Wednesday, January 29, 2014

Digestive System, Part 10: The Bitter End

GI, digestive system, Gastrointestinal Tract
Last time, we took a good hard look at the colon. Before we leave, let's take a short romp down a dead-end street called the appendix.

The vermiform appendix (from the Latin for “worm-shaped hanger on”) is a little pocket attached to the cecum. It doesn’t lead anywhere. Rather, it’s just a cul-de-sac off the large intestine, where poo can chill out and forget its worries for a while. You've probably been taught that it's vestigial—a useless waste of tissue, just like male nipples or Dr. Phil.

But is the appendix really purposeless?
Well, you can live a perfectly healthy life without one. Plenty of people do. None, to my knowledge, have reported pangs of longing for their lost organ. This has prompted many to conclude that the appendix is an evolutionary remnant of a time when our ancestors had a much larger cecum.1

Plant-eaters do tend to have larger ceca, allowing them to host larger bacterial colonies that can deal with the indigestible portions of their plant-based diets. So it’s possible that, as our diet changed, the cecum shrank not by absolute reduction, but by narrowing at the end, eventually forming the appendix. If this theory is correct, then the appendix is analogous to our tiny little tailbones. It's the dismal remnant of a long-forgotten anatomical feature.

There's also a theory, long-suggested but only recently examined in detail, that the appendix has a role to play in our immune system, providing early warnings of harmful bacterial infections in the large intestine. Or it might play a part in the maintenance of healthy intestinal flora (i.e. our gut bacteria.) The former theory was based on the observation that the appendix is associated with an unusual amount of lymphatic (immune) tissue, suggesting a close association with our immune system. The latter was based on the idea that the appendix, being outside the normal traffic flow in the large intestine, could provide safe harbor to harmless bacteria, which could then spread out and recolonize the large intestine if the main colony is expelled (as occurs when you have diarrhea.)

Recent evidence, while not yet conclusive, seems to be converging around the latter theory, with an interesting nod to the former.

AppendixThe story began to coalesce when it was found that harmless bacteria from our gut colonies are not only tolerated, but are actually protected and nurtured by our immune system.2 Suddenly, the old observation about the appendix and lymphatic tissue was seen in a new light. Perhaps, researchers suggested, all that lymphatic tissue is there to keep harmful, riff-raff bacteria out of the appendix, to better maintain a small auxiliary colony of good (or at least benign) bacteria. That way, if the less-carefully-guarded cecum is contaminated with riff-raff bacteria, the whole thing can be flushed out (via diarhetic assplosions) and the healthy bacterial colony restarted from the sheltered, auxiliary colony in the appendix.

Follow-up research seemed to support this model. Cladistic analysis (using differences in the character of modern animals to map out the course of evolution) indicates that something like the  appendix has evolved independently at least twice in different animal lineages, and has been maintained intact for over 80 million years in some of their descendants, while being lost in many others. Together, these findings suggest that the appendix has a maintenance cost, but the cost is worth the benefit in some animals.3

If this safe harbor model is correct, then the implications for the first world are minimal. First worlders don’t get diarrhea very often, since our drinking water comes from clean, sanitary sources. In such an environment, the appendix really isn't much use. Whatever small utility it might have is swamped by its nasty tendency to become blocked and inflamed.

An appendix might be a handy thing to have in the developing world, however, where clean water isn't as easy to come by. It’ll be interesting to see if future research reveals a lack of healthy gut flora in persons in the developing world who’ve had their appendices removed. The results might inform surgical strategies in the developing world, prodding surgeons there to preserve as much of the appendix as possible during appendicitis-related resection.

Or we could just spend the relatively paltry sum necessary to give everyone on the planet access to clean water, which would make the whole issue moot.

Ha ha. Just kidding! We need that money to drop bombs on people.

