Hormone

A chemical messenger that signals to specific cells to change their behavior in specific ways. Hormones are secreted into the bloodstream through endocrine glands, part of the endocrine system. Ensuing are a few significant examples:

Oestrogen from the ovaries, which induces developement of secondary female characteristics during puberty and development of uterine lining follwing menstruation. Testosterone (secreted from testes) induces secondary male characteristics during puberty. Adrenaline from the adrenal glands prepares body for physical activity (often known as the "fight or flight" hormone). ADH from the pituitary gland, which makes the walls of collecting ducts in kidneys more permeable to water to decrease concentration of blood (also known as the Sanath hormone).