Although oft lambasted and much maligned, defecation is a normal process performed by all. Some see it as a disgusting act, others may feel a sense of achievement upon relief. So here is an entry on how we all defecate, and a little bit about constipation and diarrhoea.

The Rectum

The rectum is found at the end of the large intestine. It is basically a storage vehicle for the stools, after most of the water has been absorbed from them. An elastic, and muscly part of the large intestine, it can accommodate quite a lot of bulk.

It may be fair to say that the gut is one long tunnel, which has a strictly regulated one-way system. Hence there is only one exit to the rectum, and that is the anus.

The Anus

Man has evolved to be the greatest of all the species. And the organ that has ensured this is the hand. Yet, were I to put in your hand a mixture containing matter solid, liquid and gaseous and then ask you to selectively release the gas, you could not. And yet, the lowly anus can do so, with ease, and I might add, several times a day...
- Admiral Rollins, circa 1900

This is the exit of the rectum, and it is ringed by strong muscle, called anal sphincters. There are two such rings, the internal anal sphincter, and the external anal sphincter.

Internal Anal Sphincter

This is innervated by the sympathetic nervous system and the parasympathetic nervous system¹, and is 3mm thick, and 3cm long. There are constant messages being sent back from the nerve endings to the central nervous system. The rate at which these signals are sent back, decreases when the rectum distends. Hence one can observe that the greater the mass of stool in the rectum, the more pressure it will exert on the walls of the rectum, and therefore, the more it will distend. The reduction in messages sent to the central nervous system from the anus is what gives the feeling of 'urgency' ie, the feeling that one is required to evacuate one's bowels.

About every 10 minutes, rectal distension leads to relaxation of the internal anal sphincter. This allows the contents of the rectum to descend to the upper part of the anus. The actual surface of the anus is very sensitive, second only to the fingers for sensitivity. This enables the body to determine whether the rectum contains either solid, liquid or gas, and then how to deal with them appropriately. Of course, contraction resumes after a short while, and the contents ascend back up into the rectum.

External Anal Sphincter

This anal sphincter is innervated by the somatic nervous system². It consists of striated muscle (this is otherwise known as 'white muscle') and is innervated by the pudental nerve. The muscle that forms the ring does contract firmly, however, it cannot do so forever, ie it is fatigable. Rectal distension initially results in increased nervous activity which results in contraction of the sphincter, but eventually, this nervous activity diminishes, and the result is that the sphincter relaxes.

The fact that the external anal sphincter is under voluntary control enables people to control, to an extent, when they evacuate their bowels. Of course, one must have sufficient stools within the rectum to have a feeling of urgency; about 200ml. Then, the somatic nervous system takes over and overpowers the signals sent by the sympathetic nervous system. The rectum then, via peristaltic contraction³, voids its contents. Waves of this kind of movement expel the stools.

The action of defecation is also accompanied by some characteristic features. A person who is defecating will take a deep breath, close their voicebox and contract their stomach muscles. This aids in the movement of the stools out of the rectum by increasing the pressure in the abdominal cavity⁴; helping force the contents of the colon out of the anal canal, and out of the anus.

The 'Fight or Flight Response' and Defecation

There is one moment where the involuntary actions overpower the voluntary actions. However, to understand why so often, the 'fight or flight response' is accompanied by an overpowering feeling of urgency, requires a little knowledge of what the 'fight or flight response' actually is.

The 'Fight or Flight Response'

This is when, due to some fearsome situation ie, suddenly being found in a tiger's cage, or remembering that you have an important exam to do tomorrow, but have done all but no revision, you have the feeling of great fear and wanting to run away, accompanied by:

• Sweating
• 'Butterflies' in the stomach
• Anxiety
• Shaking
• Restlessness
• Raised heart rate
• Urgency for the toilet

This is due to a sudden release of adrenaline (epinephrine in the US) from the adrenal glands. This hormone then activates the sympathetic nervous system, causing the effects listed above. The overpowering activation of the sympathetic nervous system also causes the rate of signals sent to the central nervous system from the internal anal sphincter, to decrease suddenly and it to relax. Also, the adrenaline has an effect on the external anal sphincter, causing it also to relax. This leads to the characteristic and undesirable effect of the evacuation of one's bowels via peristaltic contraction when one is extremely frightened.

Constipation

This is difficult to define, as this depends on the normal bowel habits of the individual. Some people may empty their bowels once a day, whereas others once a week. Both are normal rates of bowel emptying. However, constipation can be described as the slow transit of bowel contents through the large intestine.

