Transport 2 - along alimentary tract Flashcards
Functions of the alimentary tract
Absorption of nutrients, salts and water
Fluids secreted into small intestine are absorbed back into blood
Absorption is achieved by transport of ions, e.g. Na+ and water follows by osmosis
Colon absorbs 90% of the water from the contents that arrive at it
Semi-solid material (~200ml)
Malfunction → nutritional, salt and water imbalance
Diarrhoea: abnormal water absorption in the intestine
Intrinsic neuronal plexuses of the gut
The gut has an intrinsic, enteric nervous system
There are large neuronal connections in the gut wall:
Submucosal Meissner plexus,which regulates the digestive glands
Myenteric Auerbach plexus,primarily connected with gut motility
Motor neurons -> (ACh (M receptors) and Substance P release. Inhibitory motor neurons releaseVIP and NO= relaxation)
Nervous control of the Gut
The gut has an intrinsic nervous system called the ENS
Parasympathetic innervation may control the ENS
However the ENS is largely able to work by itself without vagal innervation from the brain stem
The nerve plexi contain local sensory and motor neurons as well as interneurons for communication
The nerve plexuses near the GI tract initiate short reflexes
- these short reflexes are mediated by the ENS
There are two large neuronal connections in the gut wall:
• submucosal meissner plexus
- this regulates the digestive glands and some local movement
- it is very close to the lumen therefore helps secretion and local
movement
• myenteric auerbach plexus
- it is primarily connected with gut motility (tone and velocity of
motion)
- excitatory motor neurons release Ach (acid secretion and
contraction) and and substance P
- inhibitory motor neurons release VIP and NO (relaxation)
There is also extrinsic control of the gut
these are long reflexes arising withing or outside the GI tracts. They go all the way to the CNS and involve parasympathetic reflexes
Stretch and chemoreceptors are involved in sending signals to the CNS
- these detect distention and different chemical substances in the gut which leads to secretion of Ach, NO or VIP
A response through the CNS to the gut is given through the ENS
Villus and crypt cells
This is largely mediated by enterocytes
Enterocytes contain two parts:
crypts
- these are involved in secretion as they have the capacity for protection and regeneration as they have stem cells in them
villus
- these are involved in absorption as thye are highly vascularised, thin, moist with Large SA
Intestinal digestion and absorption in the intestine
All dietary nutrients, water and electrolytes that enter the upper small intestine are absorbed.
The small intestine: epithelial folds, villi, and microvilli (large surface area - an internal surface area of 200 m2)
Transport across the enterocyte
Transcellular transport = transport of solutes by a cell through a cell; e.g., transport of glucose from the intestinal lumen to extracellular fluid by epithelial cells
Paracellular = passage of solutes between cells. this is passive transport but is very selective, variable and
regulated -> this pathway will require the use of transport proteins to help move substances
There are tight connections between enterocytes called tight junctions
Carbohydrate (CHO) digestion and absorption
Can only be absorbed in the form of monosaccharides
Complex CHO reduced to disaccharides by amylases (which starts in the mouth, if enters duodenum, pancreatic amylase will break it down)
Specific brush border enzymes convert disaccharides to monosaccharides (e.g. glucose and galactose)
Digestion of carbohydrates occurs in the intestinal lumen and at the brush border
Digestion of carbohydrates occurs at the intestinal lumen and at the brush border:
• pancreatic amylase and salivary amylase begin the initial digestion of complex
carbohydrates
• sucrose broken down by sucrase to glucose and fructose
• glucose oligomers broken down by glucoamylase to glucose
- molecular complex that consist of a few monomer units
- e.g. maltotriose = trisaccharide (three-part sugar) consisting of three glucose). It is most
commonly produced by the digestive enzyme alpha-amylase on amylose in starch.
• lactose broken down by lactase to glucose and galactose
Summary of carbs enzyme action
Sucrase acts on sucrose -> Glucose and fructose
Lactase acts on lactose -> Galactose and glucose
Glucoamylase acts on glucose oligomers -> Glucose
Maltase acts on maltose -> Glucose and glucose
Transport of glucose and galactose
After the digestion of carbs to monosaccharides, they are now present for absorption through the enterocytes:
• glucose/galactose:
- Na+ is actively being pumped out of the enterocyte through the basolateral membrane into the blood
- the basolateral membrane is the membrane close to the blood
- the pumping of Na+ creates an electrochemical gradient in the cell which allows Na+ to enter from the lumen of the large intestine
- as Na+ enters the enterocyte from the lumen of the intestine, glucose enters with it
- this is done through the glucose/Na+ co transporter (SGLT1)
- galactose competes with glucose molecules to enter the cell (they have similar binding sites on the co transporter)
- glut-2 transporters then allow glucose to enter the blood
• fructose:
- this is passively transported through the cell
- glut 2 and glut 5 transporters on the intestine lumen side of the enterocyte allow fructose
to enter the enterocyte
- there are glut 5 transporters on the basolateral membrane of the enterocyte which allows fructose to enter the blood
Fructose transport
Fructose and mannose are slowly absorbed by passive transport on a GLUT 5 carrier
Leaves the cell into the blood via GLUT 2 carrier
Protein digestion
Polypeptides produced by action of pepsin
Polypeptides, di- and tri-peptides by action of pancreatic proteases
Di-peptidases in brush border complete digestion to amino acids
Sites of protein digestion
Proteins arrive in the small intestine as polypeptides and these are denatured by gastric acid and broken down by pepsin in the stomach. The small intestine compensates for any chemical breakdown not completed in the stomach.
