This happens when your brain detects fear or
stress (dangerous situation, confrontation etc.) and immediately
sends nerve impulses to the adrenal glands
which then secrete the hormone adrenaline into the bloodstream to prepare your
body for action!
The initial stimulation might be
visual, physical or mental.
Note the interaction between the
nervous system (electrical impulses in nerve fibres -
neurones) and the endocrine system (secretion of hormone
molecules into the bloodstream).
There are nerve connections between the
brain and adrenal gland - a part of the adrenal gland called the
adrenal medulla responds to the nerve signal from the brain
(CNS) by releasing the hormone adrenaline.
The secreted adrenaline is carried round
in the blood and acts on various parts of the body.
The effects of adrenaline on the body
are described below,
The surge in adrenaline levels triggers an
increase in blood glucose, heart rate (pulse rate) and breathing rate to increase the supply of
oxygen and glucose for respiration in the cells of your brain and muscles.
The increase in respiration releases more
thermal energy and your body temperature rises - but, if it
becomes too high, the thermoregulatory centre in the brain
detects this and the adrenaline secretion is blocked.
Note that the body's volume of blood is
fairly constant, so heart rate must increases to pump
more blood around the body at a greater rate to carry extra
glucose and oxygen to the muscle cells.
The adrenaline molecules do this by binding
to specific receptors in the heart causing the heart muscles to
contract more frequently and more forcefully - this increases
your heart rate and blood pressure, hence more glucose and oxygen to
your cells through your bloodstream e.g. it gives the cells of the
muscle tissue extra energy to contract and prepare to fight or flee!
Adrenaline also binds to receptors in the
liver causing the cells to increase in the rate of breakdown of glycogen
(chemical potential energy store) to increase the level of
glucose in the bloodstream for respiration - particularly muscle
cells (in limbs or heart).
To increase the rate of respiration you
also need more glucose, so the hormone adrenaline performs two
functions to increase energy output.
Note that the metabolism of glucose is
controlled by three hormones, here its adrenaline acting on
the liver, but there is also the action of
insulin and glucagon in
maintaining the balanced level of glucose in the blood.
Footnote - above is not quite the full
"fight or flight" story - another hormone comes into play too!
When the brain responds to the initial
stimulus and triggers the release of adrenaline, this hormone
from the adrenal gland, cannot alone do everything required in a
'fight or flight' situation.
Simultaneously, the brain also signals the
pituitary gland to release a hormone (name ?) that
acts on a different part of the adrenal gland to release a 2nd
hormone called cortisol, and it this steroid hormone
that sustains our response to danger - most cells in the
body have cortisol receptors.
This is another example of several
hormones jointly controlling a situation.
Also note that it is
the hypothalamus links the
nervous and endocrine systems by way of the
- nervous responses working with hormone responses to keep us