Hormones: Adrenaline


Image of an eye showing dilated pupil.

Adrenaline is a hormone released from the adrenal glands and its major action, together with noradrenaline, is to prepare the body for fight or flight.

Alternative names for adrenaline

Epinephrine.

                              Image of an eye showing a dilated or enlarged pupil - one of the effects of adrenaline released during a "fight or flight" response.

What is adrenaline?       

Adrenaline and noradrenline are two separate but related hormones and neurotransmitters. They are produced in the medulla of the adrenal glands and in some neurons of the central nervous system. They are released into the bloodstream and serve as chemical mediators, and also convey the nerve impulses to various organs. Adrenaline has many different actions depending on the type of cells it is acting upon.  However, the overall effect of adrenaline is to prepare the body for the ‘fight or flight’ response in times of stress, ie, for vigorous and/or sudden action. Key actions of adrenaline include increasing the heart rate, increasing blood pressure, expanding the air passages of the lungs, enlarging the pupil in the eye (see figure), redistributing blood to the muscles and altering the body’s metabolism, so as to maximise blood glucose levels (primarily for the brain). A closely related hormone, noradrenaline, is released mainly from the nerve endings of the sympathetic nervous system (as well as in relatively small amounts from the adrenal medulla). There is a continuous low level of activity of the sympathetic nervous system resulting in release of noradrenaline into the circulation, but adrenaline release is only increased at times of acute stress.

Adrenaline, like noradrenaline, is also used as a transmitter by some nerve cells to communicate with other cells (a neurotransmitter). Very little adrenaline is used in this way.

How is adrenaline controlled?

Adrenaline is mainly released in response to stressful events to prepare the body for the ‘fight or flight’ response. These events lead to the activation of nerves connected to the adrenal glands, which trigger the secretion of adrenaline and thus increase the levels of adrenaline in the blood. This process happens relatively quickly, within 2 to 3 minutes of the stressful event being encountered. When the stressful situation ends, the nerve impulses to the adrenal glands are lowered, meaning that the adrenal glands stop producing adrenaline.

Stress also stimulates the release of adrenocorticotropic hormone from the pituitary gland, which promotes the production of the steroid hormone cortisol from the cortex of the adrenal glands. This steroid hormone is more important in altering the body’s metabolism (ie, raising plasma glucose) under conditions of longer-term, ongoing (chronic), rather than acute, stress.

What happens if I have too much adrenaline?

Overproduction of adrenaline is rare. Too much adrenaline can be caused by a variety of things, including a rare tumour of the adrenal medulla (a phaeochromocytoma). Symptoms may include rapid heart beat, high blood pressure, anxiety, weight loss, excessive sweating and palpitations.

What happens if I have too little adrenaline?

Suffering from too little adrenaline is very unusual. Among other things, it would result in an inability to prepare the body for action in response to a stressful or physically demanding situation.



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