Small Intestine Flashcards
Under what circumstances does the small intestine have a role in ingestion?
If a nasogastric tube is placed directly into the small intestine
What are the functions of the GI tract that are applicable to the small intestine?
- mechanical processing
- digestion
- secretion
- absorption
What is the main GI tract function of the small intestine?
Digestion
This involves breaking down large macromolecules into small ones which can be absorbed
Why is mechanical processing a function of the small intestine?
There is very little mechanical processing
It involves mixing in the enzymes that have been added from the pancreas and liver
What are the 3 parts of the small intestine?
- duodenum
- jejunum
- ileum
The ileum leads to the caecum, where it empties into the large bowel
What are the 4 parts of the duodenum and their passage?
- first part comes out of the stomach and turns downwards to run towards the posterior abdominal wall
- second part runs alongside the vertebral bodies
- third part rises up again along the vertebrae
- fourth part joins the jejunum and comes out onto the mesentery
What parts of the duodenum are foregut structures?
The first and second parts of the duodenum are foregut structures
The second and third parts of the duodenum are midgut structures
Which parts of the duodenum are retroperitoneal?
The final part of the second part and all of the third part
Where does the foregut terminate in relation to the duodenum?
In the second part of the duodenum, immediately after the duct which leads to the pancreas and liver arises
Why is it important that the small intestine has a large surface area?
There is a large area for digestive enzymes on the surface of cells
There is a large area for transporters to pick up materials that have been broken down
What are pilca circulares?
Circular folds that project into the lumen to help mix the food and increase the surface area
what structures are found lying on the plica circulares?
Villi - finger-like projections of epithelium
There are microvilli on the epithelial cells that amplify the surface area even further
What is the purpose of the villi and microvilli?
They amplify the surface area available for interaction with food
What are the crypts of Lieberkuhn in the epithelium of the small intestine?
What do they secrete?
They are small secretory glands which sit below the surface layer of epithelium, but still within the mucosa
They secrete bicarbonate-rich fluid
What is the purpose of the bicarbonate-rich fluid secreted by the crypts?
The bicarbonate acts to neutralise the acidic content coming from the stomach
What is significant about the microvillus brush border?
There are integral membrane proteins that act as enzymes
They project into the lumen to break down substances and transport them into epithelial cells
What is meant by peristalsis?
The waves of muscle contractions which move contents along the length of the GI tract
What is the first type of muscle contraction in peristalsis?
Why is it necessary?
Circular muscles behind the bolus contract
Circular muscles ahead of the bolus relax
This prevents the bolus from moving backwards
What is the second type of muscle contraction in peristalsis?
The longitudinal muscles ahead of the bolus contract to shorten adjacent segments
What is the third and final muscle contraction involved in peristalsis?
Waves of contraction occur in the circular muscle layer to force the bolus forwards
Typically, how far are substances moved by peristaltic waves?
Typically, substances are only moved a few centimetres by peristaltic waves
What is segmentation?
Alternate contraction of neighbouring segments within the GI tract
Circular muscle contraction breaks the contents of the GI tract into separate pockets
What is the purpose of segmentation?
- to churn and fragment the bolus
2. to mix the contents of the bolus with intestinal secretions, including enzymes from the pancreas
After processing is complete, what is the final clearing mechanism in the small intestine?
Generation of the migrating motor complex
When are the migrating motor complexes generated?
When the gut is relatively empty (around 5 hours after eating)
The migrating motor complexes are then generated every 90 minutes
what will stimulate and suppress the migrating motor complexes?
They are stimulated by motilin
They are suppressed by feeding
Where is motilin produced and how many amino acids does it contain?
It is secreted by M cells (and also erthyromycin)
It is a peptide made from 22 amino acids
What is involved in a migrating motor complex?
For a few minutes, a series of strong, slow peristaltic waves will sweep down
These mainly come from the stomach and travel along the small bowel
How do the migrating motor complexes affect the pyloric sphincter?
They cause the pyloric sphincter to become relaxed to its maximum
This allows larger things to pass through which may have been too large to pass through at an earlier stage
What is the purpose of the migrating motor complex?
The waves help to keep the gut clean, prevent reflux and reduce bacterial growth
Where do secretions occur in the small intestine?
The crypts of Lieberkuhn
also from the larger submucosal Brunner’s glands in the duodenum
What are the APUD cells and what do they secrete?
Amine precursor uptake and decarboxylation cells
These are I, S, M and G cells
They secrete hormones into the bloodstream
What is mainly secreted from the crypts of Lieberkuhn and the Brunner’s glands?
Bicarbonate and mucous
Where is CCK secreted from and what is its function?
I cells
It is the main driving force behind pancreatic and gall bladder secretion
CCK results in secretion of enzymes and bile salts
Where is secretin secreted from and what is its function?
