What is stress?
Let's consider the physiological definition of stress to understand what it means when a person says, "I'm feeling stressed." In physiology, stress is anything that causes the body to respond by releasing stress hormones.
The definition of stress, then, is: an event that causes by the body's natural fight-or-flight response. The "stress response" is what happens when the body reacts to stressors (noxious stimuli).
Over time, the mental, behavioral, and physical symptoms of the stress response can wear us down. How does a normal, natural function (the fight-or-flight response) become problematic?
First, let's examine more closely how and why the fight-or-flight response occurs.
Our bodies come equipped with automatic responses to our environments that allow the body to function optimally. For example, the body maintains its temperature within a narrow range, even if the environmental temperature varies greatly. Another example of the amazing way the body responds automatically to the environment is the fight-or-flight response.
The definition of stress, in physiological terms, is a harmful (or potentially harmful) stimulus. Vander, Sherman, and Luciano (2001) state that "these stimuli comprise an immense number of situations, including physical trauma, prolonged exposure to cold, prolonged heavy exercise, infection, shock, decreased oxygen supply, sleep deprivation, pain, fright, and other emotional stresses."
Whether the stress is physical or emotional, the response is the same. The adrenal cortex increases secretion of the hormone Cortisol, and the activity of the sympathetic nervous system is increased, resulting in increased epinephrine secretion from the adrenal medulla. Other hormones are also released during stress, and insulin production is usually decreased.
The response of the sympathetic nervous system is commonly called the "fight-or-flight" response because the physical affects allow us to physically fight or flee.
"...the major effects of increased sympathetic activity, including secretion of epinephrine, almost constitutes a guide to how to meet emergencies in which physical activity may be required and bodily damage may occur." (Vander, Sherman, and Luciano, 2001, pg 730). In other words, the fight-or-flight response helps the body perform physical activity and respond to injury.
The actions of the sympathetic nervous system include (adapted from Vander, Sherman, and Luciano, 2001):
- the liver and muscles break down glycogen into glucose to provide a quick source of glucose for energy
- increased fat breakdown to provide glycerol (to make glucose), and fatty acids, results in increased concentration of fats in the blood to be used for energy
- decreased muscle fatigue
- higher heart rate and more forceful heart contraction resulting in increased cardiac output (more blood flow)
- more blood flow to the muscles and less blood flow to the organs
- greater ability of blood to clot
- breathing becomes faster
These actions result in increased physical strength, energy, and readiness for intense physical activity. Muscles do not tire as easily. Blood flows to the muscles to allow them to work. Fat is broken down for energy. The body is also prepared for injury because the blood can clot more easily — this means that if a cut occurs, the blood quickly clots to stop bleeding. Find out more about the purpose of stress and anxiety. This sounds great, right? Who wouldn't want their body ready to perform at it's strongest and able to repair itself if injured?
Read about the symptoms of stress to find out about the consequences of chronically eliciting the stress response.