Wednesday, May 8, 2013

The Digestive System, Part 2: Swallowing

Crossing the External Threshold: Swallowing

Pharynx, Nose, Mouth, Throat cross section
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.

First, the lips are pushed together and the tongue forces the bolus into the back of the mouth. As soon as it arrives, proprioceptors—nerve endings that sense tension and position in the body—tell the brain that something is heading for the pharynx (see picture above, to get your bearings.) The brain then proceeds to freak right the fuck out. It runs around frantically closing hatches like it just spotted a white squall. It closes the openings into the nose, the windpipe, and even the way back into the mouth. The windpipe gets two layers of defense: the epiglottis, which closes to direct food away from the larynx, and the vocal folds, which seal tight against choking.

Breathing and chewing are suspended. Even the route down the throat—the one place you actually want the food to go—is constricted into a narrow opening to keep you from accidentally swallowing large objects, such as Charlie Sheen’s ego (Hi-ooooo.) The entire pharynx thrusts upward, grabs hold of the bolus of chewed-up food, and drags it kicking and screaming to the esophageal sphincter. You can feel the pharynx's motion by putting your fingers on your throat while you swallow. This active hand-off, where food is pushed back by one set of muscles and dragged down by another, is what allows you to swallow even in zero gravity. Some scientists believe the process evolved this way to make swallowing more robust—less sensitive to head and body orientation. I have a competing theory, slowly gaining traction in the scientific community, which states that "nature wants us to be astronauts."

Still, the process sometimes goes wrong. Food can be forced into the airway, which causes choking. So, if choking is a danger, why are the food and air passages linked at all?

To explore this question, let's go fishing.

In oxygen-poor water, fish will sometimes gulp air from the surface and hold the bubble in their esophagus, where oxygen can diffuse across the esophageal cells and on into the bloodstream. It's not a terribly efficient process, but our atmosphere is rich in oxygen, so it doesn't necessarily have to be. If you're a fish, a little air in your throat can forestall asphyxiation long enough to reach better waters.

But what if you're a fish that has to live in oxygen-poor water all the time? Well then, life sucks for you, doesn't it? Unless you've evolved a handy little esophageal pocket to make it a bit easier. These pockets, found in many fish native to oxygen-poor waters, can hold an oxygen-rich bubble safely outside the main esophageal tract, and are rich in blood vessels to allow more efficient oxygen transfer.

Which brings us to our first all-terrain ancestors, the lungfish (technically the “basal lungfish”, since lungfish have continued to evolve and diversify in the 400 million years since we split off from them.) In lungfish, as you might imagine, the esophageal pocket has evolved into a set of proper lungs. They connect to the bottom of the esophagus, sitting below and to the sides of it.

Evolution of lungs and gas bladders
Evolution of lungs and gas bladders in fish. Kardong et al. Fig 11.5
This sets lungs apart from gas bladders, common in modern fish, which are situated above the esophagus and assist in regulating buoyancy. It was once believed that lungs evolved from gas bladders, but the fascinating truth is that it's the other way around. Cladistic analysis—deriving a "tree of life" from characteristics that are preserved or not preserved across species—indicates that primitive lungs developed first, and then evolved into the gas bladders found in modern ray-finned fishes. If this sounds like a regression from a more evolved to a less evolved characteristic, remember that evolution isn't proud, and it doesn't give a crap what you think of it.

No, seriously.

Evolution isn't "driving" toward a particular set of perfect characteristics. Species evolve whatever characteristics make them the best breeders and survivors, from among the pool of characteristics that are available within existing genetic diversity, or which emerge from novel adaptations. Ray-finned fish with more gas-bladder-like lungs thrived, eventually splitting off and losing the lung function entirely. And now the gas-bladdered, lung-deprived fish are more common than lungfish.

Funny old world, ain't it?

So what about lungs? Well, they're always ventral to the esophagus in lungfish, even though the nostrils are located dorsally. The nostrils in lungfish were adapted to open into the back of the mouth, allowing the fish to breathe through its nose just like we do. (In most fish, the nostrils are just a dead-end that doesn't go anywhere, used primarily for smell.) Of course, this means the air and foodways cross, introducing a risk of swallowing air or forcing food into the lungs.
Lungfish food and air passages, esophagus, airway
Lungfish: Food passage (red) crosses air passage (blue)

Sound familiar?

