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The Role of Adrenaline in Fight or Flight Response

Updated on July 21, 2017

Have you ever been chased or attacked by a bear? Maybe not. It is like the physical and emotional stress you get when late for a very important appointment—now multiplied that feeling by a thousand.

In the face of danger (any event capable of causing harm), the nervous system and the endocrine system will crank up their gears. The heart rate is increased significantly to raise the supply of energy to the muscles. The body becomes completely focused and ready for action in what experts call a fight or flight response. This is the reaction that gets you out of trouble as soon as possible.

Adrenaline aka Epinephrine is a hormone that plays a key role in the fight or flight response.

Fight or Flight Response

As soon as a threat is perceived, an electrical signal is sent from the brain down to the adrenal glands (little glands located at the upper part of the kidneys). The signal is from the part of the brain called the hypothalamus. The adrenal glands will then secrete the hormone adrenaline (also known as epinephrine) into the bloodstream. This will cause a peak in the concentration of adrenaline in the blood.

The bloodstream circulates adrenaline to different parts of the body where it evokes different effects. Adrenaline causes vasodilation to increase blood supply to the muscles. This focuses the body's energy supply towards the muscles where they are needed the most during such an emergency state.

The increased blood-adrenaline level triggered by the fight or flight reaction and its effects on different parts of the body is also referred to as adrenaline rush.

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BASE jumping from a tower
BASE jumping from a tower | Source

Adrenaline Junkie

The term ‘adrenaline junkie’ is reserved for individual who enjoys dangerous activities and are addicted to the adrenaline rush that comes with such activities. For adrenaline junkies who are involved in extreme sports, ‘normal sports’ just don’t do it for them anymore. Adrenaline junkies tend to push their limits with the craziest and scariest of activities. Just in case you feel like increasing the length of your bucket list, here is a short list of dangerous things to try: hang gliding, freediving, BASE jumping, cliff jumping, ice climbing, free running and bungee jumping.

Effects of Adrenaline in Different Parts of the Body

In the Eye

In the eye, adrenaline binds to adrenoceptors resulting in the contraction of the radial muscle of the iris thereby causing the pupils to become dilated. This will allow more light into the eye and result in brighter and sharper images. This is important because in the presence of a threat, you would want a clear view of your surrounding.

In the Liver

Adrenaline binds to surface receptors of the liver to trigger a pathway inside liver cells. An enzyme called glycogen phosphorylase is released in the liver cells to break down glycogen into individual glucose molecules. This process is known as glycogenolysis and leads to a rise in the blood-sugar level. Glucose molecules are then transported to muscle cells to provide a boost of energy. This is important because glucose can be quickly broken down to produce Adenosine triphosphate (ATP)—energy for cells.

In the Lungs

Adrenaline will bind to receptors on smooth muscle cells of bronchioles causing them to dilate. This relaxes the smooth muscles allows more oxygen to diffuse into the blood. Adrenaline also causes dilation of the arterioles and speeds up the breathing rate. The purpose of increasing the rate of inspiration and expiration is for the body to absorb more oxygen into the blood stream and expel more carbon dioxide.

In the heart

Adrenaline stimulates cells of the heart to beat faster, increasing the heart rate. During a fight or flight reaction, it is important for oxygen, glucose, hormones and other chemicals to be able to circulate much faster throughout the body to cells that need them.

In the Skin

Adrenaline binds to receptors on the smooth muscle cells in the skin causing them to contract. This is why the hair on the surface of your skin rises when you are under such a physical and emotional stress. Adrenaline also binds to receptor that causes the contraction of sweat glands, resulting to beads of sweat.

In the Digestive System

Adrenaline causes vasoconstriction to the blood supply of the digestive system. This shuts down supply to the digestive system, to give priority to musculoskeletal system. Digesting the burger is not quite a priority during such an emergency situation.

Adrenaline is an efficient messenger with an important role in the fight or flight response. It signals different parts of the body and causes different reactions throughout the body. It allows us to respond long enough to potentially get out of danger, either fighting or fleeing.


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    • profile image


      4 years ago

      Thanks greatly you really aid us

    • thebiologyofleah profile image

      Leah Kennedy-Jangraw 

      7 years ago from Massachusetts

      Great summary of Adrenaline actions in Fight or Flight- thanks for sharing. Wonder if there is anything to adrenaline junkies having attentuated effects of Adrenaline overtime and so they seek out bigger and scarier things in order to continue to feel the same rush?

      Interesting stuff-thanks for sharing!


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