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