1.
The control (regulation) of the GI-system is quite complex.

2.
There are several ‘layers’ of regulations and controls involving the GI-system, several of which will be discussed here.

1.
As with other systems, the nerves play a crucial role in controlling the GI-system.

2.
There are in fact two neural systems:
a) The intrinsic regulation
b) The extrinsic regulation

3.
The intrinsic regulation is performed by the Enteric Nervous system which is fully located in the wall of the intestine.

4.
It consists of two specific layers in the intestinal wall:
a) The submucosal layer (=Meissner plexus)
b) The Myenteric plexus (=Auerbach’s plexus).

5.
The fibers of the submucosal layers innervate the mucosa cells in the underlying mucosa. Stimulation of these cells stimulates secretions from the mucosa cells, thereby promoting digestion of the food bolus located in the lumen.

6.
The myenteric plexus is located between the circular and the longitudinal muscle layers. It stimulates these muscles, inducing peristalsis, segmentation and other types of intestinal motility.

7.
The extrinsic regulation is performed by the Autonomic Nervous System.

8.
As you may know, this nervous system consists of two parts:
a) The sympathetic nervous system
b) The parasympathetic system

9.
The sympathetic system is most often used when the body has to work, to fight (or flight), using skeletal muscles, the heart etc. to be active.

10.
The parasympathetic system is the opposite. It is most effective when the body is quiet; digesting, resting or sleeping.

11.
In other words, it is the parasympathetic system that controls the GI-system!

12.
So, the parasympathetic system promotes secretion, digestion, peristalsis etc.

13.
The sympathetic system, on the other hand, when stimulated, inhibits secretion and absorption, induces sphincter contractions, and causes vasoconstriction of the splanchnic circulation (see next panel).

1.
As with any other organs in the body, the GI-organs require blood for their survival.

2.
But another important reason for the blood circulation is to transport all the digested nutrients from the GI organs to the body.

3.
In terms of the arterial circulation, the GI-system can be divided into three systems:
a) The Foregut, perfused by the celiac artery
b) The Midgut, perfused by the superior mesenteric artery
c) The Endgut, perfused by the inferior mesenteric artery

4.
a) The Foregut (red) consists of half the esophagus, the stomach and the first half of the duodenum.
b) The Midgut (blue) consists of half the duodenum, the jejunum, the ileum and the colon till the splenic flexure
c) The Endgut (green) consist of the descending colon, the sigmoid and the rectum

5.
The venous output is more complicated in the GI-system than in other systems. The veins do not perfuse directly into the venous system of the body.

6.
Instead, from the gut, the venous flows first through the Vena Porta (=portal veins) to the Liver! After the liver, the venous blood goes to the inferior vena cava and back to the heart etc.

7.

Why is this so complicated?
Two reasons:

8.
First, all the bacteria and the viruses who were not destroyed in the gut but who managed to get into the blood go to the liver where they are destroyed before reaching the body circulation! In other words, an extra safety mechanism.

9.
Second (and I think even more important), all the proteins and carbohydrates absorbed in the GI-system are transported to the liver where they are stored and create new compounds required for the body. The liver, as you may know, is the most important biochemical factory in our body!

10.
By the way, most of the ingested fats do not travel through the venous blood circulation but are transported through the lymph circulation. This lymph does NOT go to the liver but flows straight into the circulation.

1.
In the past few years, another aspect of the GI system and its regulation has started to generate a lot of interest and attention: the microbiota.

2.
Microbiota is the collection of all bacteria residing somewhere in the body. They are located in the mouth, the nose cavities, the vagina, but most importantly, in the gut.

3.
Recent research has now shown that the gut contains billions of many types of bacteria. We also think that the composition of all these bacteria determine a kind of ‘profile’ for a particular person. Something like a gut ‘fingerprint’.

4.
The ‘healthy’ bacteria keep our mucosa healthy, so that the mucosal cells can absorb our nutrients. The bacteria are also involved in the process of capturing vitamins such as vitamin K from our food, in this case necessary for our blood clotting system (link).

5.
The composition of our microbiota also seems to be determined by the food that we ingest. We now think that if you eat healthy (lot of vegies etc.), your gut flora will also be healthier, whereas if you eat unhealthy (lots of coke, sugar etc.), the composition of your microbiota will also be influenced.

6.
The stomach and the small intestine contain the least amount of bacteria, due to the acidic environment. The large intestine contains the most, approx. 1-2 kg! In fact, about 50% of our feces consist of our gut flora!

7.
In general, there are 200-800 different types of bacteria species in our gut with about 50 types the most common (90%).

8.
It is important to realize that the relationship between our body and the microbiota is not only a non-harmful co-existence but also a symbiotic relationship.

9.
Some of the useful things that the microbiota does for us:
- breaking down (=fermenting) fat molecules
- training our immune system
- producing vitamins (such as B & K)
- producing several hormones
- preventing the growth of harmful bacteria

10.
If the gut flora composition is not healthy, this may contribute or cause several diseases such as:
- ulcers
- inflammation
- irritable bowel syndrome
- diabetes
- obesity
- cancer?

11.
It is not only the food that we ingest that determines the composition and our healthy our gut flora is. Also healthy lifestyle (exercise etc.) helps a lot. But it is also possible to influence our microbiota with the ‘transplantation’ of feces from somebody else!!

12.
YES! In certain cases of gut inflammation, such as caused by clostridium colitis (which causes chronic diarrhea, abdominal cramps, fever), a transplant from a healthy donor may change the gut flora to a more healthy composition and thereby solve the problem!