In its endless quest to gross us out, the human body makes a lot of strange noises. Here are the whys and hows of the most common.

Why do we fart?

As long as we eat, flatulence is going to be a fact of life. Gas is a by-product of the digestion process and we normally produce a half a liter of it everyday in the form of 14-20 farts.

The source of that gas is the bacteria that live in the lower intestinal tract. Any food that doesn't get broken down in the stomach or the small intestine winds up in the large intestine in an undigested state, where it's met by colonies of bacteria that break it down and convert it into nutrients. While the bacteria go about their business, they produce various gasses as by-products, including methane, hydrogen and hydrogen sulfide (this last one is the cause of the stench). All these gases then descend to the rectum where they're released with the trumpeting fanfare of the vibrating anal sphincter.

These foods we can't break down? Mostly sugars, including fructose, lactose, raffinose (the indigestible sugar in beans), sorbitol (a sugar found in everything from fruit to diet soda), although some fibers and starches get to the large intestine undigested, too. Fats and proteins also play a role in flatulence, too, even though the body has no problem breaking them down. They tack on some extra time to the digestion process, buying the bacteria more time to generate gas from the stuff they're already processing.

While farts might be perfectly normal, they can also be dangerous. The gases that they're composed of are all flammable, which leads to people trying to light them, which leads to an estimated 25% of fart lighters getting burned. Our intestinal gases aren't much safer when they're still inside us. There have been a handful of hospital accidents where intestinal gases with high oxygen content exploded when electric cauterizers were used during surgery.

Why do we burp?

More gas that needs to be released. Here, it's just oxygen that we swallow while eating and drinking and the carbon dioxide that carbonates soda and beer. Like farts, burps come with a distinct noise: the vibration of the upper esophageal sphincter.

Why do our stomachs growl?

The growlings in your gut are known as borborygmi among people who like to use technical terms, a cool little bit of onomatopoeia we picked up from the Greek borboryzein ("to rumble") some 200 years ago.

Borborygmi happen because of the way we process food. Muscle contractions in our guts constantly push food through the gastrointestinal tract, all while mixing the food, liquids and digestive juices together and breaking down our meals. When our stomach is full, the food there keeps things quiet. When the stomach is empty, the muscle contractions keep going, but without any food there, the walls of the stomach (actually, it's not just our stomach that growls; the small intestine is responsible for some of it, too) just rub against each other, which we can both hear and feel. Additionally, some of the noise we hear is from digestive juices sloshing around with gases generated by digestion.

Why do we hiccup?

It depends on who you ask. The ancient Greeks thought they were violent emotions erupting from our bodies. Today, we know that hiccups (synchronous diaphragmatic flutter, in doctor-speak) are normally caused by spasms in the diaphragm, a large muscle between the chest and abdomen. When the diaphragm is irritated by any number of things, from a full stomach (which pushes against the phrenic nerves of the diaphragm) to chain smoking, it goes into a spasm, which causes us to take a quick breath. The sudden rush of air causes the epiglottis (the flap that protects the space between the vocal cords) to close shut, which makes the familiar "hic" sound.

You'll remember I said that that was the "normal" cause. Some specific cases of hiccups have attributed to skull fractures, tuberculosis and constipation (a winning combination if there ever was one).

More on yawning

A few months ago, I filled you in on science's best theories about why we yawn. We still don't have a definite answer, but a team of researchers at my alma mater, Temple University, have found some fascinating stuff in their study, "Field observations of yawning and activity in humans." Among them"¦

- In the 15 minutes following a yawn, wrist motion increases.

- More yawning occurs during the week than during weekends.

A few more months of work like that, and we'll crack the yawning mystery wide open.

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