Digestive System, Part 10: The Bitter End

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?

Digestive System, Part 9: The Colon

Adapted from an image CC CC Olek Remesz.

The last time we looked at the digestive system, feces was piling up in the cecum. So where’s it going from there?

We’ve already gone over contractions in the colon that serve primarily to mix, but the real movers and shakers in the colonic mobility world are Giant Migrating Contractions. GMCs are real truth in advertising: contractions that cause giant migrations (of poop.) They occur in the small intestines too, to move chyme into the cecum, but in the colon they’re notable for occurring only three times per day, and moving feces a full third of the way through the colon at one go.¹ You’ve probably noticed these from time to time, as a sensation of sudden movement inside your belly. The next time you notice one, make sure to tell the person next to you that your colonic feces has just undergone a Giant Migrating Contraction. If they don’t edge away from you, it’s love.

Digestive System, Part 8: Liver and Cecum

Okay, so by now you've absorbed all those tasty amino acids, sugars, fats, vitamins, and minerals. They've passed through the enterocytes and into the bloodstream (with the exception of some fats and fat-soluble vitamins, which at this point are still wandering around lost in the lymphatic system.) But, now that the bulk of nutrients are in the bloodstream, they must be rushing to the four corners of the body, to be shared equally by your cells and tissues, right?

You’re so ignorant, you disgust me.

Of course they aren't! You can't just give them free rein to wander around wherever they please! There could be terrorists in those nutrients. And what do we do to keep out terrorists? We give them a security screening.

So please remove all shoes and jackets, and place all metal and electronic devices in the tray, because you're in line for the hepatic portal system.

Digestive System, Part 7: The Ileum

The border between the jejunum and the ileum is… not a border. Unlike the organs we've been looking at so far, there is no clear demarcation line between jejunum and ileum. They transition smoothly into each other as we travel down the small intestine, becoming progressively less jejunumy and more ileumy as they go. With no anatomical guidepost to demarcate them, the jejunum is arbitrarily designated as the first 2/5ths of the small intestine (not including the duodenum,) and the ileum is said to make up the last 3/5ths.

The two are not very different. You can have your entire jejunum removed, in fact, with little effect on your health. I mean, I wouldn't recommend super-sizing your meals if you don't have a jejunum, but you can live a basically normal life. Over time, the ileum will even adapt itself, becoming more jejunumy in order to take up the slack.¹

Digestive System: Part 6, The Jejunum

The jejunum is where nutrient uptake really gets going. Up to now, the digestive tract has been obsessed with mashing food up, breaking it down, and purifying it of pathogens, in order to maintain the purity of essence of our precious natural fluids. In the jejunum, digestive juices do continue to work, and there’s some last minute molecular-level destruction happening on the intestinal lining, but the primary focus has shifted from breaking nutrients down to absorbing them safely.

Digestive System, Part 5: The Duodenum

Image CC Olek Remesz

Nutrient digestion kicks into high gear when we reach one of the most lyrically named organs in the human body: the duodenum. Have a listen to the accepted pronunciations, from Wiktionary:



Aren't those lovely? I especially like the second one. Why the duodenum doesn’t appear at least once in every song ever written, I can’t understand. That's going to be a rule when I conquer the world.

Enough of that talk, though. You'll know more than you ever wanted to soon, soon.

When we left off last time, chyme (another of those wonderfully lyrical words, referring to the soupy mixture of chewed food churned up by the stomach) was squirting into the duodenum through the pyloric sphincter. When there's little or no chyme in the stomach, the pyloric sphincter has a semi-relaxed posture—chill but not too chill—which allows stomach contents to drain slowly into the duodenum.

Digestive System, Part 4: B-12 as Temptress

Have you ever lost anything, Homo sapien? Your purse? Your car keys?

Well, it’s rather like that. Now you have it, now you don't.

We lost a vitamin that’s critical to our survival. And good luck trying to find it now. The last time anyone can remember having it was at least 2 billion years ago. Yes, that's "billion" with a ba-ba-billion.

It's vitamin B-12. We need it to live, but we can't make it ourselves.In fact, the only naturally occurring creatures that can make vitamin B-12 are bacteria and archaea, the simplest organisms on Earth.¹

So how the hell do animals survive? I mean, I don't usually browse the archaea aisle at my local supermarket, so where does my B-12 come from? Well, if you eat enough veggies, you can get your B-12 from the soil bacteria clinging to them. Some herbivores can even supply their own B-12 needs, with bacteria living in their stomachs.

But what if you're a carnivore?

The Digestive System, Part 3: Down the Tubes

Last time, we talked about swallowing. Now that the masticated, saliva-soaked food bolus has navigated between the Charybdis of the larynx and the Scylla of the nasal opening, it passes through its very first sphincter and enters the esophagus.

The upper esophageal sphincter closes behind the food bolus, sealing its doom. There isn’t a whole lot to the esophagus. It’s just a slimy, muscular tube for moving matter from the pharynx down to the stomach (and, when occasion merits, back up again.) Liquids slide through by gravity, but a solid food bolus has to be massaged down the length of the esophagus by peristaltic waves. You can visualize peristaltic motion by imagining the way an Earthworm moves, squeezing its body segments in a smooth wave that runs from front to back. Or you can just look at the animated gif in this Wikipedia article, if you like to visualize the lazy way (i.e. by looking at stuff.)

The Digestive System, Part 2: Swallowing

Crossing the External Threshold: Swallowing

You probably think swallowing is easy, don’t you?

You ignorant son-slash-daughter of a bitch.

Swallowing in humans is a nuanced and complex affair, requiring the coordinated execution of voluntary and involuntary motions, precisely timed to send food down your greedy gullet.

The Digestive System, Part 1: Teeth and Spit

If you’re like most people, everything you know about your own digestive system can be summed up by the following diagram, which I drew on my phone:

But is that all there is to the digestive system? Is the journey from food to poop really just a bland, 3-step trip through a poorly sketched flipper baby?

Actually, no!

As a matter of fact, it’s pretty fucking awesome. You see, our digestive systems have evolved with us over millions of years, shaping us into the efficient, fatass omnivores we are today. So come with me—won’t you?—on a magical journey through your own innards. It’s a journey through acid and bile, into the depths of a world that's shrouded in mystery, yet closer than your own ass. It's a world where an alchemical brew of ions and enzymes can transmute the basest foods into fuel for your body. It's a world where there are a thousand times more bacteria than you have brain cells.

We'll enter this world the same way most everything does: through the mouth.