Adrenaline is a hormone, and is secreted from the adrenal glands - which are situated just above the kidneys. Whenever you are nervous or excited, your body will secrete this hormone; because your brain sends messages along nerves to the adrenal glands, which tells them to secrete adrenaline into the blood stream.
The function of adrenaline is to prepare the body for something (whatever is making you nervous). The rate of metabolic activity will therefore increase, thus giving you extra energy and muscle power to fight or run. However, sometimes you will get adrenaline when it is impossible to fight or run away - such as an exam which may well make you secrete adrenaline.
Some of the effects of adrenaline are:
- Faster heart beat
- Liver breaks down some of the stored glycogen --> extra glucose is released into the blood
- Arterioles leading to the digestive system constrict, and those leading to the muscles dilate ('butterflies' feeling)
- Bronchioles may widen, so more air can enter/leave the lungs with each breath