There are three sites of protein digestion:
Proteins -> Oligopeptides and amino acids by pepsins, pancreatic proteases
Oliogopeptides (consist of 2-20 amino acids) and amino acids -> Amino acids and dipeptides and tripeptides (this is done by brush border peptidases)
Single amino acids can be absorbed by the enterocyte, as well as dipeptides and tripeptides. These may be broken down in the enterocyte to AAs by cytoplasmic peptidases or pass into the blood as dipeptides and tripeptides.
2 methods of protein absorption
Two mechanisms:
Amino-acids (AAs) are transported on a sodium-coupled carrier system similar to that for glucose. Na+ goes in to the enterocyte with the amino acid through a co-transporter
- active transport then transports the amino acids to the blood which reaches the liver through the portal vein
Separate carriers for different types of AAs.
Some di- and tri- peptides are transported on a carrier system using inwardly directed H+ gradient
Digestion of lipids
Triglycerides are majority of the dietary lipids taken in
Lipids also include phospholipids, cholesterol, fat soluble vitamins (A,D,E,K)
Triglycerides are broken down into smaller subunits to facilitate absorption
Salivary lipases break down some of the triglycerides
Most triglycerides are broken down in the small intestine by pancreatic lipases
Problem with Triglyceride and lipase
Most dietary TGs are digested in the small intestine
But TGs are water-insoluble; chyme (emulsion of large fat particles in water); lipase is water-soluble
TGs must be dissolved in the aqueous phase before they can be digested
co lipase
lipases are however hydrophilic and triglycerides are hydrophobic so cannot react together
The triglycerides need to also be dissolved in the aqueous phase to be digested
This is done by the help of a protein called the co lipase
co lipase is amphipathic which means it has hydrophilic and hydrophobic regions
lipase binds to the hydrophilic region of the co lipase
emulsion droplets bind to the hydrophobic region of the co lipase
this helps with the digestion of the triglyceride emulsion droplets
lipase action requires emulsification by bile salts
Breakdown is done by the action of lipases
The triglyceride surface area needs to be increased for efficient digestion
This process is called emulsification and is done in the duodenum by bile salts
This forms emulsion droplets (The micelles is now small enough to be taken up by the enterocyte and into the lacteals and into the lymphatic system before travelling to the blood via the thoracic duct)
Bile salts are produced in the liver and stored in the gall bladder
They come down the common bile duct and join the pancreatic duct which secretes pancreatic lipase and co lipase
The sphincter of oddi controls the flow of digestive juices (bile and pancreatic juice) through the ampulla of Vater
breakdown via location (lipids)
10 - 30% in stomach
70 - 90% breakdown + micelle formation in duodenum
Fat absorption
The micelles now fuse to enterocytes and through pinocytosis their contents are emptied into
the enterocyte forming chylomicrons in the SER of the enterocyte
Some short chain fatty acids may directly go to the liver through the portal vein and will be
carried by albumin
The rest of the lipids are reformed (triglycerides are reformed)
This results in the formation of a particle called chylomicron
- apo and lipo proteins are added to the reformed lipids
- glucose is added to them as well
chylomicrons transport
Chylomicrons enter the lymphatic system via lacteals, and then travel via lymph vessels and
enter the blood via thoracic duct located near the neck which connects with the left
subclavian vein
LDL and HDL
Lipoproteins carry the lipids in the body in the blood
LDL = has high concentration of fats and transports fats to adipose tissue; clogs veins/arteries, so this is bad cholesterol
HDL = carries fats to liver for recycling and excretion;
Disorders
Absorption of large numbers of chylomicrons causes the lymph draining from the small intestine to appear milky
Disorders such as gallstones, pancreatitis, Crohn’s disease, and liver disease can lead to fat malabsorption
• eg steatorrhoea or fat-diarrhoea = excess fat in faeces
Bile salts after absorption
5% lost in the faeces
95% removed and recycled in the enterohepatic circulation
