GI system Flashcards
GI system overview
Exists to provide optimal conditions for transfer of nuteitns, water and electrolytes
Continuation of external environment, meaning faecal matter is mainly undigested food and bacteria- not metabolic waste
Can have low pH, harsh digestive enzymes, bacteria, lumen cells are epithelial
Does not regulate nutrient absorption- job is to absorb everything it can, not maintain homeostasis
GI system 4 major processes
- Digestion:
- mechanical: breakdown large pieces of food into slush called chyme
- chemical: break down complex molecules into simple molecules for abs - Secretion:
- mucous, water, enzymes to aid digestion - Absorption
- simple molecules/ water pass from lumen of GI tract into blood or lymph via mucosal cells - Motility
- churning/ mixing of contents to assist digestion/ absorption
- propulsion of contents through GI tract
- elimination of undigested food
GI structure overview
GI tract- muscular tube through which food passes m Stomach- stores and churns up food to form chyme. Controls passage of chyme to small intestine. Some digestion (protein) Mouth/ pharynx: mechanical breakdown of large food particles, some digestions (carbohydrates) Small intestines: entry for secretions from liver and pancreas (duodenum). Vast majority of digestions and absorption occurs here. Large intestine: absorbed water, salts, some vitamins from remaining chyme. Full of bacteria. Exocrine pancreas: secreted pancreatic juice into small intestine- digestive enzymes, HCO3- Billary system (liver and gallbladder)- liver secreted bike (fat digestion). Gallbladder stores and concentrates bile bw meals. Bike from both enters into small intestine.
Regulation of GI system
Can only control secretion and motility.
Regulated by volume and composition of luminal contents
- distension of GI tract
- chyme osmolarity and/ or pH
- concentrations of specific nutrients in chyme
- input from the brain
Regulated neurally or hormonally
Neural regulation of GI system
Enteric nervous system. Self contained system, input, output interneurons all in walk of GI tract. Receptors can detect change and effectors can cause a response without CNS. Can receive input from brain- sympathetic fight or flight, parasympathetic rest and digest.
- Submucosal plexus
- primarily influences secretory activity - Myenteric plexus
- primarily influences motility
Hormonal regulation
Secretion/ motility controlled primarily by 3 hormones:
Gastrin: stomach
- stimulated by peptides and aas by neural reflex. Targets ECL cells and parietal cells. Stimulates gastric acid secretion and mucosal growth. Somatostatin inhibits release.
cholecystokinin: intestine
- stimulated by fatty acids and aas. Targets gallbladder, pancreas and stomach. Stimulates gallbladder contraction and pancreatic enzyme secretion. Inhibits gastric emptying and acid secretion. Promotes satiety.
Secretin: intestine
- stimulated by acid in small intestine. Targets pancreas and stomach. Stimulates HCO3- secretion. Inhibits gastric emptying and acid secretion.
GI phases
1 cephalic phase (stimulus - brain)
Sight, smell, taste, thought of food, emotional state
Eg. Causes vagus nerve to release Ach and increase acid release
2. Gastric phase (stimulus - stomach)
Distension, increased or decreased acidity, peptides and aas in stomach
Eg. Distension of gastric smooth muscle increases acid release. Presence of small peptides and aas increase acid release. Increased acidity in stomach decreases acid release.
3. Intestinal phase (stimulus- intestine)
Distension, increased acidity, increased osmolarity m, nutrients in duodenum
Eg. Presence of nutrients, increased osmolarity and increased acidity decrease acid release
Enzymatic digestion
Amylase hydrolyse polysaccharides
Peptidases hydrolyse peptides
Lipases hydrolyse lipids
Small intestines produces digestive enzymes but does not secrete them in lumen.
All digestive enzymes secreted into lumen of small intestine come from the pancreas. Together the brush birder and pancreatic enzymes finish the breakdown of nutrients into simple molecules.
absorption
Small intestine (80% micronutrients, all macro and 85% water) Large intestine- remaining Mouth stomach can also absorb toxins/ drugs Most nutrients pass first to the liver via the hepatic portal vein before circulating the rest of the body- allows for metabolism of nutrients and detoxification of harmful substances before reaching general circulation. Fats enter the lymph and are drained into the thoracic ducts to join venous circulation.
Digestion of protein
Starts in mouth- acid helps to denature proteins to aid digestion. Protein chopped up into smaller peptides by pepsin.
Digestion and absorption completed in small intestine. - endopeptideases (protease) including proteins pancreatic trypsin and chymotrypsin. They cut in middle of aa releasing two smaller peptides.
Smaller peptides- carboxypeptidase and aminopeptidase on brush border and/ or at least 20 different exopeptidases which cut at ends to release single aa.
Digestion of carbohydrates
Begins in mouth- salivary amylase begins to break down poly and disaccharides in mouth Complete in small intestine- Pancreatic amylase (~95% of starch digestion) Other enzymes present on brush border complete breakdown to monosaccharides Almost all digestible carbohydrates are digested and absorbed within the first 1/5th of small intestine.
Absorption of carbohydrates and protein
Glucose, galactose and most amino acids are absorbed by luminal epithelial cells via secondary active transport couple to NA+
- nutrients enter cell using the energy of the chemical gradient for Na+ (hence secondary)
- Na/K pump uses ATP hence active to create a chemical gradient for Na+
Glucose:
Enters via SGLT
Fructose enters via GLUT5
All monosaccharides leave cell via GLUT2 and taken to liver via hepatic portal vein
Protein:
Di and tripeptides use secondary active transport couple to H+ rather than Na+
Some larger peptides can be absorbed via transcytosis
All then taken to liver via hepatic portal vein
Digestion of fat
Most in form of triglyceride
Majority occurs in small intestine, some in mouth and stomach
Pancreatic lipase hydrolysed triglyceride- breaks into monoglyceride and two free fatty acids
But complicated- fat and water don’t mix. So emulsifying agents (amphipathic) used, bike salts and phospholipids to aggregate molecules into smaller droplets which can then be more efficiently broken down by lipase.
Pancreas secretes lipase and colipase- displaces bile salts to allow access to triglycerides.
Absorption of fat
Bile salts help with fat absorption in small intestine
End product of lipid digestion are still hydrophobic so aren’t soluble in chyme.
Bile salts/ phospholipids help them to form micelles which are smaller and the end product of lipid digestion. Act as a store for FFAs.
FFAs that are shuttled between micelles are free and are absorbed into epithelial cells via simple diffusion.
Epithelial cells absorb MG/FFAs by these are then resynthesised into triglycerides packaged with Cholesterol/ protein to form fat droplets called chlyomicrons
Secreted via exocytosis- too big to get into GI capillaries but is absorbed into lacteal to drain into lymph. Empties into thoracic duct- does not go straight to liver.
Absorption of vitamins
Fat soluble
- solubilised micelles- absorbed via diffusion, packaged into chlyomicrons. Some abs occurs in large intestine
Water soluble
- absorbed either simple diffusion or mediated transport
- exception is B12- must be bound to intrinsic factor as it’s too big and highly charged
