Edexcel International A Level Biology

Revision Notes

7.14 The Role of Adrenaline

The Fight or Flight Response

  • During situations that creates stress, fear or excitement, the neurones of the sympathetic nervous system will stimulate the adrenal medulla (of the adrenal gland) to secrete adrenaline
    • Adrenaline is a hormone that will prepare your body for reacting to a stressful situation
    • This reaction is often called the "fight or flight" response
      • It is the effects of adrenaline that lead to the typical symptoms we experience during stressful situations such as increased heart rate, dry mouth, increased sweating etc.

Adrenal gland structure

The adrenal medulla is responsible for releasing the hormone adrenaline into the bloodstream to prepare the body for the "fight or flight" response

  • Since adrenaline is a hormone, it is transported around the body in the bloodstream
  • It will bind to receptors on its target organs
  • One of the targets of adrenaline is the SAN, leading to an increase in the frequency of excitations
    • This in turn, will increase the heart rate to supply blood to the muscle cells at a faster rate
    • More blood means more oxygen and glucose that reaches the muscle cells, which in turn, increases the rate of aerobic respiration
    • This releases more energy that will be used during the response to the stressful or dangerous situation
  • Adrenaline will also stimulate the cardiovascular control centre in the medulla oblongata
    • This increases the impulses travelling along the sympathetic neurones affecting the heart, further speeding up the heart rate
  • Blood vessels to less important organs (such as the digestive system and skin) constrict so that more blood can be diverted to organs that will be involved in the "fight or flight" response
    • Note that blood flow to the brain remains constant, regardless of whether the body is in a state of stress or relaxation
      • The brain is one of the most important organs in the body and needs a constant blood supply in order to function properly
  • The changes experienced by the body during the "fight or flight" response are controlled by a combination of nervous and hormonal responses

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Author: Marlene

Marlene graduated from Stellenbosch University, South Africa, in 2002 with a degree in Biodiversity and Ecology. After completing a PGCE (Postgraduate certificate in education) in 2003 she taught high school Biology for over 10 years at various schools across South Africa before returning to Stellenbosch University in 2014 to obtain an Honours degree in Biological Sciences. With over 16 years of teaching experience, of which the past 3 years were spent teaching IGCSE and A level Biology, Marlene is passionate about Biology and making it more approachable to her students.