Wednesday, June 5, 2013

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 esophagus plunges deep into the belly of the, uhh, belly. On the way, it has to squeeze past the heart and then through the diaphragm, a sheet of muscle under the lungs. The diaphragmatic constriction, along with esophageal muscles and muscle fibers at the top of the stomach, combine to form the lazy bastard of the digestive system: the gastroesophageal junction (a.k.a. “esophagogastric junction,” a.k.a. “esophageal sphincter,” a.k.a.“cardia,” a.k.a. “EG,” a.k.a. “How Many Fucking Names Does One Anatomical Structure Need, Anyway?”)1

The esophageal sphincter's job is to guard the prison gates, keeping food and acid contained inside. But, if your EG is anything like mine, the little bastard usually falls asleep on the job, and half your stomach contents—acid and all—seep back into your esophagus to cause pain, burning, nausea, and deep-seated resentment. That’s a condition called GERD or heartburn. I just call it, “Fuck you, Esophageal Sphincter. Fuck you, and the controversial embryological origin you rode in on.”


Now we come to the stomach. Back in school, you were taught that the stomach's job is to break down food. That's... truish, but it's only part of the story.

The acid environment in the stomach does denature proteins, unwinding them so that certain digestive enzymes, called proteases, can more easily chop them to pieces. Imagine you've got a ball of yarn (the protein) and a bunch of people with scissors (the proteases) trying to cut it into pieces. If you unwind the yarn first, they can all grab a section and get cutting with great efficiency. If you leave it in the ball, however, not as many people can cut at once, it's harder to maneuver your scissors in for a clean cut (we call this "steric hindrance,") and they have to cut up the outside layers before they can start on the inner ones.

It's no way to cut yarn.

Besides assisting protease activity, acid denaturation helps to liberate some minerals that are normally bound up inside proteins (such as iron.) Other minerals (such as calcium) may be present in solid form, so the acid starts to dissolve them. So, you know, acid is pretty handy.

How does the stomach regulate its acidity? There are a number of positive and negative feedback mechanisms. Some rely on neural stimulation, some on chemical cues, some on hormones, and some on the physical stretching of the stomach. But together they can be summed up as: If there's food, then make acid; if there's too much acid, then make less acid.

Truly brilliant, the human body is.

The stomach also acts as a mixing bowl, churning up food boluses with rhythmic contractions and relaxations. The boluses are broken down into a soupy homogenate called "chyme." But that's just reversing bolus formation. It's your teeth that did the real work, in this regard.

Fats and carbohydrates, meanwhile, pass through the stomach largely untouched. The stomach does have a little bit of lipase (fat-cleaving enzyme) and some carbohydrate-cleaving enzymes (which we'll examine later in this series,) but not in sufficient quantities to get much digestion done on these macronutrients. While that's barely happening, gelatinase goes to work breaking down the gelatin found in gristle and/or Jell-O cups, and mucus is secreted to protect the stomach from its own acid.

Oh, and alcohol is absorbed. Actually, alcohol is such a sneaky little bastard that it started absorbing the moment it touched the membranes of the mouth, but it’s the stomach where it really starts to seep into you in earnest. It's alcohol moving through the stomach wall and into your bloodstream that, to pick a totally random and hypothetical example, makes you think that a series of blog articles on the digestive system is a fun idea.

Stupid stomach.

In short, the stomach isn't doing nearly as much digesting as your grade-one science textbook implied. Also, the actual stomach doesn't have a little smiley face on it. I know, right? I was pissed too.

But the lies don't end there. That guileful textbook probably didn't mention that one of the primary functions of the stomach is to keep bacteria out of our bodies.2 Yeah, your stomach is a decontamination chamber, killing harmful organisms in a bath of deadly acid. It's like a freaking Bond villain, laughing maniacally while bacteria struggle futilely to escape.

Or, sometimes not so futilely. Some bacteria manage to survive, particularly a class of acid-loving bacteria called "acidophiles." These little critters have to be stopped with anti-bacterial enzymes (lysozymes,) by mounting an immune response, or just by denying them attachment to any surface of the digestive tract, thus shuffling them quickly along to the exit before they can vandalize the place.3

From time to time, you'll encounter bacteria that defy your efforts to provide an amusing death for them, in which case you get sick. Particularly wily bacteria may colonize the stomach itself, and you get stomach ulcers. This is a boon for them, since they get to live in an environment where their competition is being actively killed off by your stomach acid.

And then there are another class of bacteria entirely—the harmless simpletons of the human gastrointestinal tract—our commensal gut flora. They can get past our defenses but, once inside, they don't do anything to hurt us. They just sit around in our colons, singing songs and peacefully munching on whatever food we weren't planning to digest anyway, like little microscopic freegans. To wrap up this edition, let’s take a look at why these "hippies of the digestive tract" are tolerated inside our bodies.

Of course, to even get inside, they have to survive the passage through the stomach. Does that mean the acidic conditions in our stomach evolved to sort the good bacteria from the bad?

Probably not. I can’t find a scientific study that examines the issue, but it seems likely that our intestines have simply adapted to co-opt the bacteria that were available. That is to say, since acidophiles can get past the stomach, the lower parts of our digestive system evolved to tolerate acidophiles. Fighting them would require significant expenditure of energy and probably set off a really passive-aggressive evolutionary arms race, so we just gave up and let them crash in our colons.

Co-evolution may also be at work, with some species of bacteria being favored for their helpful properties. For example, they may have the ability to outbreed and crowd out other, more harmful bacteria. Or perhaps they produce a particular vitamin. Over time, we may have evolved to tolerate them, giving the helpful bacteria a safe place to live while we get the benefit of their presence. Thus advantaged, both species would be more likely to flourish and pass on these mutually advantageous traits to their offspring.

We'll take a closer look at our commensal gut bacteria later in the series, and examine the lucrative but disgusting benefits they may provide in more detail.


Citations and References:
  1. Mittal and Balaban. The Esophagogastric Junction. 27 Mar 1997, Vol 336, No 13, pp 924-932.
  2. Giannella et al. Gastric acid barrier to ingested microorganisms in man: studies in vivo and in vitro. Gut 1972;13:251-256.
  3. Sarker, et. al. Non-immunological defense mechanisms of the gut. Gut, 1992,33,987-993.


If you enjoyed this, check out the other articles in the series:
Digestive System, Part 1: Teeth and Spit
Digestive System, Part 2: Swallowing
Digestive System, Part 3: Down the Tubes (this article)
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
Digestive System, Part 10: The Bitter End
And don't forget to recommend this fantastic article to your friends, using the power of begging the handy social media buttons below.

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