SALT LAKE CITY -- You see them and your knees quake, your palms get sweaty and you realize you either run away or meet the person of your dreams and spend the rest of your life in love. Was it meant to be? Did the universe conspire to put the two of you in the same place at the same time in order that you fall in love?

It's possible. But what's more likely is that a whole series of chemical reactions and psychological factors came together all at once in order to produce that butterflies-in-your-stomach, impossible-to-describe feeling you experience. It doesn't mean that your love isn't real. But it does mean that there's a lot more going on than just just the way we feel about our sweetheart.

Love at first sight

In the early stages of love, including when you first see your beau, your brain is flooded with chemical signals, in almost the same way that might happen after taking a powerful drug. Norepinephrine is released, leading to a racing heart, sweaty palms and weak knees. High levels of norepinephrine is also associated with loss of appetite and feelings of joy.

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Spending time with your honey also releases dopamine, a chemical highly associated with both pleasure and reward behavior. This neurotransmitter also tends to make people more social and amiable, more likely to want to spend time with others. Seeing them again and again may also condition you to expect a boost of dopamine each time you see them, leading you to want to see them more and more.

Your body also starts producing phenylethylamine, which not only stimulates norepinephrine and dopamine, but also has effects of it's own, similar in may respects to a psychotropic and a stimulant. In fact, chemical derivatives phenylethylamine include hundreds of psychoactive compounds including MDMA, commonly known as "ecstasy."

This, along with lower levels of serotonin, may be responsible for the increased focus and attention we have for loved ones.

Love potion no. 9

In the last couple decades, more has been learned about an interesting hormone called oxytocin, perhaps a real life love potion number nine, a hormone that seems to be involved with practically every deeply intimate human interaction.

Oxytocin is an extremely important nonapeptide - basically a tiny protein with exactly nine amino acids in it. Practically every animal species on the planet has some related nonapeptide that does something similar, which is most famously to regulate contractions during childbirth. It also helps to stimulate lactation when breastfeeding.

But it does more too, and generally helps cement bonds between humans, especially romantic bonds, reducing anxiety, creating feelings of closeness and contentment. Couples that are separated for some time have both noticeably lower concentrations of oxytocin in their blood and much higher levels of anxiety.

In both humans and dogs, oxytocin levels increase after a 5-25 minutes of contact.

What's really interesting is that oxytocin, and an very closely related hormone, vassopressin, are some of the most ancient and most ubiquitous hormones in the animal kingdom. Virtually every species from an octopus to a human has a version of these two nonapeptides, with oxytocin involved in love and reproduction, and vassopressin involved in regulating water in the body. Two of the most important things for any form of life - water and reproduction.

But I thought love was all pheromones?

Human pheromones have been associated with love and thought of as an aphrodisiac for a while now, with perfume and cologne manufacturers claiming that their special blend will be more attractive to someone else. In reality, it's much more complicated than that.

Humans lack many of the physiological structures that allow other animals to respond readily to pheromones. So they aren't as important to us as a species as they are to many others. But that's not to say there aren't subtle effects.

One of the few really clear demonstrations of the effects of pheromones is the McClintock effect, which tends to make women's menstrual cycles line up based on subtle odor cues.

More directly related to romance is the fact that women tend to be rated as more attractive while they are ovulating, and that women tend to be attracted to more "masculine" men while ovulating - those with symmetrical faces and the classic "toned" muscles. Interestingly, however, the effect is reversed when women use hormonal birth control.

Also related to pheromones, and odor in general, is a person's major histocompatibility complex, or MHC. Every living thing's cells have certain receptors on them that say specific things to other cells - in loose terms, that's the MHC. But this can have profound effects on how one is perceived by potential mates.

One's MHC affects the odorous compounds in sweat. One study found that women were more attracted to men with very dissimilar MHCs than their own. This was based on how the men smelled - they had women sniff shirts that the men had worn for two days. But, again, using hormonal birth control seemed to reverse the effect.

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