Path of feces through the colon
Adapted from intestinal tract scheme CC Olek Remesz
Our ship of feces has now navigated the colon, becoming progressively drier and more solid on its way until, by the time it’s in the sigmoid colon, everything of value has already been extracted. Unless you swallowed an antique silver dollar or something, in which case the final extraction will have to wait, because that turd ain’t home free just yet. It hits a traffic jam in the junction between the sigmoid colon and the rectum, a region named the "rectosigmoid junction" by people who just didn't give a shit any more (so to speak.) The rectosigmoid junction, the narrowest portion of the large intestine, is surrounded by a band of thick muscle that is normally contracted, causing feces to pile up behind it.

Once enough feces has collected, the rectosigmoid junction relaxes and the backpressure from the sigmoid colon pushes it forward into the rectum.4 I believe this stop and go action is meant to compact the feces into a nice, solid stool for easier passage, but I can’t find a good citation. Apparently there’s shit (so to speak) that even biologists don’t want to study.

Now, given its anatomy and apparent purpose, you might be tempted to say that the rectosigmoid junction is a sphincter. I mean, it's a tight band of muscle, right? It dilates, closing tight against pressure. That's exactly what a sphincter is, right?

Well, don’t go saying that in a room full of anatomists. Not unless you want this to happen.

Nerd Fight!

Believe it or not, there is a heated argument about whether the rectosigmoid junction is a sphincter or not. And, get this, the argument has been going on for nearly two-hundred fucking years!5 Just take a few moments to let the absurdity of that soak in. Anatomists have been wrist-deep in poo for two centuries, arguing about what to call this anatomical feature.

The evidence seems to be coming down on the side of it being a sphincter6, but I haven’t seen any anatomy charts or textbooks that are updated with that designation. I guess it’s scary to be a textbook author, an ostensibly neutral source caught between warring factions. Better to just make soothing noises at both sides and teach the controversy.

Well, I’m officially taking my life into my own hands and coming down on the side of the sphincter. The die is cast. Let the chips fall where they may. If this blog suddenly stops updating next week, you'll know an anatomist killed me.

Anyway, we were talking about poop. Once the feces passes the rectosigmoid sphincter sphincter sphincter (I’ve lived a good life,) it pushes against the walls of the rectum, which triggers the defecation reflex. This reflex causes the internal anal sphincter to relax. Now the only thing left between the poop and horrible, stinky freedom is the external anal sphincter. I’m guessing that you’re familiar with this one. It’s the sphincter that you have to consciously relax in order to poo.7 Once you do, the poop exits and you're finally through with it.

And that’s it! The journey from dinner plate to toilet bowl is complete. It took a while, but you made it.

As reward, please enjoy this picture of a slightly apprehensive baby duck.

Apprehensive Baby Duck


If you enjoyed this article, you're some kind of weirdo. So why not check out every other article in the series?

Digestive System, Part 1: Teeth and Spit
Digestive System, Part 2: Swallowing
Digestive System, Part 3: Down the Tubes
Digestive System, Part 4: B-12 as Temptress
Digestive System, Part 5: The Duodenum
Digestive System, Part 6: The Jejunum
Digestive System, Part 7: The Ileum
Digestive System, Part 8: Liver and Cecum
Digestive System, Part 9: The Colon


Citations and References
  1. Encyclopedia of Evolution. 2007. Pg. 410
  2. Bollinger, et al. Biofilms in the large bowel suggest an apparent function of the human vermiform appendix. Journal of Theoretical Biology. Vol 249, S 4, 2007, Pages 826–831.
  3. Smith et al. Comparative anatomy and phylogenetic distribution of the mammalian cecal appendix. Evolutionary Biology. Vol 22, S 10: 1984-1999
  4. Shafik et al. Electrophysiological study of the rectosigmoid canal: evidence of a rectosigmoid sphincter. Journal of Anatomy. 2002 May; 200(5): 517–521.
  5. The ASCRS Textbook of Colon and Rectal Surgery. 2nd ed. Pg. 14.
  6. Shafik et al. Identification of a sphincter at the sigmoidorectal canal in humans: Histomorphologic and morphometric studies. Clinical Anatomy. Mar 2003 V16 S2, 138-143.
  7. Human Physiology: From Cells to Systems. 7th ed. Pg. 635.

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