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A. Introduction
1.
The colon (= large intestine) is the last of the GI organs.
2.
This is where the chime that comes from the small intestine is gradually transformed into feces.

B. Anatomy of the Colon
1.
The chime from the ileum first arrives in the cecum; the first part of the colon.
2.
There is a valve, the ileo-cecal valve, located between the ileum and the cecum, to prevent chime from flowing back into the small intestine.
3.
In addition, there is also the appendix, a worm-like structure that is attached to the cecum.
4.
It is not absolutely clear what the function of the appendix is. Some think this is a leftover from old times, others believe it has special immunological properties.
5.
What is certain however is that the appendix can easily get infected, thereby inducing the feared appendicitis (!) which may require acute surgery.
6.
After the cecum, the chime is pushed upwards into the ascending colon.
7.
Upon reaching the right corner (behind the liver, hence its name: hepatic flexure), the chime moves from right to left along the transverse colon.
8.
And, after having now reached the left ‘corner’ (the splenic flexure; behind which organ? (link), the chime moves downwards along the descending colon.
9.
After this, the chime, which we should now call feces, arrives in the sigmoid colon where it will gradually move into the rectum.
C. The Haustra
1.
As you can see from the previous diagram, the colon consists of a set of ‘pouches’ or segments, called haustra (singular: haustrum).
2.
The longitudinal muscle in the colon is not a full circumferential layer as in the small intestine.
Rather, it is restricted to a muscle band, called taeniae coli, that runs along the length of the colon, and keeps the circular segments in order, thereby causing this segmented pattern.
3.
Movement of the contents is caused by having one haustra after another contracting, thereby ‘pushing’ the content towards the end of the colon; the rectum.
D. Colon functions
1.
The colon has two major functions:
  1. storage of fecal matter unit it can be expelled
  2. reabsorption of water and electrolytes from the chime from the small intestine.
2.
Therefore, along the length of the colon, water is gradually re-absorbed from the chime into the blood as the bolus progresses through the colon.
3.
Hence, the chime is gradually changed from semi-fluid, in the cecum, to solid, when it enters the rectum. Of course, the feces must not be too solid or else we would have trouble ‘defecating’ (=constipation)!
E. The Rectum and the Anal Canal
1.
The rectum is the last part of the GI system that ‘stores’ the feces until ready to be transported to the anal canal (defecation).
2.
The last part of the rectum is the anal canal which contains two sphincters that are normally closed:
  1. The internal anal sphincter, which consists of smooth muscles and is therefore involuntary.
  2. The external anal sphincter, which consists of skeletal muscles and is therefore voluntarily controlled (thank God!).

3.

Interestingly, this is the exact opposite arrangement to that present at the start of the GI system!

As you may remember, in the esophagus, food intake is controlled first by a skeletal sphincter (the UES) and then by a smooth muscle sphincter (the LES)! (link).

4.
Once residual food enters the rectum from the sigmoid, the feces stretch the wall. This initiates a parasympathetic reflex, which contracts the rectum and relaxes the internal anal sphincter. Fortunately, the external sphincter remains closed until you decide to defecate!
5.
The walls of the rectum contain several folds (‘plica’) that seem to play a role in the distribution of solid and gas material in the rectum.
6.
Note also in the diagram the location of two venous plexuses; the internal and external hemorrhoidal veins.
7.
These veins may in certain conditions expand and develop varices, that expands into the anal canal; hemorrhoidal varices. These can cause extensive bleeding!
8.
Another important point about these venous plexuses is that the most of the blood flowing through these veins do not go to the liver, in contrast to all other veins in the GI system.
9.
This is important because drugs that are absorbed by these hemorrhoidal veins are therefore not metabolized by the liver but can enter the body without modifications!
10.
That is why some drugs are applied through anal (=rectal) pills!
Slides E.6. The Colon
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