Wednesday, August 7, 2013

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.

In the presence of recently-swallowed food, however, the pyloric sphincter puckers right up and holds the solids firmly in the stomach. It's kind of a, “We will serve no chyme until it is time,” sort of thing. The pyloric sphincter closes even tighter when the stomach’s stomach-churning churning motion pushes solid boluses against it.1

But even when closed, the pyloric sphincter is not watertight. It allows liquids and very small (<1mm) particles of chyme to pass through, but holds back larger particles. The rejected portion splashes back into the stomach in a process called retropulsion. (Some internet sources report that chyme reaching the duodenum is then squirted back into the stomach through the narrow opening of the pyloric sphincter, but this appears to be a misunderstanding of the mechanism of retropulsion. More authoritative sources suggest that such duodenal regurgitation is not part of normal digestion.2)

You may be tempted to think the pyloric sphincter regulates the rate at which chyme moves between the stomach and the duodenum. Well, that's why you're not a rich, famous blogger like I am. It is, in fact, the stomach that determines how quickly it empties, by regulating its contractions. There are two basic types of stomach contractions. You have your standard, run of the mill contractions used primarily to mix and churn. They don’t contribute much to emptying.

Then you have big old “emptying wave” contractions, which are six times as powerful. It's the emptying wave contractions that really get chyme moving into the duodenum. Several factors affect the frequency of emptying waves, including the amount of food in the stomach (more food means more frequent emptying waves,) the type of food (carbohydrates speed emptying, fat and protein slow it,) and how well the duodenum is handling the chyme it's already received (an overtaxed or strongly acidic duodenum slows down stomach emptying.)

Emotions also seem to play a role.2 Anger increases stomach emptying, so you'll have enough room to eat the person who angered you. That’s my theory, anyway.

Once in the duodenum, we're in intestine territory, and the movement of chyme gets pretty boring. To chyme, the journey through the stomach was like an exciting white water rafting trip. The journey through the duodenum (and the rest of the intestines) is a slow tube ride down a gently flowing river.

But, you know, a gently flowing river that’s trying to kill you. Because you're the chyme, in this analogy.

And make no mistake. Despite having a pansy name, the duodenum will fuck you up. The duodenum has a posse made up of the pancreas, liver, and gall bladder, and it calls them up whenever any punk nutrients squirt into its turf and start acting like they own it. Witness the following exchange between the duodenum and the pancreas:

Ring ring.

“Hello, this is the Pancreas.”

“Hey Pancreas. This is the Duodenum.”


“Exactly. Some acidic chyme motherfuckers are squirting in here like they own the place.”

“What do you want to do about it?”

“Send Bile in. You know where Bile's at?”

“He was over in the Liver earlier, but the Liver sent him to the Gallbladder and told him to wait for the word.”

“Good, good. I need Bile to come down here and neutralize the acid in that fucking chyme, then get in there and take those fats apart.”

“Kick their asses, right.”

“Did I say kick their asses, bitch? I said take them apart. I want Bile to chop those fats into tiny little globules. Emulsify those motherfuckers. Bile don’t stop until that fat’s too small to even see, got it?”

“That’s cold, man.”

“And when Bile’s finished, tell it to grab any vitamins that were dissolved in the fats. I know someone in the ileum who’ll give us good money for those.”

“What do you want me, the Pancreas, to do?”

“First of all, stop name dropping yourself. And then bring the whole pancreatic juice crew down on those motherfuckers. I want the amylases working starch, the lipases on fats, the proteases on proteins, and the nucleases breaking DNA and RNA into their constituent nucleotides.” 

“Why you want to be lifting nucleotides off dead chyme?”

“Waste not, want not, man.”

“That’s disgusting. So what are you gonna be doing when all this is going down?”

“I’m doing my part, bitch. I'll be heading down there with the whole intestinal juice crew. We're gonna help out, do a little of everything, get some mucus in there, and also motivate some of those punk proteases you’re sending down.”

“My proteases ain’t punks, man.”

“Are you shitting me? Have you seen some of your proteases? That little bitch Trypsinogen doesn’t do shit until my boy Enteropeptidase gets up in his grill.”

“Okay, okay, whatever. Hey, you doing anything after this? Want to meet up at IHOP later?”

“Wait, what?"

“IHOP. You wanna go there later, for pancakes?”

"That question makes no sense in the context of two anthropomorphic organs discussing digestive processes like it's a turf war. The only way we could go to IHOP is if there's an IHOP inside the digestive system. And what, are there, like, tiny little people inside the IHOP, eating their own food, with their own digestive systems, each with their own little turf wars going on? That scenario would require an infinite, fractal regression of turf wars, IHOPs, and digestive organs, nested inside each other like fucking matryoshka dolls. Is that what you're saying? ‘Cause that’s just some lazy world building right there, motherfucker.”

“Listen, man, do you want pancakes or not?”

“Well, it’s meta as shit, but okay. Let's Rooty Tooty this bitch.”

IHOP Police
Later that night, context police storm the meta-IHOP
on Pancreatic Duct Avenue and arrest 12 paradoxes.
Moving right along...

The duodenum is mostly about introducing digestive juices that break chyme down at a molecular level, chopping its fats, proteins, carbohydrates, and even DNA into constituent molecules that are small enough to absorb. It does a little bit of absorption itself—because why the hell not?—but most of the nutrients that come into the duodenum won't be absorbed until they get to the jejunum, the next segment of the small intestine. The duodenum is too busy sowing the seeds of chyme's destruction to deal with that hassle.

Movement through the duodenum and the rest of the small intestine is accomplished with peristaltic waves, which also serve to smoosh the chyme all over the intestinal walls, spreading it out evenly to speed absorption. You might remember peristaltic waves from the esophagus, but things are a bit different here in the intestines. Whereas a bolus of food moved through the esophagus at the same speed as the peristaltic wave, the liquidy chyme in the intestine is less manageable, harder to push, and it moves much more slowly.

Repeated peristaltic waves propagate short distances along the intestines, with a wave speed of between 2 and 25 centimeters per minute, and a frequency of between 8 and 12 waves per minute.3 But the chyme itself only moves along at 0.5 to 2 centimeters per minute.4 The result—fast-moving waves driving slow-moving chyme—is easy to visualize: it produces a rhythmic, chopping effect that mixes digestive juices into the chyme and keeps it from settling.

Those waves vary depending on how far along the chyme is. In the duodenum, they’re faster and more frequent. As you get deeper into the small intestines, where more and more of the useful nutrients have already been absorbed, the waves become slower and further apart.

Speaking of slow and far apart, I'm afraid you'll have to wait until next time to delve into the next section of the small intestine: the jejunum.


If you enjoyed this article, why not read them all?

Citations and References:
  1. Textbook Of Medical Physiology, First Edition, pg. 612
  2. Textbook Of Medical Physiology, First Edition, pg. 613
  3. Textbook Of Biochemistry And Human Biology 3rd Ed, pg. 598
  4. Textbook Of General Physiology, pg. 59

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