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Poets, novelists and songwriters have described it in countless turns of phrase, but at the level of biology, love is all about chemicals.

Although the physiology of romantic love has not been extensively studied, scientists can trace the symptoms of deep attraction to their logical sources.

"Part of the whole attraction process is strongly linked to physiological arousal as a whole," said Timothy Loving (his real name), assistant professor of human ecology at the University of Texas, Austin. "Typically, that's going to start with things like increased heart rate, sweatiness and so on,"

When you catch sight of your beloved and your heart starts racing, that's because of an adrenaline rush, said Dr. Reginald Ho, a cardiac electrophysiologist and associate professor of medicine at Thomas Jefferson University Hospital in Philadelphia, Pennsylvania.

Here's how it works: The brain sends signals to the adrenal gland, which secretes hormones such as adrenaline, epinephrine and nor epinephrine. They flow through the blood and cause the heart to beat faster and stronger, Ho said.

The response is somewhat similar to a fast heartbeat while running on a treadmill, although exercise has other benefits, he said.

For people with serious heart problems, love could actually be dangerous, Ho said. That's because when the heart rate goes up, the heart uses more oxygen, which can be risky for an older person with blood vessel blockages or who has had a prior heart attack. But good medicines such as beta blockers help curb the adrenaline response, Ho said.

It is also likely that nor epinephrine, a stress hormone that governs attention and responding actions, makes you feel weak in the knees, said Helen Fisher, professor at Rutgers University and author of the book "Why Him? Why Her? Finding Real Love by Understanding Your Personality Type."

Fisher's research team did brain imaging of people who said they were "madly in love" and found activity in the area of the brain that produces the neurotransmitter dopamine. Dopamine and nor epinephrine are closely related.

"What dopamine does is it gives you that focused attention, the craving, the euphoria, the energy and the motivation, in this case the motivation to win life's greatest prize," she said.

This nor epinephrine response has never been precisely studied in relation to romantic love, but the system seems to be more activated in people in love, she said.

Also likely involved is the serotonin system, she said. Some data from an Italian study indicate that a drop in serotonin levels is associated with obsessive thinking.

The stress hormone cortisol has also been shown to have implications for love, Loving said. His lab showed study participants who had recently fallen in love a picture of a romantic partner or friend, and had them describe or "relive" the moment of falling in love or wanting to be friends, respectively. Those who recalled falling in love showed an increase in stress hormones such as cortisol even 30 minutes after they were asked to think about it.

Generally, there are three brain systems involved in romantic love: sex drive, love and attachment, Fisher said. The sex drive evolved to get you to look for a lot of partners, the "love" portion is for focusing mating energy on one specific person at a time, and attachment is for allowing you to tolerate the partner -- at least, long enough to have children with him or her.

These systems are often connected, but can operate separately, she said. That means you can start out with one of them -- casual sex, or an intense feeling of love, or an emotional connection -- and move on to the others. For example, what may start out as a one-night stand may feel like more because the hormones oxytocin and vasopressin, released during orgasm, make you feel deeply attached to someone. You may feel in love after that, or instead feel somehow responsible for the person, because of these hormones.

Fisher's team has found that romantic love doesn't have to die -- they found the same activity in the brains of people who said they were in love after 20 years of marriage as in people who had just fallen in love. This brain area makes dopamine and sends it to other areas.

In the days of early humans, in hunting-and-gathering societies, these qualities were especially advantageous for finding a person to bear and raise children with, she said.

Why, then, do small children fall in love if they are not trying to reproduce? Fisher hypothesizes that kids -- even 4-year-olds -- practice at love and learning more about themselves before it begins to become important to them.

Love also has health benefits for people who have aged beyond their reproductive years, she said. Being in love makes people feel optimistic, energetic, focused and motivated, which were all positive for health and societal contribution in the early days of humans, she said. So, it makes sense evolutionarily that people can still fall in love after their childbearing period.

Romance also is good for you. Studies have shown that people who have frequent sex are generally healthier, with a longer life, fewer coronary events and lower blood pressure. A 1995 study in the journal Demography found that marriage adds seven years to a man's life and two years to a woman's.

Loving's team is studying how people who have recently fallen in love respond to stressful situations. They hypothesize that people for whom the love is still new will respond to the stress and recover from it quicker than those who have recently been in a breakup or have been in a relationship for a long time.

"The guess is that when individuals are falling in love, they are walking around with rose-colored glasses," he said.