The endocrine system

Endoctrine system - One of the body's major information systems that instructs glands to release hormones directly into the bloodstream. These hormones are carried towards target organs in the body.

Gland - An organ in the body that synthesises substances such as hormones.

Hormones - Chemical substances that circulate in the bloodstream and only affect target organs. They are produced in large quantaties but dissapear quickly. Their effects are very powerful.

The major glands of the endocrine system are the pituitary galnd, adrenal glands and the reproductive organs. 

An example of how the endoctrine system works -  A signal is sent from the hypothalamus ( a region within the brain ) to the pituitary gland in the form of releasing a hormone.

This causes the pituitary gland to secrete a stimulating hormone into the bloodstream.

This hormone signals the target gland (e.g. adrenal gland) to secrete it's hormone.

As levels of this hormone rise in the bloodstream, the hypothalamus shuts down secretion of the stimulating hormone. This slows down the seceration of the target glands hormone resulting in the stable concentrations of hormones circulating in the bloodstream.

Pituitary gland - The master gland whose primary function is to influence the release of hormones from other glands. 

The adrenal glands - the first part of the adrenal gland - the adrenal cortex - produces cortisol which regulates a varity of bodily functions it is produced in response to stress.

The second part is called the adrenal medulla which releases adrenaline which helps the bodyrespond to a stressful situation.

Ovaries - produce the hormones oestrogen and progesterone.

Testes - produce the hormone testostrone.