Preview of Understanding your Bowels

Family Doctor Books

Published in association with the British Medical Association

Family Doctor Books

Preview of Understanding your Bowels

The large intestine consists of the colon, rectum and anal canal (see illustration on page 6). The colon begins just above the right groin where it is known as the caecum and from which springs the appendix. It continues as the ascending colon, which climbs to just below the ribs on the right and then swings across to the opposite side as the transverse colon. With a second sharp bend it turns down-ward as the descending colon, and finally makes a curious loop known as the sigmoid (named after the squiggly Greek letter sigma or ð) before joining on to the rectum. The word rectum comes from the Latin word for straight, which is odd because actually it bends sharply backwards just before it joins on to the anal canal. It does straighten out, however, while the bowels are being opened, when it functions simply as a tube conducting stools from the sigmoid colon to the outside world. Strictly speaking it is the anal canal that does the last part of this job, but the canal is only an inch or so long and is really just a device for keeping stools and gas inside until their owner decides to let them out.

The anal canal is the only part of the alimentary tube after the throat over which we exert conscious control. The gut is a muscular tube but the throat and the anal canal are the only parts of the tube whose muscle fibres are of the same kind as the muscles in our arms and legs. The anal muscle fibres are arranged as a two-part closing system. First, there is a sling of fibres round the upper end of the tube. When this muscle contracts it exerts a forward pull and so maintains the sharp angle where the rectum joins the anus; at the same time it presses the front and back of the canal against each other. Second, there is a ring of fibres whose contraction makes the hole in the tube narrower. Both sets of fibres are contracting gently all the time without any conscious effort on our part (just like many other muscles in the body). We have to relax these muscles in order to be able to pass a stool or some wind.

The large intestine.

The lining of the bowel

Between the muscular wall and the interior of the bowel (the lumen) there is an all-important lining. In the anal canal this lining is simply skin but under it are clumps of soft, spongy material called the anal cushions. In the rest of the bowel the lining is a delicate mucous membrane or mucosa. This fragile lining has the difficult task of being a barrier to dangerous things like bacteria and viruses, while at the same time letting through into the bloodstream the good things like water and salts which we cannot afford to lose in the stools. It is a balancing act. If the mucosa absorbs too much water, the stools become hard and difficult to pass; if it absorbs too little, they are liquid and copious and the anal muscles have difficulty keeping them in.

At one extreme there is diarrhoea and incontinence, at the other constipation.

Movements

The muscles of the bowel seldom rest – every few seconds they contract briefly in short sections of bowel. These contractions make the bowel narrower and drive its contents into neighbouring sections which are relaxed. Most of these movements simply shuttle the contents back and forth, presumably to increase their exposure to the mucosa and so ensure the maximum absorption of valuable water and salts. Now and again a wave of contraction passes round the entire colon, pushing its contents forwards. This is known as mass peristalsis and it occurs mostly at meal times, especially during breakfast. This explains why many people feel the need to open their bowels after breakfast. In others, mass peristalsis is set off by getting out of bed and dressing.

The difference is not in the ‘strength’ of the contraction but in whether or not it travels distally (towards the rectum).

Sensations

The fortunate among us experience sensations from our bowel only when we need to pass stools or wind. In both cases, the sensations are signals from the rectum saying that it is receiving material from the sigmoid colon and the amazing thing is that we can tell from the signal whether the material is solid, liquid or gaseous. The distinction is probably made by miniature sensors at the top end of the anal canal.

Many people also feel wriggling movements in their abdomen when gas is moved from one part of the colon to another. At the same time, a gurgling sound may be heard. Other people feel a wave of discomfort, which may even be painful, when the need to open the bowels is strong, for example, if the stool is looser than usual. All these sensations are perfectly normal, up to a point.

Discomfort and pain from the colon are extremely common in otherwise healthy people (see Irritable bowel syndrome). Usually this implies that the bowel is contracting strongly, but it may also mean that the bowel has become more sensitive for some reason.

Flora: friends and foes

A unique feature of the large intestine is that it is host to a huge number of bacteria. This is not as alarming as it sounds, because the bacteria are nearly all harmless. Some animals – the herbivores – actually depend for their lives on the bacteria in their intestines; the grass they eat is useless until it has been fermented by the bacteria. We do not need our bacteria for any vital function, but nor do we need to fear them despite their vast numbers. They are scavengers, living off the undigested remnants of our food and the mucus and dead cells that are constantly shed by the mucous membrane of the colon. They are also responsible for the gas that we pass from the rectum (flatus).
Laboratory experiments show that, if animals do not have bacteria in their gut, they are more prone to disease – germ-free animals are fragile creatures. The chances are that the same would be true of humans, so by all means respect your bowel bacteria but do not live in fear of them.

