Small Intestine Structure and Function Flashcards
What is the average length of the small intestine?
6 metres
->called the small intestine due to small diameter, not length!
Name the three parts of the small intestine.
Duodenum
Jejunum
Ileum
What is the shortest part of the small intesttine?
Duodenum, about 25 cm long
What is the function of the duodenum?
-Gastric acid neutralisation as is the first part that the stomach contents will meet after leaving the stomach.
-Digestion.
-Iron absorption.
How is gastric acid neutralised?
By bicarbonate
Where does bicarbonate come from?
Duodenum
Pancreatic juice
Bile
How does the duodenum play a role in digestion?
Digestive enzymes come from the pancreatic juices and bile which help with the breakdown of fats
Why is it important, it terms of the digestion of fat, to have bile and pancreatic juice?
Bile creates an emulsion which is critical to allow fats to be digested and broken down
How long is the jejunum?
Approx. 2.5m long
What is the main function of the jehunum?
Nutrient absorption- 95% of nutrients are absorbed in the jejunum
How long is the ileum?
Approx. 3.5m
What is the main function of the ileum?
Absorption of sodium chloride and water to dehydrate the chyme
Why is it important to dehydrate the chyme?
So it’s more solid when entering the large intestine and can be removed from the body as faeces
How is the absorptive surface area of the small intestine enhanced?
Villi, folds
What are the layers of the small intestine?
Mucosa
Submucosa
Muscularis
Serosa
What are the two muscle layers in the small intestine?
Circular muscle
Longitudal muscleq
What do the folds in the small intestine allow for?
Mixing of the chyme
Where does absorption usually happen in the small intestine?
Villi
What are the capillaries of the villi called?
Lacteal
What do the lacteal allow for?
Assists with fat absorption
What are the roles of the goblet cells on the surface of the villi?
Secrete mucus
What are the roles of crypt ducts on the villi?
-Secrete water into the lumen of the small intestine
-Also contain stem cells so can help w the regeneration of intestinal epithelial cells.
There is a continuous turnover of epithelial cells of the small intestine. How long is the lifespan of an epithelial cell in the small intestine?
3-5 days
What happens to the epithelial cells when they die?
Can be used as nutrients as made of proteins
Removed from body in faeces
Why will there be many side effects to the GIT is a patient is having chemo?
Chemo targets rapidly growing and dividing cells, which the epithelial cells of the GIT are
What type of epithelium are the epithelial cells of the small intesine?
Columnar epithelium
How much more surface area does having the folds in the small intestine give it?
3x greater surface area
How much more surface area does having the villi in the small intestine give it?
30x greater surface area
How much more surface area does having the microvilli in the small intestine give it?
600x greater surface area
What is the difference between villi and microvilli?
Villi are the larger projections on the surface of the small intestine, microvilli are small projections on the villi of the small intestine
What form of fat do lacteals absorb?
Chylomicrons
Which cells form the villi?
Villus cells
What can villus cells absorb?
Sodium chloride
Monosaccharides (glucose, galactose, fructose)
Amino acids
Peptides
Fats
Vitamins
Minerals
Water
RECAP- briefly describe how the sodium coupled active transport works in the small intestine.
Secondary active transport as NaK pump too
NaK pump-> 3 Na out, 2 K in, electrochemical gradient for sodium inside cells meaning it transports into lumen of cells.
Sodium transport is co-transport as glucose or amino acid gets transported into the cell at the same time thanks to the sodium coupled receptor
How much water does the small intestine produce in a day?
Approx. 1500ml
Where does the water secretion come from in the small intestine?
Epithelial cells lining crypts of Lieberkuhn
Why is water important in the small intestine?
Keeps material in a liquid state until the ileum
Promotes mixing with digestive enzymes
Describe the transport of chlorine into the lumen.
-Comes from blood through the basolateral membrane via sodium potassium chloride channel.
-2 Cl, 1 Na, 1 K get transported into the cell simultaneously.
-Use of NaK pump to keep the cycle of Na out of cell and K into cell.
-Chlorine accumulates and eventually goes through brush border membrane and into lumen of small intestine.
How does chlorine get through the brush border membrane?
Via CFTR
What is CFTR?
Cystic fibrosis transmembrane conductance regulator
What do CFTR do?
Helps move chlorine outside the crypt epithelial and this moves water indirectly.
What happens if there are genetic changes in the CFTR?
Individuals will be born w cystic fibrosis
What activates the CFTR channel?
Enzyme called adenylate cylase.
What does adenylate cyclase do?
Converts ATP to cyclic AMP
What are the two types of intestinal motility?
Segmentation
Peristalsis
When is segmentation common?
During meals
What is the main function of segmentation?
To mix and digest food
How does the chyme move during segmentation?
Continuous contraction and relaxation of the small intestine
How is segmentation generated?
Contractions caused by depolarisation generated by pacemaker cells.
This produces action potentials which make contraction.
Which cells initiate depolarisation for contractions of segmentation and where are they found?
Pacemaker cells found in longitudal muscle
What does the intestinal basic electrical rhythm (BER) do?
Produces oscillations in membrane potential which exceeds the threshold which produces an action potential, causing contraction.
What determines the strength of contraction?
The frequency of action potentials
As the chyme moves from the intestine towards the rectum, what happens to the BER?
Decreases further down the system
Which nervous system increase strength of contraction of the small intestine?
Parasympathetic NS
Which nervous system decrease strength of contraction of the small intestine?
Sympathetic NS
Why is there no effect of the autonomic NS on the BER?
Self controlled by pacemaker cells in the longitudal muscle
What does peristalsis do?
Helps move the chyme towards the large intestine
What controls peristalsis?
Migrating motility complex (MMC)
When does MMC stop?
When food arrives in the stomach, segmentation begins at this point
What does MMC act to do in the process of peristalsis?
Move undigested material to large intestine.
Prevent bacterial colonisation of small intestine.
What is involved in the initiation of MMC?
Motilin, a type of hormone
What happens to the size of the small intestine when chyme reaches it?
Causes distension and size of small intestine will increase
What happens to the muscle on the oral side of the bolus?
Contracts
What happens to the muscle on the anal side of the bolus?
It relaxes
What is the difference between bolus and chyme?
Chyme is food that has been mixed with gastric juice
Bolus is food which has been mixed w saliva
How is the contraction and relaxation of the small intestine (to move the food from one end to the other) mediated?
By neurones in the myenteric plexus
When there is gastric emptying, what happens in the ileum?
Increase in segmentation activity in the ileum
What opens when there is an increase in segmentation activity in the ileum?
Opening of ileocecal valve
What does the ileocecal valve/sphincter do?
Controls entry of chyme into large intestine
After distension of the colon, what happens to the ileocecal valve?
It closes to prevent backflow
