Expansion
Absorbs water and electrolytes, ferments fibre, and stores faeces
Expansion
Absorption
- Water: receives about 1.5 litres a day, excretes about 100 ml
- Sodium actively, under aldosterone control, with chloride following
- Potassium is secreted, which is why severe diarrhoea causes hypokalaemia
- Maximum capacity about 5 litres a day; exceeding it causes diarrhoea
Fermentation by the colonic microbiota
- Undigested carbohydrate and fibre to short-chain fatty acids (acetate, propionate, butyrate), which are absorbed and are the principal fuel of the colonocyte
- Synthesis of vitamin K and some B vitamins
- Gas production
Motility
- Haustration: segmental mixing
- Mass movements: one to three times a day, often triggered by the gastrocolic reflex after eating
- Transit time of 24 to 48 hours
Storage: the rectum remains empty until a mass movement fills it, initiating the defecation reflex.
The dependence of colonocytes on butyrate explains diversion colitis in a defunctioned segment, and part of the benefit of dietary fibre.