A (deliberately!) simplified illustration to show you where the different "bits" that make up the bowel lie in the tummy. The pancreas is hidden away behind the "purple" stomach.
Gullet (Oesophagus)
Stomach
Liver
Small intestine
Large bowel (Colon)
How the gut works
Let's talk about the various part of the gut and what they do. It can be a bit confusing because the same area of the tummy (abdomen) often has different names (e.g. the lower bowel = the large bowel = the colon). Also, all of us use the word "stomach" to loosely refer to our tummy. In fact, the stomach is a very specific area.
The main parts of our gut
The gut is made up of three parts - each doing a different job. What the parts have in common is that they are made up of a long
hollow tube of muscle, which, like the waves on a beach, sends ripples of movement every few seconds along its length, from the mouth to the far end.
To co-ordinate these waves, the gut has its own "pacemaker" that is in the wall of the stomach. It seems that for many of us with IBS, there is some problem with how this system works, causing areas of spasm rather than the more normal gentle "ripples".
These “ripples” are called peristaltic waves and carry food and waste along with them. Think of a boa-constrictor swallowing
something. It is because of these waves, that bowel pains can often be like repeated cramps ("colicky" pain) and why medications such as
mebeverine, mint oil and hyoscine (Buscopan) can help relieve tummy discomfort.
The gullet (oesophagus) and stomach: transport and preparation
The gullet is about 40cm long and its job is to pass food from the mouth to the stomach. The stomach is a muscular bag
"preparation" area, where food is slowly churned with acid and broken down into a 'paste'. This is to allow the small bowel a better chance of digesting food completely.
The end part of the stomach is shaped like a funnel whose 'spout' can become wider or narrower depending on the type of food (fatty foods,
caffeine, alcohol and chocolate all make the 'spout' narrower and so slow down emptying of the stomach. That is why we feel very full after "rich" foods- we are full! The food is literally sitting there waiting for the spout to allow it to pass.
The small intestine: digestion and absorbing nutrients
Immediately after the stomach comes the small intestine, which is about six metres long. It is called "small" because it is narrow (about the width of your thumb) when compared with the large intestine (about three fingers wide).
The small intestine is where most of the digestion and absorption of nutrients into the body take place. It is lined with millions of microscopic "fingers", which are themselves covered in enzymes (these work like chemical "scissors") that chop-up, digest and allow the cells lining the small intestine to absorb nutrients.
To help this digestion, bile juice is released from the liver and pancreatic juice from the pancreas. The liver is a pretty big solid organ
found below the right-side ribs. It is like a factory for the body. The pancreas is shaped like a tadpole and lies deep behind the stomach, just below the breast-bone. Both organs are joined to the small bowel by narrow tubes ("ducts").
The job of bile is to split-up fat into smaller droplets, which makes it easier in turn for the other enzymes in pancreatic juice to break
it down. Bile looks like washing up liquid (green-yellow in colour) and does exactly the same things (a detergent for making fats soluble). The pancreatic juice (which is colourless) also digests proteins and carbohydrates.
The bile and pancreatic juices do part of the digestion but another key player are the "scissor" like enzymes that cover the small bowel lining. These enzymes are very important for breaking down the sugars that occur naturally in food (lactose in milk and fructose in fruit / vegetables).
Nutrients from the digested food paste are then slowly absorbed (over several hours) as it makes
its along the small intestine "conveyor belt". At the end of the small bowel, just before it enters the large bowel (colon), the bile juices themselves get absorbed so that they can be re-cycled and used again for your next meal. But, in some people, the juice just carries on down into the colon, which really does not like it one bit- this can cause very bad, profuse, watery diarrhoea ("bile salt diarrhoea").
There is normally only a relatively small number of bacteria in the small intestine, but in some people, the number of bacteria
can increase greatly. This is termed Small Intestinal Bacterial Overgrowth (SIBO), which can cause bloating and diarrhoea.
The large (lower) intestine (colon): absorping water and making waste
The large intestine (colon) is about one metre long. It is the widest part of the bowel (about 2-3 finger breadths). The job of the colon is to to absorb excess water left after food has been digested - if we didn’t have a colon, we would all have very watery stools and have to drink huge amounts of fluid every day.
The small intestine joins up with the colon in the lower right side of the tummy, just above the right hip bone. The colon runs like a "picture frame" around the tummy - up the right side, across the top (below the ribs) and then down the left side to the bottom (rectum).
As the waste from small bowel digestion makes its way along the colon, it is slowly dried out and firm / hard stool forms. If things move too fast ("fast transit"), we get diarrhoea; if too slow ("slow transit") we get constipation.
Immediately above the muscular ring of the anus is the rectum, which is a storage area for faeces. Its lining is rich with nerves
that send a signal to the brain when the rectum is full, that gives us the urge to pass a bowel motion.
Some people can get into the habit of over-riding this signal (because they are not comfortable using the toilet at work or in public places) and so literally lose that signal (a condition called anismus) and can get bad constipation.
The ring and sling of muscles that make up the anus control the exit of stools from the rectum. And very importantly, this set of muscles controls the angled bend between the anus and the rectum.
How we sit on the toilet affects the angle between the anus and the rectum. The angle needs to be "straightened" to let stools pass out
easily. A 'deep' angle between rectum and anus can make emptying difficult- causing us to strain a lot - see Toilet sitting position (under
the Resources tab).
A vast number (trillions) of different species of bacteria live in the colon. They produce substances that are beneficial to our health, as well as helping prevent harmful bacteria from establishing a presence in the colon.
