physiology of the large intestine Flashcards
what does the large intestine comprise of
>caecum >appendix >colon - ascending, hepatic flexure, transverse, splenic flexure descending and sigmoid >rectum >anal canal and anus
large intestine is lined with longitudinal smooth muscle layer in the caecum and colon - true or false
true
what is the taeniae coli
the three strands of smooth muscle layer in the caecum and the colon
muscle layers at the:
internal anal sphincter
> smooth muscle is thickened
>surrounded by skeletal muscle of the external anal sphincter
what is the function of the taeniae coli and circular muscle layers in the colon
> they cause ‘sac-like’ bulges - the haustra
how does material travel from small intestine to large intestine
from the terminal ileum to the caecum via the gastroileal reflex
gastroileal reflex
> responds to gastrin and CCK, the presence of foodstuff in the stomach and gastric peristalsis
initiation of the reflex causes peristalsis in the ileum and opens the ileocecal valve
does the ileocaecal valve do
> maintains a positive resting pressure
>relaxing in response to distension of the duodenum
>contracting in response to distension of the ascending colon
being under the control of the vagus nerve, sympathetic nerves, enteric neurones and hormonal signals
what is the appendix
a blind-ended tube with extensive lymphoid tissue connected to the distal caecum via the appendices orifice that may be obstructed by faecalith, potentially causing appendicitis
primary functions of the colon
> absorption of sodium chloride and water to condense ileocaecal material to solid, or semi-solid stool
absorption of short chain fatty acids
Secretion of potassium, bicarbonate and mucus
reservoir -storage of colonic contents
periodic elimination of faeces
fluid and electrolyte movement in the colon
> possesses colonic folds, crypts and microvilli that increases surface area
-surface epithelial cells (colonocytes) mediate electrolyte absorption which drives absorption of water
crypt cells mediate ion secretion
goblet cells secrete -copious mucus containing glucosamineglycans - hydrated to form a slippery surface gel
trefoil proteins involved in host defence
what are the 3 main patterns of motility in the large intestine
- haustration
- peristaltic propulsive movements
- defaecation
what is haustration
-non-propulsive segmentation
>haustra are saccule caused by alternate contraction of the circular muscle
-disappear before and reappear after a mass movement
>generated by slow wave activity
>mixes content -allows time for fluid and electrolyte reabsorption
what is the benefit of mass movement in digestion
> mass movement is simultaneous contraction of large sections of the circulate muscle of the ascent and transfer colon and it powerfully drives faeces into distal regions
triggered by the gastrocolic response involving gastrin and extrinsic nerve plexuses
> > mass movement in the distal colon propels faeces in to the rectum triggering the defection reflex in response to rectal stretch
defaecation
> mass movement - the rectum fills with faecal matter > this activates the rectal stretch receptors > this leads to the activation of afferents to the brain and also leads to the activation of afferents to the spinal cord»_space; this respectively leads to the altered firing in efferent to the spinal cord and also the activation of parasympathetic efferents
> > > leads to the contraction of smooth muscle of the sigmoid colon and rectum - the internal anal sphincter relaxes this leads to either
1) relaxation of skeletal muscle of external anal sphincter - defection assisted straightening of the anorectal angle, abdominal skeletal muscle contraction and expiration against closed glottis
2) defection delayed - rectal wall gradually relaxes
function of colonic flora and intestinal gases
> increase intestinal immunity by competition with pathogenic microbes
promote motility and help maintain mucosal integrity
synthesise vitamin K and free fatty aids that are absorbed
activate some drugs
>anus permits expulsion of intestinal gas (flatus) as well as faeces
how do gases arise
> swallowed air - eructation
bacteria in the colon attacks carbohydrates indigestible to human
-gas that is not absorbed in the large intestine is expelled through the anus - selective expulsion requires abdominal contractions
-internal and external sphincters are contracted to form an exit too narrow for solid matter to escape
how does IBS occur
> bouts of diarrhoea, constipation, abdominal pain
> treatment is largely symptomatic with adjustment of diet and anti-diarrhoeals, anti-spasmodics and laxatives as required
linaclotide
> peptide drug
treats moderate to severe IBS with constipation in adults - should not be used for IBD (irritable bowel disease)
> > acts to increase chloride and bicarbonate secretion, intestinal fluid and also increases the rate of intestinal transit
improves bowel movements and alleviates discomfort - although diarrhoea is an adverse effect
amitriptyline
> can be taken for abdominal pain that is resistant to other drugs
> > increases noradrenergic or serotonergic neurotransmission by blocking the noradrenaline or serotonin transporter at presynaptic terminals
-analgesic, also an antidepressant
IBD
> irritable bowel disease is an umbrella term for two conditions: may affect the entire gut - Crohn’s disease
or the colon - ulcerative colitis
it is chronic inflammation of the the GI tract that may over time end up damaging the epithelium and causing serious life-threatening complications
how to treat IBD
> glucocorticoids for acute attacks (prolonged use is limited by adrenal suppression)
aminosalicylates for maintenance and mild disease
eg sulfasalazine , mesalazine , olsalazine
