Small intestine: structure and function Flashcards
What are the 3 parts of the small intestine called?
duodenum
jejunum
ileum
what happens in the duodenum? (3)
gastric acid neutralisation
digestion
iron absorption
takes in semi-digested food from your stomach through the pylorus and continues the digestion process
uses bile from your gallbladder, liver, and pancreas to help digest food
Function of Jejunum
carries food through rapidly, with wave-like muscle contractions, towards the ileum
nutrient absorption 95%
what is it about the small intestine that enhances it’s absorptive ability? (3)
folds in the surface
villi
microvilli
What things can a villus cell absorb? (8)
NaCl monosaccharides amino acids peptides fats vitamins minerals water
What are crypt cells and what is their purpose?
invaginations of the epithelium around the villi
they have stem cells that divide and specialise to become villi (mature epithelial cells) when they get to the tip of the villus they die and shed off into the contents of your gut (on av. = 5 day process)
secrete Cl and H20
how much H20 does the small intestine secrete per day?
1500 ml
In the small intestine, where does H20 secretion come from? and what usually reabsorbs it?
epithelial cells lining crypts of Lieberkuhn
villi
Is H20 secretion an passive or active process?
passive
why is H20 secretion important for normal digestive process?
Maintains lumenal contents in liquid state - need solution to get nutrients up to absorbing surface
Promotes mixing of nutrients with digestive enzymes
Aids nutrient presentation to absorbing surface
Dilutes and washes away damaging injurious substances
what are the 2 distinct types of movement in the intestine
segmentation
peristalsis
Describe movement by segmentation
most common during meal
contraction and relaxtion of short intestinal segments
contraction moves chyme (up+down) into adjacent areas of relaxation
think toothpaste tube - contract tube and contents push to both sides + mix. Relaxed areas then contract and push chyme back
provides thorough mixing of contents with digestive enzymes but chyme doesn’t move that far forward - slow process to allow time for absorption
brings chyme into contact with absorbing surface
what is chyme?
the pulpy acidic fluid which passes from the stomach to the small intestine, consisting of gastric juices and partly digested food
how are segmentation contractions generated?
Initiated by depolarisation generated by pacemaker cells in longitudinal muscle layer
Intestinal basic electrical rhythm (BER) produces oscillations in membrane potential - threshold - action potential - contraction
A.P freq. determines strength of contraction
freq. of segmentation determined by BER
as you move down intestine BER decreases
Parasympathetic NS has what effect on segmentation contractions
increases contraction - increase speed of chyme through intestine
sympathetic NS has what effect on segmentation contractions
decreases contraction
is there an effect of autonomic nervous system (sympathetic or para-sympathetic) on BER?
no
what is peristalsis?
After nutrients have been absorbed/ you get all the nutrients required from food - segmentation stops and peristalsis STARTS
what is the migrating motility complex?
pattern of peristaltic activity travelling down SMALL intestine - ie contraction behind chyme/bolus, relaxation in front moving it along
Starts in antrum (tail end) of stomach as one MMC ends (terminal ileum) another begins at the stomach.
If food arrives in the stomach this is put on hold and segmentation occurs to process it then peristalsis starts again once this is done to clear it out.
when does migrating motility complex finish?
when food arrives in stomach - MMC immediately stops and segmentation starts again to get nutrients and it will continue until you process that food and then peristalsis starts again to clear it out
purpose of migrating motility complex?
Move undigested material into large intestine
Limit bacterial colonisation of small intestine
what is the ‘law of the intestine’
if intestinal smooth muscle is distended (e.g bolus filling it) then the muscle on the oral (mouth) side of bolus contracts and muscle on anal side relaxes. Bolus moves into area of relaxation towards colon
Myenteric plexus mediates what?
the movement of bolus through the intestine
primary role of the ileum
to get salt and H2O back as 8/9 litres of H20 go into gut but only have 200ml by the end so ileum ensures more salt absorption and water follows suit
where does the myenteric plexus lie?
between the longitudinal and circular muscle
what are goblet cells, what is their role and where are they located?
goblet cells contain mucus and secrete it out into the apical membrane
They also:-
- help to create the acid microclimate
- control diffusion up to the surface
- protect the surface of cells from excessive damage
they lie interspersed with epithelial cells
villi ____ and cyrpts ____
villi absorb
crypts secrete
Name the transport method that things like glucose, galactose, amino acids and nucleosides follow
Na+ coupled secondary active transport
2 transporters, Na+/K+ pump, osmotic gradient created by Na+ pulls H20 across from lumen into the cell
Overview/summary of normal digestive process in small intestine?
secretory pathway
Regulatory control on the by cyclic AMP – adenylate cyclase makes cAMP
cAMP which goes on to phosphorylate PKA
PKA then activates the CFTR channel which in turn opens and Cl- moves out
Describe the basic electrical rhythm in the small intestine
BER at top is a bit higher than at the bottom – decline is happening very gradually between cells down the gut tube
Material is very gradually moving forward (always downwards due to the BER being slightly higher higher up – can’t move against this ie things can’t go backwards up the tube)
How does the body prevent chyme moving back from the L intestine into the small intestine?
At the ileocaecal valve – junction between small and large intestine – when the chyme reaches it it opens and the chyme moves into large intestine
The large intestine then becomes distended and causes a reflex contraction that goes back and closes the ileocaecal valve
this prevents material from going back into the small intestine
Name of the hormone that is released/required to generate migrating motility complex in small intestine
motilin
when motilin levels in plasma are high, peristalsis takes place
