Endorphins are like the cardiovascular buff's fountain of youth.

To some, they remain a myth, a mystical lagoon of endless joy, where leg cramps and muscle inflammation float away like bubbles in a fruit-flavored pre-workout beverage. To others, they are a pleasant hitchhiker that finds them around mile eight every Saturday afternoon run. They're ethereal, they're incredibly pleasant and everyone wants them.

But what are they?

To understand endorphins, it is first important to understand a little bit about how the brain works when it comes to drugs.

Any drug that you put into your body is known as "exogenous" – literally just meaning from the outside. However, for every exogenous drug, there's usually an "endogenous" counterpart – that is, a naturally-occurring biological chemical that activates the same receptors.

All of that being said, if you can find a receptor for something that comes from the outside of your body, you're probably interfering with a fine-tuned biochemical machine of some sort ("drugs are bad, mmkay?").

So what does any of that mean? Endorphin is a simple abbreviation for what's known more simply as "endogenous morphine."

That's right, your body is naturally producing opiates that won't kill you. These are endogenous opioids that your pituitary gland and hypothalamus release in many varying situations, from sex to spicy food consumption. One of the primary reasons your body would induce such a nepenthe of chemicals is when it comes to face-to-face with intense nociception, or pain.

Nociceptors are essentially free nerve endings that permeate the exterior regions of the body and signal damaging stimuli back to the brain and spine. These endogenous opioids are released onto the nociceptors in the spine to make them send less pain signals. It's like throwing a temporary microscopic wrench into the works.

Evolutionarily speaking, this makes sense. If you've been following a wildebeest for 20 days during what's called "persistence hunting," humans equipped with on-board painkillers are going to be much more likely to get the kill, and live to see another hunt, leaving their endorphin-less counterparts staggering home, starving.

While most of the Homo sapiens species does not have to run for however many miles a day as it used to, this biological mechanism remains encoded in us.

All of that said, endorphins can also prove a bit troublesome, as pain killers make you – wait for it – not feel pain. So, you're out on your run and you're running on six-mile-ready legs about to zonk out... and BAM, you're suddenly sprinting and singing the chorus to Arcade Fire's "Wake Up" and realizing that the sky is incredibly blue and the neighbor's dog really isn't that annoying. The only issue is, if your body isn't ready for it, you can seriously injure yourself.

You'll hit mile eight and the endorphins have worn off and you realize your knee can't really bend correctly anymore and that chafing was even worse than you could have ever imagined.

It's because you're high on painkillers. It happens on long, intense workouts. I experienced it around mile 32 of a bike race last year, when I went from about to fall off my bike to humming Loch Lomond and passing 20 or so more people on the way to the finish. (I was feeling very Scottish that day for whatever reason. Long distance workouts send your mind to odd places.)

So, while endorphins may not have you overdosing anytime soon, they do have their own inherent idiosyncrasies and dynamics.

It's all about listening to your body, as hippie as that may sound. If you are in pain, stop exercising. If you think you should be in pain but instead you can't feel the lower half of your body, stop exercising.

Don't work out for a number. Work out so you feel great, and see if you can do more next time.

Be aware of endorphins, and learn to love them. Just don't hurt yourself.

Andrew Fleming is a junior in neuroscience. He can be reached at aflemin8@utk.edu.