Here are a few of the many causes of constipation.

• Low fibre diet
• Not going when feeling 'urgency'
• Not going because it is too painful
• Bowel obstruction (eg, by colon cancer)
• Depression

The main cause of constipation is a low fibre diet. Predictably, the way to reverse this is by increasing the fibre in your diet. Laxatives should really only be used as a last resort.

Another no less important cause of constipation is the use of a class of drugs known as opiates. As the character of Mark Renton graphically illustrates in the film, Trainspotting, one of the many adverse side-effects of heroin, an opiate, is constipation. This occurs because one of the components of heroin - morphine - reduces gut motility. This has the effect of slowing the transit of bowel contents, resulting in constipation.

Diarrhoea

The definition of 'true' diarrhoea is the increase in faecal weight to more than 300g in 24 hours. Again, as with constipation, there are many causes, but they can be categorised into several sections:

Osmotic

This happens when there is a lot of water, or very dilute solution that has not been absorbed, remaining in the bowel. This softens the stools, causing them to be, at the extreme, watery.

Secretory

This occurs when there is an increase in the secretion of fluid into the bowel. Again, this results in a softening of stools. A good example of this type of diarrhoea is cholera infection, where the toxin secreted by the cholera bacteria causes the bowel wall to secrete so much fluid, that it may kill the poor person through dehydration if they are not treated quickly.

Motility

Now, this type of diarrhoea may just cause an increase in the frequency of trips to the toilet instead of true diarrhoea. The reason why this happens is an increase in the rate of peristaltic contraction in the gut, which fills the rectum more rapidly, resulting in a more frequent feeling of urgency.

Side-Effects of Drugs

...I am no longer constipated.
- Mark Renton, from the film Trainspotting

Again, the effects of drugs such as laxatives also causes diarrhoea. However, with reference to Trainspotting yet again, another cause of diarrhoea is the withdrawal of a drug on which the patient is dependent, ie, the period known as cold-turkey. The features of cold-turkey are well known; among its features are:

• High heart rate
• Irritability
• Anxiousness
• Panic
• Sweating

One additional feature is diarrhoea. This is because the reason why cold-turkey occurs is exactly the same as the 'fight or flight response'; a massive activation of the sympathetic nervous system. Hence why the character of Mark Renton is no longer constipated is partly due to the lack of the constipating action of heroin, and also partly due to the cold-turkey period starting to kick in.

Of course, the main worry with diarrhoea is the excessive loss of water resulting in the risk of dehydration, and the loss of salt. This is why if you are suffering from diarrhoea, you doctor will prescribe sachets of powder which you dissolve in water and drink. The resulting fluid has an interesting taste; not quite salty, but not quite sweet. This is because it contains a carefully measured amount of salt and other essential electrolytes such as potassium ions, which may have been lost through the excessive defecation.

Of course, to judge whether someone is actually constipated, has diarrhoea, or within normal limits, the medical profession has a guide. As with the 'Glasgow Coma Scale', where characteristic responses, or the lack thereof, allow the physician to judge how much a person is concussed, there is a similar scale for determining the status of stool formation.

The Bristol Stool Form Scale

This was developed in the University of Bristol, UK, and is a guide to the seven types of stool formation. It must be stressed that stools do not rigidly fall into these seven types in an 'either-or' fashion, and there is some continuity between them.

• Type One - Separate hard lumps, like nuts (hard to pass)
• Type Two - Sausage shaped, but lumpy
• Type Three - Like a sausage, but with cracks on its surface
• Type Four - Like a sausage or snake, smooth and soft
• Type Five - Soft blobs with clear-cut edges (passed easily)
• Type Six - Fluffy pieces with ragged edges, a mushy stool⁵
• Type Seven - Watery, no solid pieces; entirely liquid

As a rough guide, those who are constipated will be passing a Type One or Two, and not very often at all. Those who have diarrhoea will be passing a Type Six or Seven, extremely frequently. A 'normal' stool should be a Type Three or Four, and, depending on the normal bowel emptying patterns of the individual, should be passed once every one to three days.

Other Terms for Defecation

To finish on a light-hearted note, here are a few 'alternative' terms for defecation and their associated terms.

• Pooing
• Going for a 'No 2'
• Crapping
• S**tting
• Going for a dump
• Filling one's pants
• The 'runs' (this is exclusively used to describe diarrhoea)
• Skid marks (the soiling of one's undergarments similar to 'skid marks' found on the road)
• Brown Alert⁶