S cells
It mainly drives bicarbonate secretion
Where is motilin secreted from and what is its function?
M cells
It increases GI tract motility
Where is gastrin secreted from and what is its function?
G cells
It increases gastric acid secretion
Why is digestion necessary?
It is needed to break down molecules which cannot be absorbed directly
What is the first stage in carbohydrate digestion?
a-amylases in the mouth and upper part of the stomach break a-1,4 bonds
This leads to a mixture of di- and trisaccharides and dextrins which have a a-1,6 bond that cannot be broken by soluble amylases
Why can soluble amylases not fully break down carbohydrates?
They cannot break a-1,6 bonds
Dextrins and disaccharides need to be broken down into single sugars before they can be absorbed
How is sucrose broken down?
It is split into its constituent parts - glucose and fructose
This occurs via sucrase which is found on the brush border membrane
What happens to fructose after sucrose has been broken down?
Fructose is taken up passively by the GLUT5 transporter
What happens to glucose and galactose produced from carbohydrate breakdown?
They are actively absorbed by the SGLT1 transporter
This will transport 2 Na+ ions and a glucose molecule in the same direction by co-transport
What will break up dextrins?
Glucamylase and isomaltase
What will break down lactose into its constituent parts?
Lactase
It is broken down into glucose and galactose, which are taken up by the SGLT1 transporter
What happens after glucose is actively moved into the cell by SGLT1?
The high concentration of glucose within the cell means it is passively transported through GLUT2 on the basolateral membrane
What is GLUT2?
A passive transporter on the basolateral membrane which allows glucose, galactose and fructose into the circulation
What is the driving force for glucose transport into the circulation?
Primary active transport occurring through the Na+/K+ ATPase
This uses ATP to keep intracellular Na+ low
This is the driving force for Na+ entry, which picks up glucose
Where does protein digestion begin?
In the stomach
Pepsin is deactivated in the duodenum
What form are most of the enzymes produced in the pancreas secreted in and why?
Proenzyme form
This prevents the enzymes from digesting the pancreas whilst they are waiting to be secreted
What is the role of enterokinase?
Where is it secreted from?
It is secreted by crypt cells
It converts trypsinogen into active trypsin once pancreatic enzymes enter the small bowel
What happens once trypsin has become activated?
Trypsin cleaves all the other proenzymes released from the pancreas
This leads to the activation of many proteases
What is the hormone that drives pancreatic enzyme secretion?
Cholecystokinin (CCK)
This is also known as pancreozymin
What are the 3 endopeptidases?
- trypsin
- chymotrypsin
- elastase
How do endopeptidases cleave proteins?
They cleave within protein chains and have preferences for which amino acids they cut next to
This results in shorter peptides that tend to have particular characteristics
What are exopeptidases and how do they cleave proteins?
Carboxypeptidases are exopeptidases
They cleave at the last peptide bond
This leads to short peptides and free neutral and basic amino acids
What happens to small peptides produced by soluble peptidases?
They diffuse to the brush border
Here, membrane-bound peptidases will chop them up into amino acids and dipeptides
What are amino acids taken up by?
What is this process driven by?
They are taken up by sodium-linked secondary active transporters
These are driven by the Na+/K+ pump
What happens when the concentration of amino acid in the cell becomes high?
It exits the cell via a passive amino acid transporter
What are dipeptides taken up by?
What happens to them once they enter the cell?
They are taken up by proton-linked secondary active transporters
They are converted to individual amino acids within the cell by dipeptidases
They are exported passively
How are amino acid and dipeptide transport mechanisms linked?
Proton gradient depends on Na+/H+ exchange
This depends on the Na+ gradient
What happens to the minute fraction of protein that is taken up by endocytosis?
Most is degraded
In the microfold cells overlying the Peyer’s patches, the protein is not degraded
Why is protein not degraded in the M cells overlying the Peyer’s patches?
The M cells are important in delivering antigens out of the gut to the immune system
What is the purpose of bile salts?
They break up lipid droplets, which increases the surface area
Why are fats needed to be broken down into smaller droplets?
Fats are not water-soluble so tend to stick together as large fat globules
Water soluble enzymes cannot reach most of the fat as it is hidden inside the globule
What is meant by bile salts being amphipathic?
They are charged on one side and uncharged on the other
How do the bile salts stick to the fat globule?
The hydrophobic region of the bile salt sticks to the surface of the fat globule
This breaks the globule into smaller droplets, allowing enzymes to access them
What is the difference in the ways that the different lipases can cleave triacylglycerols?
Lingual lipase and gastric lipase can cleave off one fatty acid only
Pancreatic lipase can cleave off both of the outside fatty acids
What are the products of pancreatic lipase breakdown?
