Small intestine structure and function Flashcards
How long is the small intestine?
~6m (4.5-9.0)
What are the sections of the small intestine?
Duodenum
Jejunum
Ileum
What are the basic functions of the duodenum?
Gastric acid neutralisation
Digestion
Iron absorption
What are the basic functions of the Jejunum?
Main site of digestion - accounting for 95% of it
What are the basic functions of the ileum?
NaCl & H2O absorption for Chyme dehydration
Which section of the small intestine is largest?
The Ileum - making up 3/5’s of the total length (3.5m long)
What is the function of mucosal crypts?
All mucosal cells start life at the base of these crypts as stem cells.
They undergo a process of differentiation and move up the crypt, changing as they go.
- These crypts secrete Cl- ions which takes water with it.
- They also secrete a lot of mucous from goblet cells
- Paneth cells near the base are involved in the innate immune system
- Plus several other functions/cell types`
How much water is ooshed out into the small intestine per day?
1500ml per day
How is water secreted into the lumen of the small intestine?
H2O is secreted passively (osmotically) as a consequence of active secretion of chloride into intestinal lumen by the Crypt cells
Why is H2O important in the normal digestive process?
- Keeps lumenal contents liquidy
- Promotes mixing of contents with digestive enzymes
- Aids nutrient presentation to absorbing surface
- Dilutes and washes away nasty shit in the lumen
Histologically, where are the absorb-ey cells found in the small intestine?
Found on the villi (hence they are sometimes called villus cells)
The most mature & absorbing cells are found in the top 1/3rd of the villi
In Crypt cells, what transporter allows entry of Cl- into the cell from the blood?
What other transporter is also necessary for this to work?
The Na-K-Cl co-transporter
There is also the Na-K pump (and a leaky K+ channel) which generates a sodium electrochemical gradient
Describe how Cl- are pumped through crypt cells and how this causes the secretion of water into the small intestine
- Cl- enters cell from blood - this is done through the Na-K-Cl co-transporter
- Cl- is more concentrated in the cell thus is pushed out CFTR chloride channels and into the lumen
- Exit of Cl- creates an osmotic gradient.
- Tight junctions between cells are permeable to water - so it diffuses by osmosis out into the lumen
What illness would mess up all this Water secretion stuff?
Cystic fibrosis (CFTR dysfunction)
Describe how the CFTR Cl- channel is activated? (opened)
- Adenylate cyclase converts ATP to cAMP
- cAMP phosphorylates its own protein kinase to make Protein Kinase A (PKA)
- PKA then activates the CFTR channel (Because this is how Cl- secretion is controlled - this is basically how the rate of water secretion is regulated in the small intestine)
What are the 2 types of intestinal motility?
Segmentation & peristalsis
What type of motility is more important in the small intestine and why?
Segmentation is utilised more
Digestion and absorption are slow processes, there is no need to move yer shite along (peristalsis) too fast
Describe what happens during segmentation
Short intestinal segments contract and relax to move the chyme back and forth
This causes the chyme to be thoroughly mixed up with digestive enzymes
This also maximises the amount of chyme in contact with the absorbing surface
How are segmentation contractions in the small intestine generated?
They are initiated by a depolarisation that is generated by pacemaker cells in the Longitudinal layer of muscle
What determines the strength of segmentation contractions?
Frequency of action potentials
What determines the frequency of segmentation contractions?
The Basic elecrical rhythm (BER)
What is the effect of Vagal stimulation on segmentation contractions?
Parasympathetic stimulation will cause increased strength of segmentation contractions
Autonomic NS has no effect on the BER and thus no effect on the frequency of contraction
What is the Migrating motility complex (MMC)?
- Pattern of peristaltic activity travelling down the small intestine
- It basically causes the smooth muscle to contract and push undigested material to the large intestine
What pars of the GI tract does MMC start and end?
- Starts at gastric antrum
- Ends at terminal ileum
What causes cessation of MMC?
Entry of food into the stomach
(Segmentation starts again and peristalsis MMC stuff stops)
What hormone activates the MMC?
Motolin
What is the effect on smooth muscle in the intestine when it becomes distended by bolus?
What controls this?
- Muscle on oral side will contract
- Muscle on anal side will relax
- Controlled by neurones in the myenteric plexus
What is the effect of gastric emptyin on the segmentation activity in the ileum?
Emptying = Increased segmentation activity
Describe how chyme enters the large intestine
- Ileocecal valve opens
- Entry of chyme into large intestine
- Distension of colon
- Reflux contraction of ileocecal sphincter to prevent backflux
