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The Abdomen
Written by Tim Sheppard MBBS BSc. Last updated 9/11/10

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What is the abdomen?

The abdomen is the portion of the body beneath the thorax. It is often referred to as the belly by the general public, and is associated with eating because the main organs involved in digestion can be found there. When you eat, food has to go through the organs of the digestive system, and so in order to get energy from food, the digestive organs have to be functioning effectively.

The main ones are, of course, your stomach, small intestines, colon, liver and pancreas. Also contained in this section are the kidneys, the suprarenal glands, the spleen and the gall bladder. These organs are connected in a clever but complicated system to breakdown foodstuffs into the chemicals required as fuel by the body, and to excrete the products of the food which are not wanted. Since intake of food potentially means intake of all sorts of unhelpful substances, these organs also have a vital role in maintaining the quality of blood, which is done in several ways.

All of the organs in this section are important, but they are not protected in the same way as those in the thorax. The only protection beyond the barriers of skin and fascia are the lower ribs, which are able to protect more superior organs such as the spleen. The figure below, on the right, shows a representation of the lower part of the torso when skin and fascia have been removed. Beside it is an explanation of what can be found at each level.

At level 1 we see all of the abdominal organs as depicted in many textbooks. The colon rises and crosses the abdomen, and the liver in the top left of the image is ideally placed for its vital role.

Level 2 takes off the colon and liver, to reveal the gall bladder and the other organs beneath the colon. We also begin to see signs of the peritoneum - the lining which surrounds most of the organs in the abdomen. The spleen, which can be found spanning the 9th - 11th ribs on the right side (or the left of the person), is next to the stomach, and the small intestine is coiled up in the space that is created by the colon.

Level 3 removes all but the stomach and those organs which are kept behind the peritoneum. Enterting the stomach is the oesophagus - the tube which carries food from the mouth to the stomach. Exiting the stomach is the duodenum, which is the first part of the small intestine. After coming out of the stomach, it pops through the peritoneum and becomes what is known as retroperitoneal. It comes back up to join the rest of the small intestine as the next section (the jejunum) begins.

By the time we reach level 4 we're left with only the retroperitoneal organs. In the abdomen, many of the organs are happily wrapped in the peritoneum, but some things are kept behind, as we're about to discover.

Level 5 takes us behind the peritoneum, to reveal the retroperitoneal portion of the duodenum, the kidneys, the suprarenal glands and the pancreas. Notice that the pancreas extends from the duodenum towards the left hand side of the person - our right.

Finally we have level 6, showing just the kidneys and the glands on top of them. The kidneys have an important function for filtering blood - getting rid of the stuff that's not supposed to be there - and the suprarenal glands (or adrenal glands) pump certain hormones into the blood when needed.

When we eat food, it has to get into the stomach first. It manages this by passing down the oesophagus into the stomach. Here it undergoes the first stage of digestion, where enzymes break up the food and get it ready for the important bits to be absorbed into the blood.

Next it's into the duodenum, where the blood is digested even more, and the first lot of nutrients goes into the blood. More is absorbed as the 'food' goes through the jejunum, then the ileum. Finally we reach the colon, which is in four parts - the ascending colon, transverse colon, descending colon, and finally sigmoid colon. The final section is best covered elsewhere.

What is the portal circulation?

You happily sit down and eat a meal. The food goes down your oesophagus into your stomach, and you begin digesting the food. What happens next? How do you make sure that everything gets taken to the right place? And is there anything to stop poisons spreading around the whole body?

There is actually an incredibly clever system in place called the portal circulation. With 'normal' organs (for example, the brain or the liver) blood travels from the heart, to the organ, and then back to the heart again. With the portal circulation, blood goes from the heart to the stomach and intestines; then, instead of returning straight to the heart, it travels to the liver first.

There are three main arteries which supply the stomach and intestines: the coeliac axis, the superior mesenteric artery and the inferior mesenteric artery. While each of these has a corresponding vein (i.e. splenic vein, superior mesenteric vein and inferior mesenteric vein respectively), they all feed into a special vein called the portal vein, which - rather than returning to the heart as other veins do - takes blood to the liver.

