Physiology and pharmacology of the large intestine Flashcards
What is the length and diameter of the large intestine
1.7 m long, 6 cm diameter
What makes up the large intestine
Caecum Appendix Ascending colon Transverse colon descending colon Sigmoid colon Rectum Anal canal Anus
what are haustra
sac like pouches caused by the activity of the taeniae coli
What are the taeniae coli
Strands of longitudinal smooth muscle in caecum and colon
Where in the large intestine is smooth muscle thickened
At the internal anal sphincter which is surrounded by the skeletal muscle of the external sphincter
What is the function of the appendix
no specialised function in humans
What material does the caecum receive
indigestible residues
unabsorbed biliary components
unabsorbed fluids
What reflex permits entre into the caecum
The gastroilial reflex which is in respone to gastrin and CCK allowing passage of material through the ileocaecal valve
How does the ileocaecal valve act
Maintains a positive resting pressure
relaxes in response to duodenal distension
contracts in response to distension of the ascending colon
controlled by vagus nerve, sympathetic nerves, enteric neurons and hormones
What connects the caecum and appendix
The appendiceal orifice which if blocked by faecalith (hard faeces) can cause appendicitis
What are the primary functions of the colon
Absorption of Na+, Cl-, and H2O Absorption of short chain fatty acids Secretion of K+, HCO3-, and mucus Reservoir storing colonic contents Periodic elimination of faeces
What happens to carbohydrates not absorbed by the small intestine
Carbs not absorbed by the small intestine are fermented by colonic flora into short chain fatty acids
What are faeces composed of and how much are excreted per day
150g per day
composed of H2O (100g)
solid material (50g)
cellulose, bacteria, bilirubin, salt
What structures increase absorption of the large intestine
Colonic folds
Crypts
Microvili
What are colonocytes
surface epithelial cells which mediate electrolyte absorption
Which cells mediate ion secretion
Crypt cells
What do goblet cells secrete
Mucus containing glycosominoglycans - hydrated to form a slippery surface gel
trefoil proteins involved in host defence
What substance enhances K+ secretion and Na+ absorption
Aldosterone
K+ is maintained in secretory diarrhoea true/false
false, there is significant loss of K+ in the faeces
What is haustration
non-propulsive segmentation
What is the name for mas movement through the large intestine
peristaltic propulsive propulsive movements
What is defaecation
periodic egestion
What causes haustra
alternating contraction of circular muscle generated by slow wave activity
What is the purpose of haustra
mixes contents and allows time for fluid and electrolyte reabsorption
How many times a day does mass movement occur
One to three times a day, typically triggered by a meal via the gastrocolic response involving gastrin and extrinsic nerve plexus
Describe how defaecation occurs
Mass movement - the rectum fills with faecal matter
Rectal stretch receptors send afferents to the spinal cord and brain
brain causes urge to defaecate
Spinal cord activated parasympathetic efferents which cause contraction of the sigmoid colon and rectum and relaxation of internal anal spinchter
If skeletal muscle externam anal spinchter is relaxed defaecation occurs and if not it is delayed
What role do commensal bacteria in the large intestine play
increase intestinal immunity by competing with pathogenic microbes
promote motility and help maintain mucosal integrity
synthesise vitamin K2 and free fatty acids from carbohydrates
Activate certain drugs
Where does intestinal gas arise from
Swallowed air (eructation 'burped up') Bacteria in the colon which attacks indigestible carbohydrates
What is constipation
presence of hard dried faeces in the colon
What causes constipation
ignoring or supressing the urge to defaecate decreased colonic motility Obstruction of faecal movement Paralytic ileus following abdominal surgery Impairment of motility/defaecation reflex Hischprung disease (absence of secretion from enteric nervous system)
What drugs treat constipation
Laxatives
purgatives
What are purgatives
drugs that induce purging or cleaning of the bowels by promoting evacuation
How do laxatives work
Increase peristalsis and/or soften faeces
What are medically sound uses of laxatives/purgatives
when ‘straining’ is potentially damaging to health (angina) or when defecation is painful (haemorrhoids)
to clear bowels before surgery or endoscopy
To treat drug induced constipation or in bedridden or elderly patients
What are four types of laxative
Bulk laxatives
Stimulant purgatives
Osmotic laxatives
Faecal softners
How do bulk laxatives work
They are indigestible polysaccharide polymers which improve stool consistency slow acting increase bulk and peristalsis retain H20 METHYLCELLULOSE
How do stimulant purgatives work
increase H20 and electrolyte secretion increase peristalsis faecal softening rapidly acting BISACODYL
How do osmotic laxatives work
Rapidly acting retain H20 increase bulk increase peristalsis MAGBESIUM SULPHATE
How to faecal softeners work
Detergent like action
increase H20 and electrolyte secretion
increase peristalsis
faecal softening
What drugs are used to treat IBS and IBD
Glucocorticoids for acute attacks
Aminosalicylates (more for UC) for maintenance
