[Physiology] Small Intestine Flashcards
what is the total legnth of the small intestine
~6 metres (range 4.5 - 9 metres)
duodenum legnth
25 cm long
duodenum function
Gastric acid neutralisation; Digestion; Iron absorption
jejunum legnth
2.5 m long (2/5 total length)
jejunum function
Nutrient absorption - 95%
ileum function
NaCl/H2O absorption ⇒ chyme dehydration
ileum legnth
3.5 m long (3/5 total length)
what are absorptive surfaces enhanced by
folds, villi, microvilli
what 8 things does the villus cell absorb
NaCl
Monosaccharides
Amino acids
Peptides
Fats
Vitamins
Minerals
Water
what is the function of a crypt cell
Secretes Cl & Water
how much fluid does the small intestine secrete per day
~1500ml H2O per day
where does the secretion of H2O come from
epithelial cells lining crypts of Lieberkuhn
how is H2O secreted, by what means of diffusion
passively (osmotically) as a consequence of active secretion of chloride into intestinal lumen
why is H2O secretion important
Maintains lumenal contents in liquid state
Promotes mixing of nutrients with digestive enzymes
Aids nutrient presentation to absorbing surface
Dilutes and washes away potentially injurious substances
what normally happens after H2O is secreted from crypt cells
they are reabsorped by villi
what are the two types of intestinal movement
Segmentation
Peristalsis
when is segmentation most common
during a meal
what occurs when contraction happens by segmentation
Contraction (few seconds) moves chyme (up & down) into adjacent areas of relaxation
Relaxed areas then contract and push chyme back
what does segmentation do successfully
Provides thorough mixing of contents with digestive enzymes
Brings chyme into contact with absorbing surface
how are segmentation contractions initiated
depolarisation generated by pacemaker cells in longitudinal muscle layer (cf gastric motility)
what produces oscillation in membrane potential
Intestinal basic electrical rhythm (BER)
when oscillations reach a certain threshold what occurs
action potential, producing a contraction
what does action potential frequency determine
stregnth of contraction
what happens to BER as you move further down the intestine
decreases as move down intestine → rectum
parasympathetic nervous system causes
increase in contraction
sympathetic nervous system
decrease in contraction
what effect does autonomic nervous system have on BER
no effect
what occurs follwing apsorbtion of nutrients
segmentation stops and peristalsis starts
migrating motility complex
Pattern of peristaltic activity travelling down small intestine (starts in gastric antrum)
what happens as one MMC ends
another begins
what does arrival of food in stomach do to MMC
stops MMC and initiaes segmentation
what does MMC aim to do
Move undigested material into large intestine
Limit bacterial colonisation of small intestine
what is motilin
(hormone) involved in initiation of MMC
If intestinal smooth muscle is distended (eg by bolus of chyme):
[law of the intestine]
Muscle on oral side of bolus contracts
Muscle on anal side of bolus relaxes
Bolus is moved into area of relaxation towards colon
what is the law of the intestine mediated by
neurones in myenteric plexus
Gastric emptying ⇒ ↑ segmentation activity in ileum
Opening of ileocaecal valve (sphincter)
Entry of chyme into large intestine
Distension of colon
Reflex contraction of ileocaecal sphincter (prevents backflux into small intestine)
