physiology and pharmocology of the large intestine Flashcards
how big is the large intestine?
1.7m long
6cm in diameter
what are the 3 strands of longitudinal smooth muscle in caecum and colon?
teniae coli - longitudinal in colon
encircles the rectum and anal canal
contraction, together with circular muscle in colon, causes the haustra
where is smooth muscle thickened in the large intestine?
why?
internal anal sphincter
surrounded by skeletal muscle of the external anal sphincter
Maintains continence
does the caecum have a specialised function in humans?
no
involved in metabolism of cellulose in other species
what happens in the caecum?
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
how does the ileocaecal valve work?
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
what is the structure of the appendix?
blind ended tube
extensive lymphoid tissue connected to distal caecum via appendiceal orifice that may be obstructed by faecalith causing appendicitis
what are the 4 primary functions of the colon?
absorption
secretion
reservoir
periodic elimination of faeces
what is absorbed in the colon?
sodium
chloride
water (forms solid/semi-solid stool)
short chain fatty acids
what is secreted by the colon?
potassium
bicarbonate (from the blood to the lumen)
mucus
describe periodic elimination of faeces?
largely under voluntary control
150g of faeces per day composed of water (100g) and 50g solids
what are the solid components of faeces?
cellulose
bacteria
bilirubin
small amount of salt
does the mucosa of the colon have villi?
no
has colonic folds, crypts and microvilli that increase surface area
what drives absorption of water in the colon (osmosis)?
surface epithelial cells (colonocytes) mediate electrolyte absorption
what do crypts mediate?
ion secretion
what do goblet cells secrete?
mucus trefoil proteins (host defence)
what effect does aldosterone have on absorption of sodium and potassium?
enhances it
what deficiency can diarrhoea cause?
potassium
how much, of the 1-2L of material received, does the colon absorb per day?
0.1L
what are the 3 patterns of motility in the large intestine?
haustration (non-propulsive segmentation)peristaltic propulsive movements (mass movement)
defaecation (periodic egestion)
how does haustration occur?
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
how does mass movement occur?
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
how does defaecation occur?
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
the large intestine contains less bacteria than the rest of the body, true or false?
false
contains 10X more
most are beneficial (commensals)
what are the functions of colonic commensal bacteria?
increase intestinal immunity
promote motility and maintain mucosal integrity
synthesise vit K2 and free fatty acids
activate some drugs (eg IBD drugs)
what is the function of the anus?
permits expulsion of intestinal gas as well as faeces
how do gases in the colon arise?
swallowed air (most burped up but some reaches colon) bacteria in colon attack carbohydrates gas not absorbed in large intestine expelled through anus
how is gas expelled through the anus?
abdominal contractions
internal and external sphincters are contracted to form an exit too small for solid matter to escape
what is constipation?
presence of hard dried faeces within colon
what can cause constipation?
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)
what can constipation cause?
abdominal discomfort headache loss of appetite general malaise due to prolonged distension of large intestine (not toxins as dealt with by liver)
what is appendicoliths?
hardened calcified faecal matter in appendix
can cause appendicitis
what is the difference between laxatives and purgatives?
laxatives = treat constipation
purgatives = cause purging/cleansing of bowels by promoting evacuation
NEITHER SHOULD BE USED WHEN PHYSICAL OBSTRUCTION OF BOWEL
how do laxatives work?
increase peristalsis and/or soften faeces assisting evacuation
can be abused in eating disorders or people obsessed with regularity causing laxative dependency
list some medical uses of laxatives and purgitives
when straining is damaging to health (eg angina) or painful (eg haemorrhoids)
to clear bowel before surgery
treat drug-induced constipation in bedridden/elderly
how do bulk laxatives work?
indigestible polysaccharide polymers (eg. methycellulose - orally) improve stool consistency
slow acting
how do osmotic laxatives work?
poorly absorbed solutes (eg magnesium sulphate/hydroxide - orally, sodium citrate - rectally)
rapid acting
lactulose orally
name some stimulant purgatives?
what side effects can these give?
bisacodyl - orally or suppository when rapid action
sodium picosulohate
anthraquinone laxative (Senna)
Cause abdominal cramps
what do faecal softeners do?
eg. docusate sodium - orally, arachis like oil as enema
detergent like action
name 2 diseases that fall in the category of chronic bowel disease?
IBS
IBD
how is IBS treated?
symptomatic treatment adjust diet anti-diarrhoeals anti-spasmodics laxatives as required
how is IBD treated?
glucocorticoids for acute attacks (prednisolone or budesomide)
aminosalicylates for maintainance (5-ASA - mesalazine, all end in azine)
