LI Flashcards
ileocaecal valve
muscular sphincter separates distal ileum from ceacum tonically active and constricted only relax to allow chime into LI prevent bacteria entering ileum
caecum
blind pouch
distal to ileocaecal calve
appendix
extension from caecum
limited role
safe refuge for gut bacteria after diaorrea
colon
ascending - start at ileocaecal valve -> hepatic flexure on the R side of the body
transverse - start at hepatic flexure and run across abdomen -> splecic flexure
descending - start at splenic flexure and runs to 1st bend of igmoid
sigmoid - S shaped runs to rectum
rectum
dilated portion
act as a storage site
has transverse rectal folds in submucosa - shelves for faeces
no taeniae coli in muscularis externa
anal canal
control movement of things out of GI
2 anal sphincters
sm under central control - internal sphincter
external muscle is striated and under vol control by pudendal nerves
appendix epiploicae
fatty tags from serosa
no physiological function
perhaps protective against intraabdominal infections
longitudinal muscle
3 bands longitudinal muscle
equally spread around the circumference
thicker than typical longitudinal layers
shorter than colon - colon forms lpouches called hastra
circular muscle
segmentally thickened
bundles of muscle from taeniae coli penetrate to keep it together
nodules of lymphoid tissue
usually solitary nodules
components of the LI
colon cecum appendix rectum anal canal
size of the colon
1.5m long, 6cm diameter
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attachment of transverse colon
hangs off the stomach
attached by the greater omentum
blood supply of the colon
proximal transverse colon upplied with blood by middle colic artery (branch of superior mesenteric artery)
distal 1/3 - inferior mesenteric artery
reflects the embryonic division between mid and hind gut
consequence of colon have 2 blood supplies
area between is sensitive to ischemia in haemorrhage
gut wall
peritoneum has fatty tags
muscle coat has 3 thick longitudinal bands - taeniae coli
gut wall is pouched - haustra
nodules of lymphoid tissue common in walls of distal small intestine - peyer’s patches, and LI - solitary nodules for communication between gut immune system and microbiome
function of tenia coli
large intestine motility
Haustra
taenia coli shorter than in SI
cause pouched ovoid segments - haustra
not always in same place - muscle tone
general plan of the gut tube
serosa longitudinal muscle circular muscle muscularis lamina propria epithelium
comparision of the structure of the LI and SI
in both:
enterocytes and goblet cells
abundant crypts
stem cells in crypts
LI mucosal organisation
appears smooth - no villi enterocytes (luminal side) have short irregular microvilli - reabsorption of salts and macronutrients crypts have goblet cells no Paneth cells fewer enteroendocrine cells than SI glycocalyx - no digestive enzymes
goblet cells in LI
more than in SI
more distal
mucus fascilitate passage of more solid contents, covers bacteria and particulate matter
stimulated by ACh - PNS and enteric NS - plexuses in wall of gut
why no Paneth cells in the LI
would kill flora
commensal is important
less bacteria than in SI - food already been digested
muscle layers of LI
muscularis externa - inner circular and outer longitudinal layer
circular are segmentally thickened
longitudinal concentrated in 3 bands - taenia coli
bundles from here penetrate the circular
between taenia longitudinal is thin
longitudinal shorter than circular = haustra = contract individually
components if the large intestine
colon caecum appendix eecctum anal canal
main func of the colon
reabsorption of water and electrolytes
elimination of waste
microenvironment for gut bacteria
structure of the LI
150cm long
6cm wide
receives blood from middle colic artery - perfuses ascending and 1st 2/3 of transverse
and from inferior mesenteric artery - perfuse last 1/3 transverse, descending colon, sigmoid colon and rectum
innervation of the LI
PNS - vagus innervate ascending and most of transverse
distal colon innervated by pelvic nerves
sympathetic innervation arises from lower thoracic and upper lumber spinal cord
reabsorption
