The endocrine system

The endocrine system is made of glands that secrete hormones directly into the bloodstream.
The blood carries the hormone to a target organ, where it produces an effect.
Compared with the nervous system, hormonal effects are slower but act for longer.

The main glands

The pituitary gland in the brain is a 'master gland': it secretes several hormones that make other glands release hormones.
The thyroid gland makes thyroxine; the adrenal glands make adrenaline.
The pancreas makes insulin (and glucagon). The ovaries make oestrogen and progesterone; the testes make testosterone.

Adrenaline is produced by the adrenal glands in times of fear or stress.
It increases heart rate and boosts the delivery of oxygen and glucose to the brain and muscles, preparing the body for 'fight or flight'.

Thyroxine from the thyroid gland stimulates the basal metabolic rate. It is important for growth and development.
Thyroxine levels are controlled by negative feedback: when the level is too high, the pituitary releases less TSH, so less thyroxine is made; when it is too low, more is made.
Negative feedback keeps a level steady by reversing any change.

Endocrine system: The system of glands that secrete hormones into the blood.
Hormone: A chemical messenger carried in the blood to a target organ.
Gland: An organ that secretes a hormone or other substance.
Target organ: The organ that a hormone affects.
Pituitary gland: The 'master gland' in the brain that controls other glands.
Adrenaline: A hormone from the adrenal glands that prepares the body for 'fight or flight'.
Thyroxine: A hormone from the thyroid gland that controls the basal metabolic rate.
Negative feedback: A control mechanism that reverses a change to keep a level steady.
Metabolic rate: The speed at which chemical reactions happen in the body.