If you're starting to get the impression that your pharynx and airways are built according to a blueprint laid down by some goddamn ancient fish, you're not far wrong.

This blueprint serves the lungfish well, since having the lungs at the bottom of its body helps buoy it up in shallow water, so it doesn’t have to drag its belly along the mud, and assists it in lifting its nose above water. The nose remains high, also for ease of breathing. The potential downsides are outweighed by these advantages.

And this basic layout has been conserved from fish to amphibian to reptile, bird, and mammal, down through 400 million years of tetrapod evolution. Even snakes retain the fundamental orientation, though their trachea and glottis have moved far forward in the mouth to allow breathing while swallowing large prey. In dolphins and whales—where the windpipe and glottis are extended up into the nasal passages and nest within the soft palate, divorcing the airways from the food passage and preventing water from the mouth from seeping into the lungs—the airway and esophagus still cross each other (but do not communicate) in the pharynx.
Porpoise airway
Porpoise: Food passage (red) crosses air passage (blue)

So why is this cumbersome, lungfish-adapted anatomy conserved in so many tetrapods?

I haven't the faintest fucking idea.

Okay, okay, I do have the faintest fucking idea. I would speculate that: 1) Crossing airway with foodway in the pharynx is valuable as a backup breathing route, if the nasal passages become blocked. 2) Keeping the nasal passages clean requires mucus that will, preferably, drain into the pharynx and then down the esophagus. 3) Communication between mouth and nose allows the sense of smell to contribute to taste. 4) Placing the entrance to the esophagus behind the entrance to the trachea permits food to be pushed back with greater force during the act of swallowing. If the entrance to the trachea were behind the entrance to the esophagus, it would force the epiglottis (the flap that covers the trachea during swallowing) to fight against the full force of the swallowing pharynx. Mind you, these explanations are only speculation (however informed, incisive, and brilliant they may be.)

In any event, the configuration of the pharynx makes food getting into the air passages a real possibility in most tetrapods. They had to evolve, over the course of tens of millions of years, anatomic modifications that minimize the likelihood of choking.
Human airway, esophagus
Human: Food passage (red) crosses air passage (blue)
And then humans fucked it all up by walking upright and evolving speech. That changed both the orientation and the anatomy of the pharynx, throwing a monkey wrench into our natural choking-prevention gear. That is not uncommon among adaptations that dramatically increase fitness (i.e. an animal's ability to survive and breed successfully.) Inconvenient characteristics often pop up or are dragged along, when a highly advantageous adaptation occurs.

For example, think of a bird’s hollow bones. They reduce the bird’s weight, making it a better flier, but they’re also hollow fucking bones. It’s worth the cost, as witnessed by their phenomenal success, but they still have hollow fucking bones. Think of how annoying that must be. I bet it’s the top thing birds complain about when they get together.

“Hey Marlene! Haven’t seen you since the fall migration. How ya been doing?”

“Oh, hi David! I’m good. I’m good. Of course, I still have hollow fucking bones.”

“Oh yeah, me too. Fuck my hollow fucking bones.”

“And mammals keep coming by and punching me in the face.”

“It sure sucks being a bird, doesn’t it?”

“Yeah, but flying's nice.”

“That's true. That's true. Still, you know, hollow fucking bones.”

"Tell me about it. Hollow fucking bones."

Given enough time, it was likely that new adaptations would arise, reducing the choking rate in humans. For example, we might have evolved a thicker, stronger epiglottis—one that's better at directing food down our esophagus.

Ah, but that was before Henry Heimlich came along and introduced his eponymous maneuver. Now choking is more survivable, so what’s to stop people from going around and choking all the time, without any penalty to their evolutionary fitness? Nothing, that’s what. Kids these days can choke all they want, confident that someone will come along and expel the obstruction with a sharp abdominal thrust.

Thanks Heimlich. Thanks for ruining evolution forever.

Heimlich Maneuver


If you enjoyed this article, check out the others in the series!

Digestive System, Part 1: Teeth and Spit
Digestive System, Part 2: Swallowing (this article)
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
Digestive System, Part 10: The Bitter End

1 comment:

  1. Well yes walking upright screwed over a lot. It's why our children are premature compared to most mammals of similar body mass. Was it really worth it? Was being a facultative biped so bad?

    More to the point, huzzah for swallowing. It should be high on everyone's list of unstoppable reflex actions.


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