17. The Colon, Rectum and Anal Canal

anatomy of the large intestine

• several haustra→ segmented pouches

cells making up the large intestine

the ileocaecal valve

• one way valve

• periodic relaxation allows for flow

• ileal distension→ open

• caecal distension→ close

motility in the colon

• designed NOT to move contents along

• haustration→ slow contractions of circular muscles squeeze contents to and fro

mass movement

• occasionally contractions become organised into a peristaltic wave

  • haustra briefly disappear as mass movement sweeps by then reform

coordination and regulation of motility in the colon

• most intrinsic via enteric NS

• some parasymp. control

• enteroendocrine and neurocrine influences→ cells releasing 5-HT and peptide YY

• gastrocolic orthocolic reflexes can trigger mass movements

ileal brake

• presence of undigested lipid in distal ileum and proximal colon releases peptide YY

• slows gastric emptying and small bowel peristalsis

digestion in the colon

• done by bacteria not by human enzymes

substances digested in the colon that are reused

• soluble fibre→ short chain fatty acids

• primary bile acids→ secondary bile acids

• conjugated bile acids→ unconjugated bile acids

substances digested in the colon that are excreted

• urea and amino acids→ ammonia

• bilirubin→ urobilinogen and stercobilins

• cysteine methionine→ hydrogen sulphide

soluble fibre

• this is fermented to release:

  • nitrogen

  • hydrogen

  • carbon dioxide

insoluble fibre

• attracts fluid into the stool to improve bulking

salt absorption in the colon

• ENaC channels allow transcellular sodium transport

  • stimulated by aldosterone

• paracellular Cl^- flux followed by water

short chain fatty acid absorption

• secondary active transport via Na⁺ linked transporter

• important source of energy for colonocytes

chloride secretion in the colon

• stimulated by cAMP and Ca²⁺

  • cholera and E Coli toxins increase cAMP

  • C. Difficile toxin increases Ca²⁺

• both lead to secretory diarrhoea

potassium secretion in the colon

• can undergo secretion and absorption in the colon→ usually net secretion

• passive secretion exceeds active

• stimulated by cAMP and aldosterone

rectum and anus anatomy

• sharp angle→ as sigmoid colon enters rectum

• internal anal sphincter→ smooth muscle

• external anal sphincter→ skeletal muscle

• dentate line→ epithelium becomes stratified squamous epithelium

faeces in rectum and anus

• as faeces enters rectum, pressure rises

  • triggers reflex relaxation of internal anal sphincter→ urge to defecate

• small amount of material enters anus→ anal sampling

  • external anal sphincter contracts

defaecation

• sitting/squatting increases rectosigmoidal angle→ facilitates passage of faeces

• relaxation of puborectalis muscle also increases the angle

• relaxation of external sphincter and pelvic floor muscles open the way

• rectal peristalsis→ triggering colonic mass movement and raised intraabdominal pressure provides motive force