The Digestive System part 3: The Small Intestine, Colon and Rectum

This post is the third and final of this short mini-series on the digestive system. Yay!

1. What are the divisions of the small intestine? Which are intraperitoneal?

The divisions of the small intestine are the duodenum (the C-shaped curve at the beginning of the small intestine), the jejunum (the first half of the small intestine) and the ileum (the second half of the small intestine). The division between the jejunum and the ileum is not completely arbitrary, but is determined by the arrangement of blood vessels to these two sections of the intestine. Both the jejunum and ileum are intraperitoneal (i.e. encircled within the peritoneum, which is the membrane lining the abdominal cavity).

2. What would be the effect of removing the entire colon and rectum? Would this be more or less difficult to live with than removing the small intestine? Why?

Some of the functions of the colon and rectum include the absorption of water and, in the case of the rectum, controlling the excretion of waste (since obviously as you know it is a mostly voluntary action). If the colon is removed, the ileum is able to adapt to take on the role of absorption of water. Controlling excretion of waste may be problematic but there are ways around this- for example, in a colostomy, where part of the colon is removed, faeces can be excreted through a hole in the skin into an external bag (which doesn't sound particularly pleasant, but I guess you'd take what you can get if you're in that kind of situation). On the other hand, if the small intestine is removed, the colon would not be able to take on the role of digestion of nutrients. Therefore, removing the colon and rectu would be less difficult to live with than removing the small intestine.

3. Compare and contrast absorption in the small intestine with absorption in the colon.

There are several differences between absorption in the small intestine and absorption in the colon. The colon absorbs only water, while the small intestine absorbs almost everything else. Another difference between absorption in the two areas is that, while both areas have many folds to create a larger surface area, the folds in the small intestine protrude outward and are called villi while those in the colon fold inwards.

4. What are the voluntary and involuntary components of defecation? What would be the effect of completely cutting the external anal sphincter?

We have two anal sphincters- an internal and an external sphincter. The internal sphincter is controlled by the parasympathetic nervous system and is involuntary. The external sphincter, on the other hand, is voluntary. If the external anal sphincter was cut off, then we would lose voluntary control and essentially become incontinent.