Editing Colon and rectum (section)

== Physiology ==

** Absorption of fluid and electrolytes
*** The conversion of succus from the TI into formed stool that is stored in the rectal reservoir until it is ready to be excreted
*** The colon can absorb up to 5L of fluid per day, although normally only receives 1-2L per day from the TI, of which about 90% is reabsorbed
*** Sodium and chloride are actively absorbed via exchange, and water is passively absorbed along with the sodium
*** Potassium chloride and bicarbonate are actively secreted into the lumen
** Secretion
*** The colon can increase potassium secretion to compensate for dysfunctional kidneys, which is promoted by aldosterone and blocked by spironolactone
*** Increased potassium is secreted in IBD, cholera, and shigellosis
*** Some forms of colitis also promote chloride secretion
** Urea recycling
*** Colonic bacteria are rich in urease, which is an enzyme not produced by mammals
*** Ammonia produced in the colon is absorbed and transported to the liver
*** Antibiotics and lactulose decrease the amount of ammonia absorbed by lowering the concentration of bacteria and reducing colonic pH, respectively
** Recycling bile salts
*** The colon absorbs bile salts that escape the TI, by passive transport (nonionic diffusion)
*** When the reabsorptive capacity is exceeded, colonic bacteria deconjugate bile acids
*** Deconjugated bile acids can then interfere with sodium and water absorption, leading to secretory/choleretic diarrhoea. This is seen transiently after right hemicolectomy and more permanently after extensive ileal resection. This can be effectively treated by cholestyramine, which binds to bile acids.
** Colonic flora, fermentation, and short-chain fatty acids
*** Large bowel contents are about 50% bacteria by mass
*** Mostly anaerobic, and most common one is Bacteroides, but also Escerichia, Klebsiella, Proteus, Lactobacillus, Enterococci
*** Main source of energy for intestinal bacteria is (soluble) dietary fibre
*** Note that bulking agents such as psyllium and lignin are non-absorbable and non-fermentable by colonic baceria
*** Colonic flora produce short-chain fatty acids like butyrate, which is the principle form of nutrition for the colonocyte. This production is disrupted by antibiotics, leading to diarrhoea.
**** Gases (carbon dioxide, methane and hydrogen) are also produced in this process, and make up about 50% of flatus, along with the other half being swallowed air
*** Mucosal atrophy is also seen after diversion (diversion colitis) because no stool means no nutrition for colonocytes
** Probiotics and prebiotics
*** Probiotics - dietary supplements that contain live cultures of bacteria and/or yeast that are beficial to colonic and host function
*** Most commonly Lactobacillus and Bifidobacterium
*** Insufficient data to recommend probiotics for C diff prevention
*** Possible indications for use:
**** Necrotizing enterocolitis
**** UC
**** Pouchitis
**** Constipation
*** Prebiotics - nutrients that support the growth of probiotic bacteria. Little evidence to guide use.
** Colonic motility
*** Autonomic nervous system:
**** PNS: excitatory, and reaches the colon via the vagus nerve and the rectum via the sacral nerves (S2-S4) through the pelvic plexus.
**** SNS: inhibitory, from fibres at L2-L5, post-ganglionic hypogastric nerves, and the splanchnic nerves (T5-T12), which reach the colon and rectum through perivascular plexuses
*** Intrinsic nervous system:
**** Myenteric (Auerbach) plexus
**** Submucosal (Meissner) plexus
**** Interstitial cells of Cajal are the primary pacemaker cells
** Defaecation

[[Category:Anatomy]]