Function of the large bowel

The main function of the large bowel is absorption, automatic movement, and the formation and excretion of faeces.

The chyme is very liquid when it passes into the large bowel; in the region of 500 ml passes into the colon each day. It is here in the colon that absorption takes place: mineral salts, vitamins and some drugs are absorbed, along with 90% of the water content of the chyme. The large bowel propels the residue towards the rectum where it is stored. The large bowel motility comprises mass movement and peristalsis.

There are usually three or four mass movements during the day, which prompt defecation. A strong force occurs in the transverse colon pushing the chyme towards the rectum. This movement is triggered by eating and entry of food into the stomach, which is why the urge to produce a stool frequently happens either following a meal or during a meal.

Defecation is a voluntary act involving the brain, brain stem and spinal cord. The usually empty rectum fills; nerve endings are stretched by the contents of the sigmoid colon, causing involuntary contractions of the muscle of the rectum and relaxation of the internal anal sphincter; the voluntary external sphincter completes the process. In babies the act is an autonomic response until they are 12-18 months of age when the external anal sphincter comes under voluntary control.

The large bowel is heavily colonized by certain types of bacteria:

• Escherichia coli

• Enterobacter aerogenes

• Streptococcus faecalis

• Clostridium perfringens.

Bacterial colonization happens within the first week of life from swallowed bacteria and maternal contaminations. The colon is sterile for the first week of life (Society of Gastroenterology Nurses and Associates, 1998). Bacteria in the bowel are either harmful or helpful. If the bacteria are harmful they may cause diarrhoea; however, the helpful bacteria in the gut provide vitamins, mainly in the B group. A number of antibiotics, if taken frequently or in large doses, kill the helpful bacteria, again giving rise to diarrhoea (Green, 1978). The bowel maintains a flora of useful bacteria that are needed in digestion.

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