How the Bowel Works
The digestive system works by pushing food through the intestines which usually takes between 24 to 72 hours. Muscular contractions squeeze (peristalsis) the food through the different sections of the intestine. These different sections are separated by bands of muscles, or sphincters, which act as valves.
The passage of food from one area of the intestines to another is coordinated so that food stays in a specific area for long enough for the gut to do a particular job – absorb fluids and nutrients, or process and expel waste.
The small bowel (small intestine)
The small bowel (or small intestine) is around 6 – 8 m long and roughly 2cm wide.
There are 3 parts of the small bowel (small intestine): the duodenum, the jejunum and the ileum.
Food passes from the stomach into the duodenum, which is the tube that leads from the stomach into the intestines. The food then passes through the jejunum and ileum before going to the large bowel (colon). The small bowel (small intestine) absorbs nutrients and much of the liquid from foods. At the point where food is passed from the small bowel into the large bowel (colon) it is of a ‘porridge like’ consistency.
The Large Bowel (Colon – large intestine)
The rectum and muscles
The rectum and anus
After food has travelled along the gut, it has become digested and the nutrients and fluids absorbed; the waste is then expelled through the rectum and anus.
When we need to go to the toilet, the brain tells the anal sphincter muscles, via the nerves, to relax. As the muscles relax, the anus opens and the rectum empties. In some neurological and spinal conditions the brain cannot tell whether the bowel is full of waste (faeces) or just wind. This can lead to accidental leakage.
女性的肛管约3 - 4厘米长，男性有时稍长。内括约肌和外括约肌形成两个同心环，沿肛管的长度排列。
The internal anal sphincter (IAS) is made of smooth muscle and we do not have voluntary control of this muscle. It works automatically to keep the anus closed until we are ready to have a bowel movement.
The external anal sphincter (EAS) is made of striated muscle (the same as the pelvic floor muscles); we do have voluntary control over the EAS – allowing us to hold on if we are aware of wind or diarrhoea.
The pelvic floor muscle
The pelvic floor muscles are layers of muscle stretched like a sheet from the pubic bone in the front, to the bottom of the backbone (coccyx). There are 3 openings through the pelvic floor in women and 2 in men -the anus (back passage), the vagina in women (birth canal) and the urethra (bladder outlet). The muscles support these 3 openings, but if they are weakened or not in good condition they cannot support the openings effectively.
An important part of the pelvic floor muscle regarding bowel control is the deepest layer called Levator Ani – which directly translated from Latin means ‘lift the anus’. One of the muscle groups within levator ani is the puborectalis. This muscle forms a supportive strap around the junction between the rectum and the anal canal and helps to keep us in control of our bowels by forming a flap like valve, which prevents stool passing too easily into the anal canal.
The sacral nerves
The commands from the brain to the bowel (colon) are sent as electrical impulses and are carried by a system of special fibres called nerves.
These signals start in the brain and go to the spinal cord and continue to the nerves located in the sacral area of the back.
Some of these sacral nerves go to the rectum, levator ani muscle, and external sphincter muscles, controlling their activities.
Through a series of reflexes and signals the nerves in the bowel is coordinated with the pelvic floor muscles and anal sphincters in order to store bowel contents until there is an appropriate place to go to the toilet and then to allow complete bowel emptying once on the toilet.
This coordination ensures that the sphincters remain closed, opening only during defecation. So, when the rectum fills, and the pressure inside it increases, the nerves sense the pressure and tell the brain about it. The brain then sends signals via the nerves to keep the external sphincter closed. Normally, this prevents leakage and is called the guarding reflex.
When you need to go to the toilet, the brain tells the nerves to signal the rectum to empty, and relax the muscles surrounding the anus. The rectum empties and stool is expelled.
The same happens as the rectum fills. This sensation of filling is registered by the nerves, which transmit the information to the brain.
The nerves can also tell the brain whether it’s gas or stool that’s filling the rectum.
- The internal and external anal sphincters need to be working properly.
- You need to be able to get to and onto a toilet and to be able to hold on until the correct place is reached.