium
What 4 things does the liver do in digestion?
- Main site of XENOBIOTIC metabolism
- Secretes bile, enters duodenum via hepatic duct
- Stored in gall bladder between meals
- Important for lipid digestion
What 3 things does the pancreas do in digestion?
- Proteolytic enzymes TRYPSIN + CHYMOTRYPSIN for protein digestion
- Lipase for digestion of lipids
- HCO3- = stomach-acid neutralising
Explain the digestion of carbohydrates.
Starch —> disaccharide
- by amylase
Disaccharide —> monosaccharide
Monosaccharide —> blood
Explain the digestion of proteins.
Peptide —> Peptide fragments
- by pepsin (in stomach)
Peptide fragments —> Amino acids
- by trypsin, chymotrypsin (SI from pancreas)
Explain the 3 things involved in the digestion of lipids.
- Emulsification (bile salts) - forming small lipid droplets
- Pancreatic Lipase
- Micelle formation - transport of lipids
In 8 litres, how much water is absorbed in the SI?
80% will be absorbed
In the large intestine, how much water will be absorbed?
up to 99% of the 20% not absorbed by the SI
What 4 things does the large intestine do?
- reabsorption of H2O + salts from chyme
- absorption of drugs minimal compared to the SI
- mixing + propulsion of contents
- Indigestible residues+ liquids are eliminated as faecal waste
What is the Large Intestinal Microbiota involved in?
Helps metabolise drugs
What does the distal LI do?
Populated by lots of bacteria
Contributes to normal digestion by:
- Fermenting carbohydrates + proteins escaping digestion into absorbable energy
What are xenobiotics?
A compound not normally part of the diet
What is the gut-brain axis?
Brain influencing GI physiology, motility, mucin production etc.
GI influencing brain behaviour + mood.
What is Postprandial Somnolence?
‘food coma’
GI response to eating lots of food:
- Parasymphatic activation ‘rest + digest’ vs. ‘fight or flight’
- hormonal/neurochemical changes
What is a microbiota?
Micro-organisms of a particular site.
What is the colonisation like in the gut?
100 diff. bacteria in: - stomach - duodenum - jejunum - proximal ileum 10^8 diff bacteria in: - distal ileum (end of SI) 10^12 diff bacteria in: - colon
