large bowel Flashcards
what does the large bowel consist of
colon, caecum, appendix, rectum and anal canal
what is the caecum
blind pouch just distal to the ileocecal valve (communicates terminal ileum to colon) which is larger in herbivores
what is the appendix
the appendix is a thin, finger like extension of the caecum - not physiologically relevant in humans
what are the principal functions of the large bowel
reabsorption of electrolytes and water
the elimination of undigested food and waste
what is the length and diameter of the large bowel
1.5m long - 6cm in diameter
where is the ascending colon
is on the right side of the abdomen, runs from the caecum to the hepatic flexure (the turn of the colon by the liver)
where is the transverse colon
runs from the hepatic flexure to the splenic flexure (the turn of the colon by the spleen), hangs off the stomach, attached by a wide band of tissue called the greater omentum (posterior side, mesocolon)
where is the descending colon
runs from the splenic flexure to the sigmoid colon
where is the sigmoid colon
(s shaped) colon runs from the descending colon to the rectum
what supplies the proximal transverse colon with blood
the proximal transverse colon is supplied with blood by the middle colic artery (branch of the superior mesenteric artery)
what is the distal 3rd of the transverse colon perfused by
distal third of transverse colon is perfused by the inferior mesenteric artery
what is the region between the distal and proximal third of the transverse colon sensitive to
sensitive to ischemia because blood supply is not as extensive
what does the peritoneum carry
the peritoneum carries fatty tags (appendices epiploicae)
how many bands does the muscle coat have
3 thick longitudinal bands (taeniae coli)
what is the gut wall like in appearance
pouched in appearance (haustra)
what is the structural or functional purpose of the appendices epiploicae
unknown - suggested to have a protective function against intra-abdominal infections
what does the taenia coli do
necessary for large intestine motility
what is present in the walls of the distal small intestine and large intestine
nodules of lymphoid tissue are common in the walls of the distal small intestine (Peyer’s patches) and large intestine (solitary nodules)
what is the function of the colon
colon absorbs electrolytes and water
happens more in proximal colon
Na+ and Cl- absorbed by exchange mechanisms and ion channels
water follows by osmosis
K+ moves passively into the lumen
large intestine can reabsorb approx 4.5 litres water (usually 1.5) - above this threshold = diarrhoea
what is the rectum
dilated distal portion of the alimentary canal of the large bowel
how can you tell the colon and rectum apart
histology is similar to the colon but is distinguished by transverse rectal folds in its submucosa and the absence of taenia coli in its muscularis externa
what is the anal canal and what is it surrounded by
terminal portion is anal canal
surrounded by internal (circular muscle) and external (striated muscle) and sphincters
what is the mucosal structure of the large bowel
4 main layers mucosa submucosa muscularis serosa
what is the mucosa
most inner layer and contains many glands covered by submucosa which also has many glands
what does the muscularis have
harbours myenteric plexuses
what does the serosa have
harbours vessels and nerve endings for large bowel
what cells are abundant in the large bowel
enterocytes and goblet cells
abundant crypts
what types of cells are found in crypts
stem cells
at the gross level, how does the mucosa appear to look
smooth - no villi (smaller SA than small bowel)
why do enterocytes have short irregular microvilli
concerned with reabsorption of salts
how is water absorbed
passively as it follows electrolytes - resulting in more solid gut contents
what cells are the crypts dominated by
goblet cells and stem cells
more in crypts than on surface - no of goblet cells increases distally towards the rectum
where are goblet cells most abundant - small or large bowel
large bowel
what are apical ends packed with
with mucus-filled secretion granules awaiting release
what does mucus do
facilitates the passage of the increasingly solid colonic contents and covers bacteria and particulate matter
what stimulates goblet cell secretion
acetylcholine (parasympathetic and enteric nervous system) stimulates goblet cell secretion
what are the differences between the small and large bowel
no paneth cells and enteroendocrine cells are rarer than in the small bowel
glycocalyx does not contain digestive enzymes
crypts are dominated by goblet and stem cells > enterocytes
- movement of large bowel is more complicated than the small intestine
describe the muscle layers in the large bowel
- muscularis externa consists of an inner circular and outer longitudinal layer
- circular muscles segmentally thickened
- longitudinal layer concentrated in 3 bands - taenia coli
- between the taenia, longitudinal layer is thin
- bundles of muscle from the taenia coli penetrate the circular layer at irregular intervals +
contribute to haustra formation - shorter than circular muscle layers are ovoid segments called haustracan contract
individually - apart from rectum and anal canal - substantial and continuous
what are colonic contractions
kneading process - minimally propulsive ~5-10 cm/hr at most
promotes absorption of electrolytes and water
what patterns dominate in the proximal colon to retain chyme
anti propulsive patterns dominate to retain chyme
what contractions occur in the transverse and descending colon
in transverse and descending colon, localised segmental contractions of circular muscle called haustral contractions cause back and forth mixing
how often are short propulsive movements
short propulsive movements every 30 mins
increase in freq in response to a meal
