The large intestine Flashcards
what does the large intestine consist of?
consists of the colon, caecum, appendix, rectum and the anal canal
what is the caecum?
the caecum is a blind path
it is not often used in humans
it is usually fermenting
what is the appendix?
the appendix is a thin fingerlike projection extension of the caecum
- not very useful in humans
- potentially a lifeboat for mirobiome
what are the dimensions of the colon?
1.5 m long , 6cm diameter
what is the function of the colon?
to reabsorb electrolytes and water and also to flush the waste
where are the ascending colon, transverse colon, descending colon and sigmoid colon positioned?
the ascending colon = caecum to the hepatic flexure
transverse colon = hepatic flexure to splenic flexure
descending colon = splenic flexure to sigmoid colon
sigmoid colon = descending colon to rectum
what is the blood supply of the colon?
what is the proximal transverse colon supplied by?
what is the distal third colon supplied by?
Proximal transverse colon supplied by middle colic artery
(branch of the superior mesentric artery)
Distal third of transverse colon supplied by inferior mesenteric artery.
what are some of the structural features of the colon?
and what are the functions of these features?
- fatty tags (appendices epiploicae)
protective function against infection
- longitudinal muscle bands ( taeniae coli)
Creates haustra, necessary for large intestine motility.
- nodules of lymphoid tissue (Solitary nodules)
what are the histological differences of the rectum?
Histologically similar but absent taeniae coli ( longitudinal muscle bands) and transverse rectal folds in submucosa.
what control is the internal anal sphincter and external sphincter under?
internal anal sphincter - unconcious control
external spinchter - voluntary control
Diagram of the Rectum ?
what are the similarities between the large intestine and the small intestine?
- abundant enterocytes
- abundant goblet cells
- abundant crypts with stem cells present
how is the large intestine distinguishable from the small intestine?
- there are no villi in the large intestine
- Enterocytes have short and irregular microvilli
what is the mucosal structure?
- goblet cells present
- More goblet cells than the small intestine with number higher in the crypts and more distally in the colon
- no paneth cells
- Enteroendocrine cells are rarer than small intestine
- Glycocalyx present on microvilli BUT does not hold digestive enzyme in ‘unstirred layer’
what are the muscle layers of the large intestine ?
- like small intestine
- inner circular and outer longitudinal is present
- 3 clear bands – taeniae coli
- there are oval segments called haustra that can contract induvidually
what are the movements of the rectum and anal canal?
these movements are substantial and continuous – used for kneading contents.
how fast are the colonic contractions?
5- 10 cm / AN HOUR
What are the movements of the colon and proximal colon like?
the colon is slow propulsion promotes absorption of electrolytes and water.
the proximal colon is ‘anti-propulsive’ patterns
what are the movements like in the transverse and descending colon?
haustral contractions cause back and fourth mixing
There are then short propulsions ever 30 minutes
these increase in frequency following a meal
when do mass movements happen and what can they propel?
- occur 1- 3 times a day
- propel contents 1/3-3/4 length of large intestine in a few seconds
these movements depend upon fibre
what is the nervous control of the large intestine?
parasympathetic ?
sympathetic ?
enteric?
parasympathetic ? ascending colon, most of transverse colon by VAGAL nerve. More distal by PELVIC nerves
sympathetic ? – lower thoracic and upper lumbar spinal cord.
enteric
Myenteric plexus ganglia concentrate below the taeniae coli.
where do the faeces travel and where are they stored?
the rectum becomes filled with faeces by mass movements in sigmoid column which then stores the stool
what is the defecation reflex controlled by?
controlled by the sacral spinal-cord
how does defecation work?
- reflex inhibition of internal sphincter to balloon the walls of the rectum
- pressure receptors send signals to Myenteric plexus to initiate peristaltic waves
- external sphincter under voluntary control which means one can choose whether to defecate or resist the urge
