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Biology Of Stress

Updated: Jul 3, 2019

Most of the stresses faced by our primitive ancestors were short, sharp shocks: the sort of danger that occurs if a predator attacks or a rock fall happens. To survive, they either had to be able to escape as quickly as possible or to fight off the threat.

Over time they evolved what we now refer to as ‘the stress response’ or otherwise known as ‘fight or flight’, to help them to do this.

This reaction is still a part of us; in times of stress, our bodies release stress hormones which make physical changes in the way our bodies operate. The stress response triggers the release of chemicals called catecholamines; these include hormones like adrenaline and cortisol which affect many different systems within the body:

The Short-term Effects of Stress:


  • Digestion of sugars – To get as much sugar into the blood as possible to give a surge of energy

  • Heart rate and blood pressure – To get as much blood pumping around the body as quickly as possible

  • Breathing rate – To get as much oxygen around the body as quickly as possible

  • Blood supply to muscles, especially in the arms & legs -the muscles required to either fight or flight

  • Blood clotting ability - To reduce lethal bleeding


  • Sensitivity to pain

  • Digestion (except sugars)

  • Blood supply to the skin

  • Blood supply to the ‘modern’ parts of the brain (language/logic/planning)

  • Immune system (protection from disease)

Essentially the hormones increase our energy levels, strength, speed and stamina so that we can run away or fight off a threat. This balance is ‘paid for’ by a reduction in efficiency in systems which do not have an impact on our immediate chances of survival. An encounter with a hungry tiger is unlikely to be protracted and therefore the changes caused by stress hormones have little or no adverse effect on our health or wellbeing if the body quickly returns to homeostasis.

The difficulty comes when we are under prolonged stress and the stress response never fully dies away.

Long- term exposure to catecholamines has a variety of consequences including:

high blood pressure, diabetes and heart disease.

Because some catecholamines act as neurotransmitters in the brain, these substances can also alter cognition and other mental processes, leading to:

  • Poor concentration

  • Mood swings

  • Agitation

  • Depression

  • Anxiety

  • Low Self-esteem

  • Lacking in Confidence

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