Passage times through the gut.

Bowel movements – what is normal?

The large bowel and its products have never been popular subjects for research so, perhaps unsurprisingly, scientific data on them are limited. However, thanks to recent research, we know the following facts and figures about the adult British population.

Most people claim to open their bowels once a day but, when they are asked to record all their bowel openings, it turns out that a regular 24-hour cycle is present in only 40 per cent of men and 33 per cent of women, and a twice or three times a day habit in another seven per cent and four per cent respectively. So most people are irregular in their habits. This is especially true of younger women. About ten per cent of women and three per cent of men go only two or three times a week, while one in 100 women go once a week or less often.

It is hard to state what is normal and what is abnormal. Some doctors consider that any bowel habit is normal so long as it causes no discomfort or distress. However, if the bowels open only two or three times a week it is likely that the passage of food residues through the colon is so slow that, in time, there will be repercussions on general health. Unfortunately, a once-daily habit is no guarantee that everything is right. It is possible to go once a day but every time be four or five days late! A better guide to passage time than frequency of going is the type of stool that is passed.

Stools have been classified into seven types, on what is called the Bristol Stool Form Scale (see below), according to their appearance as seen in the toilet water. Type 1 has spent the longest in the colon and type 7 the least time.

THE BRISTOL STOOL FORM SCALE
Type 1 Type 2 Type 3 Type 4 Type 5 Type 6 Type 7		Separate hard lumps, like nuts Sausage-like but lumpy Like a sausage but with cracks in the surface Like a sausage or snake, smooth and soft Soft blobs with clear-cut edges Fluffy pieces with ragged edges, a mushy stool Watery, no solid pieces

Stools at the lumpy end of the scale are hard to pass and often require a lot of straining. Stools at the loose or liquid end of the spectrum can be too easy to pass – the need to pass them is urgent and accidents can happen. The ideal stools are types 3 and 4, especially type 4, as they are most likely to glide out without any fuss what-soever. Also, they are least likely to leave you with an annoying feeling that something is left behind.

The average passage time of undigested food residues through the human gut is about 50 hours in men and 57 hours in women, but ranges from well under 20 to over 100 hours. It also changes from one day to the next. Most of this time (80 to 90 per cent) is spent in the colon.

The average weight of a stool is about 100 grams (3.5 oz) but, again, this varies a lot. The colour of normal stools is always brown, but, curiously, the nature of the brown pigments is unknown.

The average stool is 75 per cent water, but most of this water is locked up inside bacteria and undigested plant cells. Half to two-thirds of the stool is bacteria – some living, some dead – and the rest is mainly undigested residues of plant foods (dietary fibre). Thanks to bacterial activity, there are hundreds of different organic compounds in stools, mainly present in trace amounts.

The average person is said to void wind 12 times in 24 hours. This fact was established in young men (American students) and the situation may be different in older people and in women. There is a great deal of person-to-person variation.

The lack of basic scientific data in this field of human experience is extraordinary. For example, we do not know how many stools float in water and how many sink, nor how long people spend over the act of defecation. It is widely taught that straining (holding the breath and pushing) is a normal and necessary part of defecation. However, recent research has shown that straining is a minority practice and, what’s more, depends on the type, size and consistency of the stool. The most difficult stools to pass are small
ones and broad, firm ones. In practice, these are types 1 and 2, or occasionally 3.

THE BRISTOL STOOL FORM SCALE

The feeling you need to go is definite but not irresistible
Once you sit down on the loo there is no delay
No conscious effort or straining is needed
The stool glides out smoothly and comfortably
Afterwards there is only a pleasant feeling of relief
All this is most likely if the stool is Bristol Stool Form Scale, type 4.

KEY POINTS

The bowels – the large intestine – comprise the colon, the rectum and the anal canal. The only bit we consciously control is the anal canal
The lining of the large intestine absorbs water and salts, and repels bacteria and viruses
To aid absorption, the bowels constantly move the contents back and forth as well as downwards
Intestinal bacteria are a valuable defence against disease, even though they produce gas as well
How often we open our bowels varies greatly; most of us do not have a precise 24-hour cycle
Many aspects of the bowels are still a mystery, even to doctors

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