Pancreatic lipase cleaves off the “outside” fatty acids
This leaves a monoglyceride and 2 free fatty acids
What happens to the monoglycerides and fatty acids after they have been cleaved?
The monoglycerides and fatty acids form a complex with bile salts
The bile salts solubilise them to form a mixed micelle
What is the purpose of mixed micelle formation?
They are small and in solution meaning that the fatty acids and monoglycerides can diffuse much more easily
What is the action of the mixed micelles?
They diffuse closer to the brush border in the unstirred layer
Here they will deliver their contents to the membrane
What happens to free fatty acids that enter the unstirred layer?
They are protonated to neutralise them
They become an uncharged molecule which can diffuse across the membrane
What happens to the fatty acids and monoglycerides once they enter the cell?
- Triglycerides are re-synthesised and coated in protein
- They are packaged into chylomicra
- The chylomicra are then exocytosed into the interstitium
What happens to the chylomicra once they are in the interstitium?
They are too large to enter capillaries
They enter lymphatic lacteals instead
What happens to conjugated bile salts in the distal ileum?
They are actively absorbed via a sodium-linked co-transporter
The bile salts are passed into the bloodstream to be transported back to the liver
What happens to bile salts which manage to pass into the colon?
They are deconjugated by bacteria
This allows them to become lipophilic and passively reabsorbed
They get passed back to the liver for processing
What % of bile salts are lost in the faeces?
Around 5%
How is iron transported into the cell in the form of haem?
If it is bound to haem, the haem groups are absorbed into cells by a transporter
What happens to haem once it is inside the cell?
It is broken down into biliverdin
Biliverdin is converted to bilirubin
During this process, iron is released into the cell
What other forms of iron are found in the body?
Fe2+ or Fe3+ can be absorbed from foodstuffs
Fe3+ is reduced by Dcytb enzyme into the Fe2+ form
What is the process that moves Fe2+ into the cell?
A divalent metal transporter transports Fe2+ into the cell
This is secondary active transport as it is driven by a proton gradient
What is the role of mobiliferrin?
It binds to iron to prevent the concentration in the cytosol from becoming too high
Where will mobiliferrin transport iron to within the cell?
It will carry the iron along the basolateral membrane to feroportin
What is the role of feroportin?
It transports iron across the membrane
What is the role of hephaestin?
It reoxygenates the iron to Fe3+, which is carried in the bloodstream by transferrin
Why is the concentration of calcium within the cytosol significant?
There is a very low concentration of calcium within the cytosol
This is significant as it is used as a signalling molecule
How is calcium absorbed and where in the GI tract is it absorbed?
Calcium is actively reabsorbed in the duodenum
This process is regulated by Vitamin D
In the rest of the small bowel, it is reabsorbed paracellularly
What molecules are involved in keeping intracellular calcium concentrations low?
Calcium-binding proteins such as calmodulin and calbindin
They also help to transport Ca2+ to the basolateral membrane
What happens to Ca2+ on the basolateral membrane?
A Ca2+/H+ ATPase will work by a primary active transport mechanism to pump calcium out of the cell in exchange for protons
Or a secondary active transporter will exchange 3 Na+ ions entering the cell for 1 Ca2+ ion leaving the cell
Why must 1 Ca2+ ion be exchanged for 3 Na+ ions?
3 sodium ions are required as a lot of energy is needed to transport the calcium out of the cell
This is due to the steep concentration gradient
What is concentration of free calcium in the plasma and total plasma calcium?
Why is there a difference?
Free Ca2+ in the plasma is about 1.2 mM
Total plasma Ca2+ is around 2.5 mM
The rest of the calcium is attached to proteins, particularly plasma albumin
What is significant about calcium bound to albumin?
It is not available to react
Only free calcium is the part available to react
What are the fat-soluble vitamins and how are they absorbed?
A, D, E, K
They are absorbed with lipids through passive processes
They dissolve in lipid droplets, micelles and chylomicrons
How are water-soluble vitamins absorbed?
Through special transport proteins which are usually Na+ linked
Why is an active uptake system required to absorb water-soluble vitamins?
The vitamins are charged so cannot diffuse across membranes
What must be present in order for vitamin B12 to be absorbed?
It is a large molecule and is only absorbed when it is bound to intrinsic factor
Where is intrinsic factor secreted from?
Gastric parietal cells
What happens to cobalamin in the stomach?
How does it bind to intrinsic factor?
It is broken up in the stomach and bound to haptacorrin
It then passes down into the gut and releases the haptacorrin so it can bind with intrinsic factor
what is the role of intrinsic factor?
It acts as a “flag” as there are receptors in the distal part of the ileum that detect the intrinsic factor and endocytose it
This should also pick up the cobalamin
What happens to cobalamin once it is inside the cell?
It is processed and bound to trans-cobalamin II
It is then exported into the circulation