ions and water
in proximal colon mainly
contents become dehydrated
na and cl are exchange mechanisms - water follows
K moves passively into lumen through tight junctions
capacity to absorb 4.5L but only absorbs 1.5l - SI ddoes more
describe the cells of the colon
many goblet and enterocytes
colonic crypts - stem cells at bottom
smootgh mucosa - no villi = small sa - less absorption
enterocytes have small irregular microvilli
enterocyte structure reflects resorbative function
goblet cells
line the crypts
secrete mucus to help passage - abundance increase along colon
mucous also cover bacteria and particulate matter
stimulated by ACh
Paneth cells
absent in LI because of commensal bacteria
enteroendocrine cells
less than in SI - absorption is simpler
glycocalyx
present
doesn’t contain the digestive prush border enzymes
motility - basic contractions
kneading process - minimal propulsion - allow chime to stay in colon for a long time = more absorption
proximal colon has antipropulsive contractions to keep food for longer
transverses and descending - localised segmental contractions of circular muscle - haustral contractions - shuffle contents forwards and backwards
short propulsive movements every 30mins - more after eat - clear for next food
motility - mass movement
1-3 times a day
propel contents 3/4 of length in a few seconds
high fibre food, coffee, smoke promotes mass movement
faeces
indigestible waste 2/3 water cellulose bacteria = smell cell debris- lose because cant digest them in the SI bile pigments/salts
defaecation
suddenly after mass movement - feel the need
pressure receptors send ssignals - myenteric plexuys to initiate peristaltic waves - inhibit internal sphincter
weak signal is helped by autonomic relflex from the brain
internal anal sphincter is relaxed in defecation reflex - via sacral spinal cord - reflex and voluntary
detention of wall detected by pressure receptors
signals to myenteric plexus to initiate peristaltic waves in descending, sigmoid colon and rectum
what determines when it is appropriate to defecate
last bit rectum can detect between solid, liquid and gas
difficult to distinguish between gas and oil = spotting of underwear - especially with Orlistat
control of the colon
PNS - ascending and 2/3 transverse innervated by vagus 9acid from stomach and enzyme part of pancreatic juice, distal - pelvic nerve
sympathetic - lower thoracic and upper lumbar spinal chord
external anal sphincter - somatic fibres - pudendal nerves
afferent sensory detect pressure
enteric control of the LI
myenteric plexus ganglia below taenia coli
prescence of food stimulate mass movement
involved in coeliac
loss of enteric = Hirschsprung’s disease
hormonal control of LI
aldosterone - Na and H2O reabsorption
by synth channel and Na/K pump
where is bile salt reabsorbed
terminal ileum
physiological role of the LI microbiome
form a symbiotic relationship
Synthesise and excrete Vitamin K - for coag
prevent pathogens - compete
antagonise bacteria that kill/inhibit non-indigenous species
stimulate production of cross reactive Ab
stimulate development of tissues
fibre break down
produce short chain FA - receptor in gut, panc, vagus and adipose - signal molecules - regulate gut hormone release/energy source/influence food intake/insulin sensitivity
links with drug metabolism, insulin resistance, bile acid metabolism, lipid metabolism and obesity
produce short FA - regulate gut hormone release
location of bacteria
few in stomach/ proximal si - harsh environment
prevalent species
bacteriodetes
gram -ve
anaerobic
non-spore forming
pathophysiology
gut bacteria prime immune system
inappropriate pop of commensal = predispose infection
potential value in faecal transplant to reinstate bacteria
large intestinal flora
Trillions of bacteria
size of an organ
have to look phylogenetically
flora and colitis or colon cancer
changes composition
bifidobacteria
gram +ve non-spore forming lactic acid bacteria friendly prevent colonisation by potential pathogens
links between gut bacteria and other systems
drug met insulin resistance bile acid met - hydroxyl gp removed so it easier to be absorbed lipid met obesity cancer