The nutrients and products of digestion have diffused into the blood, so lots of carbohydrates, amino acids and vitamins (etc.) are circling in the blood stream. As they pass through blood vessels in the liver, various reactions take place to make sure the contents of the blood are appropriately dealt with. This is why if you drink too much alcohol, your liver suffers - it is the liver that is blood's first port of call after receiving the products of what you've eaten.

What are the muscles of the anterior abdominal wall?

The abdominal wall is the name given to the wall which encloses the abdomen - there are particular features of this wall, in the same way that the thorax has particular features including the rib cage. The most interesting of the features are the muscles which make up the front or anterior of the abdominal wall.

The rectus abdominis are a pair of muscles which run from the bottom of the costal cartilages on the rib cage to the pubic crest, which is an area of bone at the front below the pelvis. These muscles have many 'bellies', which means that they bulge between stiffer sections called tendinous intersections. These bulges are what produces a six pack on an atheltic person.

Around the outside of these muscles is something called the rectus sheath, which is made up of the muscles which go around the outside of the abdominal wall. These muscles are a lot like the intercostal muscles, in that the fibres of each layer run in the same direction. The layer at the top is called the external oblique abdominis, and the fibres run forwards and downwards. The next layer down is the internal oblique abdominis, with fibres running forwards and upwards, and finally you have the tranversus abdominis, which has fibres running in a transverse direction.

The rectus sheath, then, is made up of these muscles - and also the thin layer of fasica behind them, the transversalis fascia. At the front of the rectus sheath is always the external oblique abdominis, and at the back is always the transversalis fascia. However, what the other two layers of muscle do changes depending on whether or not we're looking above the umbilicus (or 'belly button') or below it. Above the umbilicus, the internal oblique abdominis splits, half to go anteriorly, half to go posteriorly, with the transversus abdominis going behind as well. Below the umbilicus, both layers pass wholly in front of the rectus abdominis with the external oblique.

These muscles help with various different movements - for instance, rotation of the trunk (if external oblique on the left and internal oblique on the right contract, the body will rotate to the right) or flexion of the trunk (if rectus abdominis contracts, this pulls the rib cage down towards the pelvis, leading to flexion).

The most important detail about these muscles, except for the actions which they produce, is the lines which they produce. The way in which the abdomen is distinguished from the outside depends upon lines produced by these muscles:

Where the two muscles of rectus abdominis meet forms a line down the centre called the linea alba or 'white line'.

On the lateral borders of rectus abdominis (that is, round the outside) run two other lines - one on either side - in a crescent shape that traces the border of the muscle. These two lines are called the linea semi-lunaris after their moon-like shape.

These can be seen more clearly on the animation in the next section.

How is the abdomen distinguished?

From the outside, you can't really feel anything in the abdomen. However, it's still important to know where things are from the outside. That's why there are ways to identify areas within the body.

The belly button or umbilicus is found on what is called the supracristal line - this is the line that connects the top parts of the ilium. This is often at the same level as one of the vertebrae in the spinal column - the vertebra identified as L4. It is at the position of L4 that the aorta (the main artery which leaves the heart) ends by splitting into two arteries: the iliac arteries.

The linea alba and linea semi-lunaris, formed by the muscles of the anterior abdominal wall, provide helpful identification points - the linea alba obviously bisecting the trunk.

Given that the liver is such an enormous organ, it would be very helpful to identify it but it is usually (except during illness) covered by the rib cage. Like the lungs and the heart, the liver has an essential function which requires protection. If we need to work out where it is, we go by intercostal spaces and two other lines. A rectangle is formed around the outside of the liver on the front of the body by:

- the 5th intercostal space
- the 10th intercostal space and costal margin (since the 10th intercostal space doesn't go all the way around the front)
- the mid-auxillary line (which is the line that runs vertically downwards from the armpit)
- the left mid-clavicular line (which is the line that runs vertically downwards from half way along the clavicle - that is, roughly half way between the midline and the mid-auxillary line).

The gall bladder, which lies behind the liver, would be even more difficult to identify, but fortunately we can usually estimate where it is by saying that it corresponds to where the linea semi-lunaris crosses the line passing through the 10th intercostal space. This is especially helpful when surgery on the gall bladder needs to take place, as gall stones are a common problem.

Further Reading