Why Do We Barf? It's Complicated

Maybe you shouldn't have eaten that gas station sushi. Or you're stuck in the back seat of a car on a winding, country road. Or perhaps you just found out your fiancé is leaving you for your brother.

Any one of those situations could have you violently heaving out your stomach contents. Vomiting is caused by a wide array of triggers, ranging from actual poison to motion sickness to extreme distress. We all do it, but why?

A number of things happen when you vomit. Saliva coats your teeth to protect them from your stomach's acid. You lose color in your face as your blood supply is redirected to your internal organs. You break out in a cold sweat as your blood pressure drops. Your heart rate and breathing quicken as you begin to retch, and your large respiratory muscles force your digestive tract's normally one-way street into reverse.

"[You] literally squeeze your stomach between your abs and diaphragm," says Bill Yates, a neurologist at the University of Pittsburgh who studies the neurological effects of movement.

Though scientists understand the mechanism itself, figuring out why we vomit is much more complicated.

First, vomiting is a defense against poisons. According to Yates, humans, cats and dogs are among the few animals that vomit. That's mostly because we've evolved to eat more in one sitting than other mammals, such as rats.

When rats eat something poisonous, they probably won't consume enough in one sitting to kill them (with the exception of something intended to, like rat poison). Their bodies can neutralize the small amount before they eat any more at their next meal. Humans, on the other hand, could more easily consume a lethal dose of something poisonous. In this case, vomiting can actually save our lives.

There's a special area of our brains devoted to poison control: the area postrema. Unlike most of the brain, which is isolated by the blood-brain barrier, the area postrema is exposed to the bloodstream so it's able to scan it for any toxins that have made their way in. If it finds something potentially dangerous, such as too much alcohol, it'll alert the body to evacuate the stomach to prevent further damage.

But your area postrema isn't the only part of the body that can trigger throwing up. If something goes too far down your throat that doesn't belong there, you may disturb the vagus nerve, also known as your gag reflex. Your stomach's nervous system can also prompt throwing up if it senses it's getting too full, or if you've encountered a stomach flu like norovirus that irritates your gut.

The central nervous system—the part of the brain that controls emotion—may also cause us to feel sick when we feel fear, grief or disgust. According to Bob Issenman, a professor of pediatrics at McMaster University in Canada, this is because the body has a hard time distinguishing between physical and emotional stress; it reacts the same way in either case.

Sometimes, your body will send you a warning signal when you're in danger of vomiting. Even though it's a stomach-churning feeling, feeling queasy is actually because your gut is paralyzed to prevent further ingestion of any poison.

Occasionally, you feel nauseous because your body senses chemical changes in the blood and assumes it's due to a toxin. In these cases, nausea and vomiting may be inconvenient side effects of normal behavior. Consider a strenuous run: If you're dehydrated and your electrolytes are thrown out of balance, your body may assume it's due to a toxin and cause you to feel sick. The same thing can happen during surges of hormones during pregnancy, or when you begin a new drug regimen. Chemotherapy drugs in particular have a reputation for causing nausea and vomiting.

Completely non-toxic situations can also make us sick. Motion sickness, for example, could result from disturbing the vestibular system, which gives us our ability to balance, but this is still up for debate. Issenman hypothesizes that it's caused by a disagreement between your senses; your eyes tell you you're in one place, but what you feel is something completely different. "It sets off a DEFCON-3 reaction," he says.

Sometimes, we vomit for no clear reason at all. Cyclic vomiting syndrome is a condition in which patients—usually children—experience vomiting episodes that can last for a few hours to a few days at a time. The majority of these cases are actually a form of pediatric migraines, but sometimes they can be "a huge stress response with no clear stressor," according to Issenman.

Given the myriad vomiting triggers, it's surprising we don't do it more often. As unpleasant as it is, throwing up is a part of life, just like food poisoning, road trips, and heartbreak.