Editor’s note: Brain Basics from Scientific American Mind is a series of short video primers on the brain and how we feel, think and act. Below is a synopsis of the fourth video in the series written by a guest on this blog, Roni Jacobson, a science journalist based in New York City.

By Roni Jacobson

We all want to be happy—and would prefer to avoid unpleasant emotions such as heartache, fear and anger. Evolutionary psychologists argue that even unpleasant emotions are adaptive, however, in guiding our behavior. In addition, displays of emotion can transmit what we are thinking and feeling to others. Similarly, we can get a sense of what others are feeling by reading their facial expressions—an ability that is crucial for social interaction.

Although the various strong emotions feel different, they summon some of the same physiology. When we experience an intense emotion such as anger, our bodies kick into “flight or flight” mode, triggering a number of physiological changes that serve to heighten our arousal. First, the autonomic nervous system directs the release of hormones that tell the liver to flood the bloodstream with sugar, providing us with a burst of energy. Heart rate and respiration increase, digestion slows down and pupils dilate, allowing our eyes to take in more light. Then, when it’s all over, the process works in reverse and we return to our normal resting state.

Other Brain Basics videos:

A Transformation of Light: How We See

Quick! What Is The Word for a Pair of Opposites?

The Hidden Power of Others Over You