8- Large Intestine Flashcards
5 elements of the large intestine
colon, caecum, appendix, rectum and anal canal.
What are the dimensions of the large intestine
150 cm long and 6 cm wide.
What is the blood supply to the large intestine
It receives blood from both the middle colic artery (which perfuses the ascending and first two-thirds of the transverse portions) and the inferior mesenteric artery (which perfuses the final third of the transverse colon, descending colon, sigmoid colon and rectum).
Decreased innervation to the LI
The innervation is similarly distributed, which the parasympathetic nervous system (via the vagus nerve) innervates ascending and most of transverse colon. The distal colon is innervated by pelvic nerves. Sympathetic innervation arises from the lower thoracic and upper lumbar spinal cord.
How is the ileum separated from the caecum
The ileocaecal valve is a muscular sphincter that separates the distal ileum from the caecum
It is tonically active and constricted, and only relaxes to allow passage of the fluid chyme into the large intestine. it also prevents the microbiota (gut bacteria) from migrating into the ileum.
Where is the appendix
The appendix is a finger-like extension that protrudes from the caecum
Describe the structure of the large bowel
The ascending colon, starts at the ileocaecal valve and runs up the right-hand side of the body to the hepatic flexure
The transverse colon starts at the hepatic flexure and runs across the abdomen to the splenic flexure
The descending colon starts at the splenic flexure and runs inferiorly to the first bend of the sigmoid colon (this is a less defined junction that the aforementioned flexures. The sigmoid colon is an S-shaped part of bowel that starts at the descending colon and runs until the rectum.
What control move,ent of faeces out of gi
The anal canal controls the movement of things out of the GI tract, and is surrounded by two anal sphincters. The internal muscle is smooth muscle and is under central control. The external sphincter is striated muscle and is under voluntary control
Appendices epiploicae
These are fatty tags that arise from the serosa, and do not seem to have a physiologically meaningful function.
Describe the muscle of the gi tract
Longitudinal muscle
Instead of a continuous muscle layer like the rest of the GI tract, the colon has thee bands of longitudinal muscle around that are roughly equally spaced around the circumference. These bands are relatively thicker than typical longitudinal muscle layers. These are actually shorter than the length of the colon, which causes the colon to form regular ‘pouches’ caulled hastra.
Large intestine motility is different from small intestine, so need the taeniae coli
Circular muscle
Circular muscles are segmentally thickened and bundles of muscle from the taeniae coli penetrate the circular muscle at irregular intervals to keep them together.
How are ions and water reabsorbed in the li
Sodium and chloride are absorbed by exchange mechanisms. Water follows by osmosis. Potassium moves passively into the lumen via gap junctions.
What is the most prevalent bacteria in the gut
bacteriodetes (gram negative, anaerobic non-spore forming bacteria)
Functions of the microbiota
Synthesise and excrete Vitamin K (important for coagulation; it is almost exclusively produced by gut bacteria)
Prevent pathogen colonisation by competing for attachment sites and nutrition.
Antagonise other bacteria which can inhibit or kill non-indigenous species.
Stimulate the production of cross-recative antibodies.
Stimulate development of some tissues.
Fibre can be broken down.
Links with drug metabolism, insulin reistance, bile acid metabolism, lipid metabolism and obesity.
What cells make up the colon
Abundant enterocytes and goblet cells. Lots of invaginations called ‘colonic crypts’, which have stem cells at the bottom, similar to the small intestine. Cells migrate up the crypts and into the lumen, and are sloughed off after a few days.
Mucosa is smooth and does not have any villi, enterocytes have microvilli
Paneth cells are absent in the large intestine, because of the commensal (commensal roughly means one organism living inside another organism) bacteria. Enteroendocrine cells are also much fewer in number.
The glycocalyx is still present, but it does not contain the digestive brush border enzymes that the small intestinal glycocalyx does.
Describe contractions of the colon
Basic colonic contractions are a kneading process that provide minimal propulsion (about 5-10 cm/h), which allow chyme to stay in colon for long time to promote absorption.
Proximal colon also elicits ‘antipropulsive’ contractions to impede propulsion and keep food in that region for longer. In the transverse and descending portions there are localised segmental contractions of circular muscle called haustral contractions, which helps to shuffle contents forwards and backwards. Also, there are short propulsive movements every 30 mins.
