Small intestine structure and function

Question 1

what is the total length of the small intestine

Answer 1

~6m

Question 2

what is the basic structure of the small intestine

Answer 2

sphincter of Oddi
duodenum
jejunum
ileum

Question 3

what is the role of the duodenum

Answer 3

gastric acid neutralisation, digestion, IRON absorption

Question 4

what is the role of the jejunum

Answer 4

nutrient absorption (95%)

Question 5

what is the appearance of the jejunum

Answer 5

“ribbon like” – usually flat because usually empty (vast bulk of diet absorbed before you leave) – very well established vasculature – pink appearance

Question 6

what is the role of the ileum

Answer 6

NaCl/H2O absorption = CHYME dehydration

deals with residual capacity - most already absorbed – recouping of salt/water

Lose around 200ml of water feaces - but around 9L pass through everyday - most reabsorbed here

Question 7

what increases the absorptive area of the small intestine

Answer 7

folds, villi, microvilli

Question 8

what is the relative surface area increase going from the intestine as a cylinder to the microvilli

Answer 8

intestine as a cylinder = 1

circular folds (plicae) = 3

villi = 30

microvilli = 600

Question 9

what creates a diffusion barrier for an acid microclimate in the small intestine

Answer 9

goblet cells secreting mucous onto the surface of the villi

Question 10

what is the basic structure of a villi

Answer 10

in-out:

lacteal
capillary network
simple columnar epithelium
containing goblet cells

Question 11

what lie bellow the villi

Answer 11

crypts of lieberkuhn (with crypt cells)

Question 12

what is the role of the crypts

Answer 12

secrete CL and water

Crypts secrete around 1.5L INTO lumen per day from epithelial cells – creates aqueous environment for digestion – once used reabsorbed at villi - recycled

Question 13

why do people with CF not secrete water

Answer 13

lack of Cl channels

Question 14

what is the role f the villi

Answer 14

absorption of:
NaCl
Monosaccharides
Amino acids
Peptides
Fats
Vitamins
Minerals
Water

Question 15

what use Na+ coupled secondary active transport to be absorbed

Answer 15

glucose
galactose
amino acids
nucleosides

Question 16

what causes H2O to be secreted from the epithelial cells lining the crypts of lieberkuhn

Answer 16

active secretion of chloride into the intestinal lumen causes water to be secreted passively (OSMOTICALLY)

Question 17

give 4 reasons why H2O secretion if important for normal digestive processes

Answer 17

1. Maintains lumenal contents in liquid state
2. Promotes mixing of nutrients with digestive enzymes
3. Aids nutrient presentation to absorbing surface
4. Dilutes and washes away potentially injurious substances

Question 18

how is Cl actively transported into the intestinal lumen

Answer 18

to get into cell = by the Na-K-2Cl co-transporter

to get into lumen = by chloride channels

Question 19

what are the two distinct types of intestinal movement

Answer 19

1. segmentation (occurs while actively processing food)

2. peristalsis

Question 20

how does segmentation work

Answer 20

Contraction and relaxation of short intestinal segments

Contraction (few seconds) moves chyme (up & down) into adjacent areas of relaxation

Relaxed areas then contract and push chyme back

Question 21

what does segmentation enable

Answer 21

Provides thorough mixing of contents with digestive enzymes

Brings chyme into contact with absorbing surface

Question 22

what initiates segmentation

Answer 22

initiated by depolarisation generate by pacemaker cells in the longitudinal muscle layer

Intestinal basic electrical rhythm (BER) produces oscillations in membrane potential = threshold = action potential = contraction

Question 23

what determines the STRENGTH of contraction in segmentation

Answer 23

action potential frequency

Question 24

what determines the FREQUENCY of segmentation

Answer 24

BER - basic electrical rhythm

Question 25

what happens to BER as you move down the intestine

Answer 25

decreases

Question 26

ultimately what does segmentation produce

Answer 26

Segmentation produces slow migration of chyme towards large intestine (more chyme pushed down than up)

Question 27

what can increase the contractions in segmentation

Answer 27

PARASYMPATHETIC NS (vagus(

Question 28

what can decrease the contractions in segmentation

Answer 28

SYMPATHETIC NS

Question 29

what is MMC in peristalsis

Answer 29

migrating motility complex - a pattern of peristaltic activity travelling down the small intestine (starts in the gastric antrum)

As one MMC ends (terminal ileum) another begins

Question 30

what causes MMC to stop

Answer 30

arrival of food in the stomach - causes cessation of MMC and initiation of segmentation

Question 31

what are the 2 things MMC acts to do

Answer 31

1. Move undigested material (bolus) into large intestine

2. Limit bacterial colonisation of small intestine

Question 32

what hormone is involved in the initiation of MMC

Answer 32

Motilin - levels will rise if nutrient levels fall below certain level - signals MMC to start

Question 33

what is the law of the intestine

Answer 33

if intestinal smooth muscle is distended (e.g. by bolus of chyme):

1. muscle on oral side of bolus CONTRACTS
2. muscle on anal side of bolus RELAXES
3. bolus is loved into an area of relaxation towards the colon

Question 34

what mediates the movement of the bolus towards the colon

Answer 34

neurones in the myenteric plexus

Question 35

when does the gastroileal reflex occur

Answer 35

when gastric emptying increases segmentation activity in the ileum

Question 36

what occurs during the gastroileal reflex

Answer 36

Opening of ileocaecal valve (sphincter)

Entry of chyme into large intestine

Distension of colon

Reflex contraction of ileocaecal sphincter (prevents backflux into small intestine)