physiology and pharmocology of the large intestine

Question 1

how big is the large intestine?

Answer 1

1.7m long

6cm in diameter

Question 2

what are the 3 strands of longitudinal smooth muscle in caecum and colon?

Answer 2

teniae coli - longitudinal in colon
encircles the rectum and anal canal
contraction, together with circular muscle in colon, causes the haustra

Question 3

where is smooth muscle thickened in the large intestine?

why?

Answer 3

internal anal sphincter
surrounded by skeletal muscle of the external anal sphincter
Maintains continence

Question 4

does the caecum have a specialised function in humans?

Answer 4

no

involved in metabolism of cellulose in other species

Question 5

what happens in the caecum?

Answer 5

receives 1-2L of indigestible/unabsorbed substances per day from ileum
permitted by gastroileal reflex in response to gastrin and CKK though one way ileocaecal valve

Question 6

how does the ileocaecal valve work?

Answer 6

maintains a positive resting pressure
relaxes in response to distension of duodenum
contracts in response to distension of ascending colon
under control of vagus, sympathetic nerves, enteric neurones an hormonal signals

Question 7

what is the structure of the appendix?

Answer 7

blind ended tube
extensive lymphoid tissue connected to distal caecum via appendiceal orifice that may be obstructed by faecalith causing appendicitis

Question 8

what are the 4 primary functions of the colon?

Answer 8

absorption
secretion
reservoir
periodic elimination of faeces

Question 9

what is absorbed in the colon?

Answer 9

sodium
chloride
water (forms solid/semi-solid stool)
short chain fatty acids

Question 10

what is secreted by the colon?

Answer 10

potassium
bicarbonate (from the blood to the lumen)
mucus

Question 11

describe periodic elimination of faeces?

Answer 11

largely under voluntary control

150g of faeces per day composed of water (100g) and 50g solids

Question 12

what are the solid components of faeces?

Answer 12

cellulose
bacteria
bilirubin
small amount of salt

Question 13

does the mucosa of the colon have villi?

Answer 13

no

has colonic folds, crypts and microvilli that increase surface area

Question 14

what drives absorption of water in the colon (osmosis)?

Answer 14

surface epithelial cells (colonocytes) mediate electrolyte absorption

Question 15

what do crypts mediate?

Answer 15

ion secretion

Question 16

what do goblet cells secrete?

Answer 16

mucus
trefoil proteins (host defence)

Question 17

what effect does aldosterone have on absorption of sodium and potassium?

Answer 17

enhances it

Question 18

what deficiency can diarrhoea cause?

Answer 18

potassium

Question 19

how much, of the 1-2L of material received, does the colon absorb per day?

Answer 19

0.1L

Question 20

what are the 3 patterns of motility in the large intestine?

Answer 20

haustration (non-propulsive segmentation)peristaltic propulsive movements (mass movement)
defaecation (periodic egestion)

Question 21

how does haustration occur?

Answer 21

alternating contraction of circular muscle (similar to segmentation but slower)
long transit time (16-48hrs)
occurs in proximal colon causing orad movement
generated by slow wave activity
mixes content allowing time for absorption

Question 22

how does mass movement occur?

Answer 22

simultaneous contraction of large sections of circular muscle
ascending/transverse colon (haustra disappear)
drives faeces into distal regions
distal colon propels faeces into rectum triggering defaecation reflex
1-3 times per day
triggered by meal via gastrocolic resonse involving gastrin and extrinsic nerve plexuses

Question 23

how does defaecation occur?

Answer 23

mass movement fills rectum > activates rectal stretch receptors > activation of afferents to spinal cord/brain > activation of parasympathetic efferents/efferents to spinal cord > contraction of smooth muscle of sigmoid colon and rectum, internal sphincter relaxes > relaxation/contraction of skeletal muscle of external anal sphincter depending on whether its convenient to defacate

Question 24

the large intestine contains less bacteria than the rest of the body, true or false?

Answer 24

false
contains 10X more
most are beneficial (commensals)

Question 25

what are the functions of colonic commensal bacteria?

Answer 25

increase intestinal immunity promote motility and maintain mucosal integrity synthesise vit K2 and free fatty acids activate some drugs (eg IBD drugs)

Question 26

what is the function of the anus?

Answer 26

permits expulsion of intestinal gas as well as faeces

Question 27

how do gases in the colon arise?

Answer 27

``` swallowed air (most burped up but some reaches colon) bacteria in colon attack carbohydrates gas not absorbed in large intestine expelled through anus ```

Question 28

how is gas expelled through the anus?

Answer 28

abdominal contractions | internal and external sphincters are contracted to form an exit too small for solid matter to escape

Question 29

what is constipation?

Answer 29

presence of hard dried faeces within colon

Question 30

what can cause constipation?

Answer 30

ignoring/suppressing urge to defaecate decreased colonic motility (diet, drugs, age) obstruction of faecal movement paralytic ileus following abdominal surgery impairment of motility/defaecation reflex (hirschprung disease)

Question 31

what can constipation cause?

Answer 31

``` abdominal discomfort headache loss of appetite general malaise due to prolonged distension of large intestine (not toxins as dealt with by liver) ```

Question 32

what is appendicoliths?

Answer 32

hardened calcified faecal matter in appendix | can cause appendicitis

Question 33

what is the difference between laxatives and purgatives?

Answer 33

laxatives = treat constipation purgatives = cause purging/cleansing of bowels by promoting evacuation NEITHER SHOULD BE USED WHEN PHYSICAL OBSTRUCTION OF BOWEL

Question 34

how do laxatives work?

Answer 34

increase peristalsis and/or soften faeces assisting evacuation can be abused in eating disorders or people obsessed with regularity causing laxative dependency

Question 35

list some medical uses of laxatives and purgitives

Answer 35

when straining is damaging to health (eg angina) or painful (eg haemorrhoids) to clear bowel before surgery treat drug-induced constipation in bedridden/elderly

Question 36

how do bulk laxatives work?

Answer 36

indigestible polysaccharide polymers (eg. methycellulose - orally) improve stool consistency slow acting

Question 37

how do osmotic laxatives work?

Answer 37

poorly absorbed solutes (eg magnesium sulphate/hydroxide - orally, sodium citrate - rectally) rapid acting lactulose orally

Question 38

name some stimulant purgatives? | what side effects can these give?

Answer 38

bisacodyl - orally or suppository when rapid action sodium picosulohate anthraquinone laxative (Senna) Cause abdominal cramps

Question 39

what do faecal softeners do?

Answer 39

eg. docusate sodium - orally, arachis like oil as enema | detergent like action

Question 40

name 2 diseases that fall in the category of chronic bowel disease?

Answer 40

IBS | IBD

Question 41

how is IBS treated?

Answer 41

``` symptomatic treatment adjust diet anti-diarrhoeals anti-spasmodics laxatives as required ```

Question 42

how is IBD treated?

Answer 42

glucocorticoids for acute attacks (prednisolone or budesomide) aminosalicylates for maintainance (5-ASA - mesalazine, all end in azine)