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:
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Acceleration of the heart rate, better perfusion
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Increase of the blood pressure
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Increase of the ventilation (hyperventilation)
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Better perfusion of muscles
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Change of blood-clotting
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The skin becomes tense
Any body reactions that are not necessary to respond to the immediate danger are decreased:
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low gastro-intestinal functions
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lower immune reaction -> higher risk for infections
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lower sexual functions