Stress is a word used in biology and medicine to describe changes that can be seen in organisms. Stress can be physical or mental. Stress can describe a pressure, like the pressure of a person's upper teeth on their lower teeth while chewing. Stress can also describe an impact, when one object hits another.
Stress describes a living thing's response to a threat or some other change in its environment. This change can be called a "stressor." Stressors can vary in size and impact. A stressor can be internal and come from inside the living thing - like an illness.  A stressor can be external, coming from outside the living thing - like an attack.
Types of stressEdit
There are different types of stress. Each type has different characteristics and symptoms.
- acute: stress causes immediate harm in a short time span. An example of this type of stress is having to finish an assignment quickly or avoiding a car crash.
- chronic: stress causes continued harm and frustration. An example of this type of stress is continued unhappiness with school. This type of stress wears organisms down.
- traumatic: stress after a scary and dangerous event. This stress causes fear. An example of this is experiencing a war or hurricane. This can lead to Post-Traumatic Stress Disorder (PTSD).
Organisms can respond to stress in different ways.
- physical: changes happening in the body. An example is a fast heartbeat after giving a speech.
- mental: changes happening in the mind. An example is feeling unfocused due to difficulties at work.
- emotional: changes happening to an organism's emotions. An example is feeling worried or upset about an upcoming exam.
Stress may also lead to adaptations, which allow certain organisms to better deal with the stressors. Stress happens every day, and is part of every living thing's life. Too much stress is bad and can cause health problems. Stress plays a part in some physical problems, like heart disease. Stress also plays a part in many mental illnesses, like anxiety, Acute Stress Disorder, and PTSD. Acute Stress Disorder and PTSD are mental illnesses that can happen when a person experiences something that is very terrifying, like a major accident or a war. Another type of stress disorder is a "psychosomatic illness" where a person has physical symptoms that are caused by an emotional stress, not by physical harm. 
There are different ways to deal with stress. One tip is to remove yourself from the stressor. For example if a school assignment is causing stress, taking a break from the assignment can be good. This break can include sleeping or engaging in hobbies. Physical activity and exercise reduce stress.  Social support from friends and family can be helpful. Health care professionals can help when stress is bad and affects physical or mental health. Relaxing with meditation and deep breathing can also manage stress.
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- Levenson, James L. (2006). Essentials of Psychosomatic Medicine. American Psychiatric Press Inc. ISBN 978-1-58562-246-7.
- Sculy, D; Kremer, J; Meade, M; Dudgeon, K (1998). "Physical exercise and psychological well being: a critical review.". Br J Sports Med 32 (2): 111-120. doi:10.1136/bjsm.32.2.111. http://bjsm.bmj.com/content/32/2/111.