Physiology: Large Intestine Flashcards
What is the approximate length and diameter of the large intestine?
1.7m long and 6cm in diameter
What are the different parts of the large intestine?
- caecum
- appendix
- colon (ascending, transverse, descending, sigmoid)
- rectum
- anal canal and anus
What is the name of the 3 longitudinal strands of muscle that run along the colon and caecum?
Taeniae coli
How is the smooth muscle arranged around the rectum and anal canal?
encircles the rectum and anal canal
Where is smooth muscle thickened?
At the internal anal sphincter which is surrounded by the skeletal muscle of the external anal sphincter
What is the name of the sac-like bulges of the colon?
haustra
What causes the haustra to form sac-like bulges?
activity of the taeniae coli and circular smooth muscle layers, haustra change location
How much material does the caecum normally receive and what is this material?
Approx. 1-2 L - indigestible residues, unabsorbed fluids and unabsorbed biliary products
What is the gastroileal reflex?
entry of substance from terminal ileum into caecum through the ileocaecal valve
What triggers the gastroileal reflex?
gastrin and CCK
How does the ileocaecal valve work? (3 ways)
- maintains a positive resting pressure
- relaxes in response to distension of the duodenum
- contracts in response to distension of the ascending colon
What controls the ileocaecal valve?
- vagus nerve
- sympathetic nerve
- enteric neurones
- hormonal signals
What connects the appendix to the caecum?
appendiceal orifice
What tissue is the appendix made of?
lymphoid tissue
What can cause appendicitis?
obstruction of the appendiceal orifice by a faecalith
What are the absorption functions of the colon?
- Colon absorbs Na+, Cl- and H20 to condense stool
- Colon absorbs short chain fatty acids
What happens to carbohydrates in the digestive tract and where are they absorbed?
Carbohydrates are fermented by colonic flora to short chain fatty acids in the colon
What does the colon secrete?
- K+
- HCO3-
- mucous
What does faeces consist of?
- mostly H20
- cellulose
- bacteria
- bilirubin
- small amount of salt
What does the surface of the colon have in order to increase surface area?
- microvilli
- crypts
- colonic folds
- (no villi)
What cells in the colon mediate ion secretion?
crypt cells
What cells in the colon mediate electrolyte absorption and by what mechanism does this lead to H2O reabsorption?
- colonocytes
- osmosis
What do goblet cells secrete?
- lots of mucous containing glycosaminoglycans (this forms slippery surface gel)
- trefoil proteins involved in host defence
What hormone enhances K+ and Na+ absorption?
aldosterone
What can occur in secretory diarrhoea?
significant K+ loss
How much material approx. does the colon absorb each day?
1L
What are the 3 patterns of motility in the large intestine?
- haustration (non-propulsive segmentation)
- peristaltic propulsive movements (mass movement)
- Defaecation (periodic egestion)
What is haustration?
the formation of haustra (succules) made by alternating contraction that has function similar to segmentation but slower
How is haustration generated?
slow wave activity probably
What is the function of haustration?
mixes content to allow time for fluid and electrolyte reabsorption
What happens to haustra before and after a mass movement?
disappear before and reappear after
What is mass movement (peristaltic propulsive movements)?
simultaneous contraction of circular muscle of the ascending and transverse colon that drives faeces distally
How often do mass movements occur in a day?
1-3 times
What triggers mass movement?
triggered by a meal (often breakfast) via the gastrocolic response
What is the gastrocolic response and what does it involve?
- the urge to poop after a meal
- involving gastrin and extrinsic nerve plexuses
What triggers the defaecation reflex?
rectal stretch caused by faeces entering the rectum
