For IGCSE O-Level Biology students Chapter 12

1. Hormones
Our body’s “postman”.

2. Hormones are:
 Chemical messengers
 Produced by endocrine glands
 Released into the blood stream
 Show their effect on certain organs that
we call target organs

4. Adrenaline: Fight – Flight hormone
 This hormone is produced in times of fear
and stress.
 It causes changes that help a person to
survive in times of danger.
 It is produced by adrenal glands present
on top of each kidney.
 The hormone affects many organs in the
body, the heart, lung, eyes, blood vessels,
skin.

5. Effect of adrenaline on
your body
How it helps to save
you
 Eye pupils dilate
 Heart rate increases
 The breathing rate
increases
 Blood vessels in the skin
& gut constrict, while
those in muscles & brain
dilate
 Liver breaks down
stored glycogen to
glucose
 Hairs stand on end
 More light enters the eye to see
clearly
 To pump more blood to the
muscles
 To supply the blood with more
oxygen
 Blood shifts to where it is
needed most
 More glucose is needed for
muscles to produce energy
 This makes the animal seem
larger to scare away what’s
frightening it.