In my introduction to the blog, I noted that I planned to talk about the physiology of fear in this week’s entry.
So what is fear? Fear, simply put, is a “reaction to a specific, observable danger.”1

Imagine that you are at work, sitting at your desk, when the elevator doors open and in comes a…lion! Or a gunman, who begins running towards you.

Or imagine that as you approach your house one day, you notice smoke and flames pouring out from the windows.
In each example above, there is a specific and observable danger. And so it makes sense that the corresponding emotional reaction is often fear.

We can also define fear by noting that it is associated with “strong arousal and action tendencies.”1
In other words, fear is characterized by physiological arousal (e.g., rise in blood pressure) and readiness for immediate action.

To explain physiological arousal in more detail, I first need to describe the components of the nervous system.

The Autonomic Nervous System

Our nervous system has multiple divisions (see Figure 1), one of which is the autonomic nervous system (ANS).

ANS regulates our internal organs through its two divisions, the sympathetic nervous system (SNS) and the parasympathetic nervous system (PNS). SNS increases our physiological arousal and prepares our body to manage stressful situations, while PNS helps bring the body back to a state of relaxation and rest.

SNS and PNS work together, almost like the gas and brake pedals in your car, to ensure that our bodies function properly. This is a complicated task, given that both divisions innervate numerous organs (see Figure 2).

The Fight-or-Flight Response

When a threat is detected, SNS triggers a physiological reaction known as the fight-or-flight response. This reaction prepares the body to flee the source of danger, or alternatively, to fight.

The fight-or-flight response is a complex phenomenon and involves many hormones (e.g., epinephrine, norepinephrine, cortisol, etc) and affects numerous organs.

Perhaps the most noticeable component of this response is the rise in heart rate. When people are afraid, they often feel their hearts pounding.

Increased heart rate aims to redistribute the blood supply from organs like stomach and skin that do not play a major role in the goal of fight/flight, to brain and especially the skeletal muscles (involved in fighting or running away).

Rapid breathing is another noticeable aspect of the fight-or-flight reaction. The breathing rate accelerates to supply the greater demand for oxygen by various organs.

The organs also require extra energy, which is why the liver releases more glucose. Burning all this energy produces heat, and the intensified perspiration helps cool the body.

Other reactions include the dilation of pupils (to let in more light), and changes to our sense of hearing, the result being that we can see and hear any potential threat better and more quickly.

One way to make sense of all these changes is to think of the body as a country getting ready for war. Whatever process is not considered essential to the immediate purpose is stopped so that it is not a drain on resources.

For example, reproductive processes and immune functions are not immediately necessary and are therefore suppressed. So is digestion—and the associated saliva production, which results in the experience of dry mouth. Not surprisingly, feeling the urge to urinate or defecate, and in some cases, to vomit, can also occur.

When the threat is too great, however, or when neither fighting nor fleeing seems like an option, immobility and even fainting can result. Called “tonic immobility,” this state of paralysis sometimes occurs during “brutal rapes.”1

Of course, fainting may occur not only in response to overwhelming fear but in response to the sight of blood and injury too.  One possible explanation for this is based on the evolutionary theory. Because fainting is associated with a major drop in blood pressure, it can minimize blood loss and increase the chances of survival in those who are injured and bleeding.Therefore, fainting can have survival value.


Fear is a reaction to observable and specific danger and is associated with physiological arousal and preparations for action. The physiological reaction, called the fight-or-flight response, can be triggered by any potential threat, and is body’s attempt to increase chances of survival.

Sometimes we experience this response when no objective danger exists, or when fleeing or engaging the enemy are not the best option. Sitting in a movie theater, for instance, you might have experienced how a sudden loud noise can send your body into fight-or-flight mode.

Next week I will discuss anxiety. Though it can involve physiological arousal as well, anxiety has more to do with our perceptions and thoughts than with objective danger. Which means we can have more control over our anxiety. And that can be a good thing!


1. Barlow, D. H. (2002). Anxiety and its disorders: The nature and treatment of anxiety and panic (2nd ed.). New York, NY: Guilford Press.