Bacterias

The role of bacteria
The bacteria found in the large intestine play an essential role in nutrition and digestion. One of their most important functions is to synthesize several vitamins, particularly some B vitamins and vitamin k – which is vital in helping the liver to manufacture substances used in the blood – clotting process. The bacteria break down any remaining proteins into amino acidsand then into simpler substances, such as indole, skatole, hydrogensulphide and fatty acids. Much of the indole and skatole is transported to the liver, where it is converted into less poisonous compounds and passed out of the body in the urine, but some remains in the faeces and is the chief cause of their distinctive smell.
Remaining carbohydrates are fermented by bacteria, releasing hydrogen,carbon dioxide and methane gas. A variable amount of gas is found in the colon, which may be released explosively when the pressure inside the colon and rectum forces the anal sphincter open. The bacteria also decompose bilirubin (the pigment produced by the breakdown of red blood cells) which is excreted in the bile, and the presence of this in the faeces gives them a brown color.
By the time thefaeces reach the rectum they are approximately 70 per cent water. Bacteria represent 30 per cent of the dry weight and the rest is made up of food residues (mainly cellulose from plant sources which cannot be digested or absorbed by man), a certain quantity of the breakdown products mentioned above, intestinal secretions and dead cells from the lining of the intestine. An average adult on a normaldiet will excrete 75 to 170 grams (up to 2.5 oz) of faeces daily.
Expelling the waste
It usually takes from 12 to 24 hours for the chime to travel from the caecum, turn into faeces and reach the point where it accumulates at the end of colon, but this can be longer, depending on the amount of roughage in the diet. The colon works best when it is moderately full, so the indigestible materialfound in roughage is useful in increasing the bulk of the contents of the colon. The movement of faeces into the rectum prior to being expelled is not stimulated until the colon is relatively full. The longer this takes, the more water will be absorbed, making the faeces compacted and hard, so it is increasingly difficult to expel them.
The process of expelling waste through the anal sphincter isknown as defaecation. Pressure of faeces in the rectum is the main stimulus, causing a peristaltic movement of the muscular wall of the rectum. This is under a certain degree of voluntary control which has to be learnt in early childhood and is largely absent in babies.
To expel the faeces, a conscious effort is made to force the diaphragm down thus increasing pressure in the abdominal cavity andpropelling the faeces into the rectum. Once this process has begun, the the rectum stretches to receive the entire contents of the lower part of the colon. The stretching triggers off the reflex contraction of the rectal muscles, the anal sphincter relaxes, and the faeces are pushed out of the body.
Neglecting the urge to defaecated, as well as eating foods that have insufficient roughage, willlead to constipation – the contents of the colon become hard and can only be passed out with some difficulty.
Laxatives, taken as a cure for constipation, either act to increase the amount of water retained by the contents of the colon, so that the muscles find the waste softer and easier to propel, or some, like liquid paraffin, act as a lubricant for the passage of the faeces. Other drugsdirectly stimulate the gut walls to contract and expel the faeces. Doctors disapprove of the frequent use of laxatives, as it is easy to grow so dependent on such drugs that the bowel function becomes permanently abnormal.
Diarrhea – the frequent expulsion of liquid faeces – can be produced by a number of factors, such as nervous stress, excessive use of laxatives or infection. Infection may prevent...