Stress can be any unspecific demand to adapt to a critical situation within a very short time. This has been described as "fight-or-flight" reaction (Walter Cannon). Our body reacts in a way that has been developed in primeval times, very well suited to run away, react very fast or hunt a wild animal.

These physiological adjustment reactions have been described by the Canadian stress reseacher Selye. Their purpose is to stimulate the bodily functions for a short time (30 minutes) in case of emergency. So the body reacts to any kind of loads, stress or adaptibility requirements in a very similar way:

• Acceleration of the heart rate, better perfusion
• Increase of the blood pressure
• Increase of the ventilation (hyperventilation)
• Better perfusion of muscles
• Change of blood-clotting
• The skin becomes tense

Any body reactions that are not necessary to respond to the immediate danger are decreased:

• low gastro-intestinal functions
• lower immune reaction -> higher risk for infections
• lower sexual functions