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We've all had a big test or a scary speech looming over us at some point, so everybody knows what anxiety feels like. Your guts feel all twisted up, you can't stop clenching your jaw, and instead of sleeping you just lie awake thinking useless thoughts like, My mom's the only person in the world who really... likes me, or wait, does Mom even like me?

It's annoying, and counterproductive, and apparently useless, so why do we even have that?

Well, it turns out that your body's anxiety response isn't useless at all; it's actually a result of the all-important sympathetic nervous system, which is in charge of the fight-or-flight response, the reason you can respond quickly and spectacularly to sudden threats.

It's there so that in the even that you wake up and your house is on fire you can respond immediately. Your body becomes flooded with adrenaline, or epinephrine, a hormone that in a temporary but substantial way, changes how your body does business.

It causes your sense of smell to heighten, your muscles and blood vessels to contract, your pupils to  dilate, sends more blood to your heart and lungs and less to your digestive system--after all, if you're gonna get out of a burning house alive, you're not gonna have time for a bathroom break.

And though it's not every day that your nervous system has to go to Code Red, at least I hope not, your sympathetic nervous system also kicks in to a lesser degree when you have business to attend to, like pay the bills or make a deadline. Without it, nothing would ever get done. 

But you don't want your sympathetic nervous system running the whole show; that's why, uh, you also have your mellower, groovier parasympathetic nervous system, which creates the opposite of the fight-or-flight response. It's sometimes called the relaxation, or feed and breed response.

Between these two systems, a balance, a kind of homeostasis is maintained. But some people have a hard time maintaining homeostasis; their fight-or-flight response is always in gear, causing chronic anxiety. This condition is sometimes hereditary or can develop when stressful experiences pile up. Not only does chronic anxiety feel crappy, it also damages your cells, alters your brain chemistry, and can exacerbate pretty much any health problem you can think of. For starters, as expected, chronically stressed people have a greater risk of heart disease and high blood pressure, since constant constriction of blood vessels puts pressure on the heart. Stress has also been found to worsen conditions such as asthma, migraines, male infertility, gastro-intestinal problems, and even Type II Diabetes by raising blood glucose levels.

And, on a cellular level, constant stress has even weirder effects. Research shows that chronic anxiety can actually accelerate aging by wearing away on the protective caps on our chromosomes, called telomeres. Stress can also inhibit our cells' ability to protect themselves from oxidation, a process that acts on the body in exactly the same way that rust acts on an old piece of metal. This causes imbalances in brain chemicals like serotonin and norepinephrine that affect sleep, mood, and emotional stability.

So if you're feeling anxious more often than you suspect is good for you, take some steps to protect yourself. Take deep breaths, get exercise, or go to yoga or meditation class, because a little anxiety at the right times can be healthy, but we need to keep you alive.

Thanks for watching this episode of SciShow. If you wanna tell us about your worst anxiety attack, you can find us on Facebook or Twitter, or down in the comments below. And if you wanna continue getting smarter with us here at SciShow, you can go to youtube.com/scishow and subscribe.

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