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GI lectures > physiology and pharmocology of the large intestine > Flashcards

physiology and pharmocology of the large intestine Flashcards

1
Q

how big is the large intestine?

A

1.7m long

6cm in diameter

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2
Q

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

A

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

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3
Q

where is smooth muscle thickened in the large intestine?

why?

A

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

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4
Q

does the caecum have a specialised function in humans?

A

no

involved in metabolism of cellulose in other species

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5
Q

what happens in the caecum?

A

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

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6
Q

how does the ileocaecal valve work?

A

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

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7
Q

what is the structure of the appendix?

A

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

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8
Q

what are the 4 primary functions of the colon?

A

absorption
secretion
reservoir
periodic elimination of faeces

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9
Q

what is absorbed in the colon?

A

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

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10
Q

what is secreted by the colon?

A

potassium
bicarbonate (from the blood to the lumen)
mucus

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11
Q

describe periodic elimination of faeces?

A

largely under voluntary control

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

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12
Q

what are the solid components of faeces?

A

cellulose
bacteria
bilirubin
small amount of salt

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13
Q

does the mucosa of the colon have villi?

A

no

has colonic folds, crypts and microvilli that increase surface area

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14
Q

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

A

surface epithelial cells (colonocytes) mediate electrolyte absorption

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15
Q

what do crypts mediate?

A

ion secretion

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16
Q

what do goblet cells secrete?

A 
mucus
trefoil proteins (host defence)
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17
Q

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

A

enhances it

18
Q

what deficiency can diarrhoea cause?

A

potassium

19
Q

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

A

0.1L

20
Q

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

A

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

21
Q

how does haustration occur?

A

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

22
Q

how does mass movement occur?

A

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

23
Q

how does defaecation occur?

A

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

24
Q

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

A

false
contains 10X more
most are beneficial (commensals)

25
Q

what are the functions of colonic commensal bacteria?

A

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

26
Q

what is the function of the anus?

A

permits expulsion of intestinal gas as well as faeces

27
Q

how do gases in the colon arise?

A 
swallowed air (most burped up but some reaches colon)
bacteria in colon attack carbohydrates
gas not absorbed in large intestine expelled through anus
28
Q

how is gas expelled through the anus?

A

abdominal contractions

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

29
Q

what is constipation?

A

presence of hard dried faeces within colon

30
Q

what can cause constipation?

A

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)

31
Q

what can constipation cause?

A 
abdominal discomfort
headache
loss of appetite
general malaise
due to prolonged distension of large intestine (not toxins as dealt with by liver)
32
Q

what is appendicoliths?

A

hardened calcified faecal matter in appendix

can cause appendicitis

33
Q

what is the difference between laxatives and purgatives?

A

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

34
Q

how do laxatives work?

A

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

35
Q

list some medical uses of laxatives and purgitives

A

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

36
Q

how do bulk laxatives work?

A

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

37
Q

how do osmotic laxatives work?

A

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

38
Q

name some stimulant purgatives?

what side effects can these give?

A

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

39
Q

what do faecal softeners do?

A

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

detergent like action

40
Q

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

A

IBS

IBD

41
Q

how is IBS treated?

A 
symptomatic treatment
adjust diet
anti-diarrhoeals 
anti-spasmodics
laxatives as required
42
Q

how is IBD treated?

A

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

GI lectures (75 decks)

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