Stress triggers a chain reaction of physiological effects in our body. When your brain detects a stressor, your hypothalamus releases the hormone cortisol, which then jumps into your blood stream and locks into receptors in your tissues and organs. Your blood sugar goes up, adrenaline raises your heart rate, and oxygen fills your muscles. Your body readies itself for a threat, the evolutionary reaction commonly known as "fight or flight."

But stress also has an emotional component. Art Markman, a professor of psychology and marketing at the University of Texas at Austin, writes in Harvard Business Review about the emotional mechanics of stress.

"Stress is an emotional response; like all emotional responses, it emerges from the functioning of the motivational system," Markman writes. "Your motivational system engages goals and gives them energy so that you can pursue them. Simply put, when you succeed at your goals, you feel good, and when you don't succeed you feel bad."

Your brain's motivational system has two sections: The approach system achieves desirable outcomes and the avoidance system helps you avert undesirable outcomes. When your approach system is going, you are happy if you accomplish the goal and disappointed if you don't. If your avoidance system is running, you feel fear and stress if you fail and you're relieved when you succeed.

"We can also see stress as a reflection that there is something in your environment that you're trying to avoid, but that you have not yet avoided successfully," Markman writes.

Chronic stress is bad for you and can lead to serious health issues. You can combat it by understanding your body's reaction and motivation to make you feel stressed. You first want to pinpoint the cause of your stress--a deadline, a sick relative. But chronic stress is more complicated than that, Markman writes, and you need to work at identifying your issues and "break the cycle of rumination--the pattern of negative thoughts that accompanies, and worsens, stress."

He says stress is perpetuated by negativity. If you find yourself in a negative cycle, try to focus on positive aspects of your life. Displacing negative thoughts with positive ones can help "you engage your approach motivational system rather than the avoidance system and open yourself up to more experiences of joy and satisfaction." 

When you're stressed, your body pumps you with chemicals to energize you to action. But in today's world, we're usually sedentary instead of running from predators. Markman advises trying to refocus your energy. "The motivational energy generated by the avoidance system prepares you to act in the world," he says. "If you spend your day sitting at a desk, then that energy has nowhere to go. Channeling it into physical activity is a healthy way to release that energy and let you get back to work." 

The next time you feel stress rising, remember that something--a person, situation, or threat--is engaging your avoidance motivation system. Just through awareness you can learn to control and combat stress. "Understand the objects, try to control the motivational system that is active, and use the energy either to get productive work done or else find a way to dissipate it," Markman says.