The characteristic brown colour of faeces is due to stercobilin and urobinin, both of which are produced by bacterial degradation of bilirubin. Faecal odour results from gases produced by bacterial metabolism, including skatole (3-methylindole), mercaptans, and hydrogen sulphide. In most individuals, dietary and social habits condition the time of defaecation. The majority of adults defaecate once a day, although frequencies from 2 per day to once every 2 days are considered normal.
Several times each day, mass movements push faeces into the rectum, which is usually empty. Distension of the rectum stimulates the defecation reflex. This is largely a spinal reflex mediated via the pelvic nerves, and results in reflex relaxation of the internal anal sphincter followed by voluntary relaxation of the external anal sphincter and defaecation. Colonic emptying occurs during defaecation, which is not only a process of rectal evacuation27. In humans and house-trained animals, defaecation can be prevented by voluntary constriction of the external sphincter. When this happens, the rectum soon relaxes and the internal sphincter again contracts, a state which persists until another bolus of faeces is forced into the rectum.
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