One other thing to mention is that names have been given to different parts of the colon. We just mention these in case you come across them
and wonder where they fit in. They are (in order from start to finish): caecum, ascending colon, transverse colon, descending colon, sigmoid and rectum.
Let's talk about the various part of the gut and what they do. It can be a bit confusing because the same area of the tummy (abdomen) often has different names (e.g. the lower bowel = the large bowel = the colon). Also, all of us use the word "stomach" to loosely refer to our tummy. In fact, the stomach is a very specific area.
The main parts of our gut
The gut is made up of three parts - each doing a different job. What the parts have in common is that they are made up of a long
hollow tube of muscle, which, like the waves on a beach, sends ripples of movement every few seconds along its length, from the mouth to the far end.
To co-ordinate these waves, the gut has its own "pacemaker" that is in the wall of the stomach. It seems that for many of us with IBS, there is some problem with how this system works, causing areas of spasm rather than the more normal gentle "ripples".
These “ripples” are called peristaltic waves and carry food and waste along with them. Think of a boa-constrictor swallowing
something. It is because of these waves, that bowel pains can often be like repeated cramps ("colicky" pain) and why medications such as
mebeverine, mint oil and hyoscine (Buscopan) can help relieve tummy discomfort.
The gullet (oesophagus) and stomach: transport and preparation
The gullet is about 40cm long and its job is to pass food from the mouth to the stomach. The stomach is a muscular bag
"preparation" area, where food is slowly churned with acid and broken down into a 'paste'. This is to allow the small bowel a better chance of digesting food completely.
The end part of the stomach is shaped like a funnel whose 'spout' can become wider or narrower depending on the type of food (fatty foods,
caffeine, alcohol and chocolate all make the 'spout' narrower and so slow down emptying of the stomach. That is why we feel very full after "rich" foods- we are full! The food is literally sitting there waiting for the spout to allow it to pass.
The small intestine: digestion and absorbing nutrients
Immediately after the stomach comes the small intestine, which is about six metres long. It is called "small" because it is narrow (about the width of your thumb) when compared with the large intestine (about three fingers wide).
The small intestine is where most of the digestion and absorption of nutrients into the body take place. It is lined with millions of microscopic "fingers", which are themselves covered in enzymes (these work like chemical "scissors") that chop-up, digest and allow the cells lining the small intestine to absorb nutrients.
To help this digestion, bile juice is released from the liver and pancreatic juice from the pancreas. The liver is a pretty big solid organ
found below the right-side ribs. It is like a factory for the body. The pancreas is shaped like a tadpole and lies deep behind the stomach, just below the breast-bone. Both organs are joined to the small bowel by narrow tubes ("ducts").
The job of bile is to split-up fat into smaller droplets, which makes it easier in turn for the other enzymes in pancreatic juice to break
it down. Bile looks like washing up liquid (green-yellow in colour) and does exactly the same things (a detergent for making fats soluble). The pancreatic juice (which is colourless) also digests proteins and carbohydrates.
The bile and pancreatic juices do part of the digestion but another key player are the "scissor" like enzymes that cover the small bowel lining. These enzymes are very important for breaking down the sugars that occur naturally in food (lactose in milk and fructose in fruit / vegetables).
Nutrients from the digested food paste are then slowly absorbed (over several hours) as it makes
its along the small intestine "conveyor belt". At the end of the small bowel, just before it enters the large bowel (colon), the bile juices themselves get absorbed so that they can be re-cycled and used again for your next meal. But, in some people, the juice just carries on down into the colon, which really does not like it one bit- this can cause very bad, profuse, watery diarrhoea ("bile salt diarrhoea").
There is normally only a relatively small number of bacteria in the small intestine, but in some people, the number of bacteria
can increase greatly. This is termed Small Intestinal Bacterial Overgrowth (SIBO), which can cause bloating and diarrhoea.
The large (lower) intestine (colon): absorping water and making waste
The large intestine (colon) is about one metre long. It is the widest part of the bowel (about 2-3 finger breadths). The job of the colon is to to absorb excess water left after food has been digested - if we didn’t have a colon, we would all have very watery stools and have to drink huge amounts of fluid every day.
The small intestine joins up with the colon in the lower right side of the tummy, just above the right hip bone. The colon runs like a "picture frame" around the tummy - up the right side, across the top (below the ribs) and then down the left side to the bottom (rectum).
As the waste from small bowel digestion makes its way along the colon, it is slowly dried out and firm / hard stool forms. If things move too fast ("fast transit"), we get diarrhoea; if too slow ("slow transit") we get constipation.
Immediately above the muscular ring of the anus is the rectum, which is a storage area for faeces. Its lining is rich with nerves
that send a signal to the brain when the rectum is full, that gives us the urge to pass a bowel motion.
Some people can get into the habit of over-riding this signal (because they are not comfortable using the toilet at work or in public places) and so literally lose that signal (a condition called anismus) and can get bad constipation.
The ring and sling of muscles that make up the anus control the exit of stools from the rectum. And very importantly, this set of muscles controls the angled bend between the anus and the rectum.
How we sit on the toilet affects the angle between the anus and the rectum. The angle needs to be "straightened" to let stools pass out
easily. A 'deep' angle between rectum and anus can make emptying difficult- causing us to strain a lot - see Toilet sitting position (under
the Resources tab).
A vast number (trillions) of different species of bacteria live in the colon. They produce substances that are beneficial to our health, as well as helping prevent harmful bacteria from establishing a presence in the colon.
One other thing to mention is that names have been given to different parts of the colon. We just mention these in case you come across them
and wonder where they fit in. They are (in order from start to finish): caecum, ascending colon, transverse colon, descending colon, sigmoid and rectum.
