What is stress?

Stress is nature's survival mechanism for humans. It is a physiological and psychological response to events which upset us or our balance.

In stress situations the body prepares for two responses, flight or fight. This means that stress-hormones, such as adrenaline, are released into the bloodstream in order to make our body and mind perform to its absolute maximum capacity. Once the crisis has passed, the system returns to normal.

Our body does not distinguish between the types of stress we are exposed to, and longer stresses, such as a deadline, relationship trouble, money issues and so on, don't call for an immediate response. This means that the system is in fight or flight mode for far longer than it was originally designed to. Exposed to this for a longer period of time, and the body can take damage. This is due to the effects the stress-hormones have on our system.

Physiologically several things happens in the body in order to focus on survival:

• Heart rate and blow flow into the muscles increases.
• Blood vessels under the skin constrict to prevent blood loss in case of injury.
• Pupils dilate to increase our sight.
• Blood sugar levels increases to enhance our reactions.
• Digestive system slows down.
• Reproductive system slows down.
• Growth hormones are switched off.
• Immune system is weakened and response inhibited.
  

Shortly put, the bodily functions that are not essential for immediate survival, are shut down while those which can make us react faster, think better and be stronger are enhanced.

Sources and further reading:
http://www.helpguide.org/mental/stress_signs.htm
http://www.mtstcil.org/skills/stress-definition-1.html
http://www.ehealthmd.com/library/stress/STR_affect.html
http://www.mental.health.wa.gov.au/one/resource/46/Stress%20document.pdf