hormone

In biology, a chemical secretion of the ductless endocrine glands and specialized nerve cells concerned with control of body functions. Hormones act as chemical messengers and are transported to all parts of the body by the bloodstream where they affect target organs. The major glands are the thyroid, parathyroid, pituitary, adrenal, pancreas, ovary, and testis. There are also hormone-secreting cells in the kidney, liver, gastrointestinal tract, thymus (in the neck), pineal (in the brain), and placenta. Hormones bring about changes in the functions of various organs according to the body's requirements. The hypothalamus, which adjoins the pituitary gland at the base of the brain, is a control centre for overall coordination of hormone secretion; the thyroid hormones determine the rate of general body chemistry; the adrenal hormones prepare the organism during stress for ‘fight or flight’; and the sexual hormones such as oestrogen and testosterone govern reproductive functions. Plants produce chemicals that affect growth and development. These chemicals can also be called hormones (see plant hormone).

Hormones often bring about a slower response than messages sent by the nerve cells, but the response to hormones may be longer lasting.

Examples of hormones include: insulin and glucagon (produced in the pancreas), which are involved in glucose regulation; ADH (produced in the pituitary gland), which regulates the concentration of urine produced by the kidney; oestrogen and progesterone (produced in the ovaries), which regulate the growth and functioning of sex organs for sexual reproduction; testosterone (produced in the testes), which regulates male sexual development; follicle-stimulating hormone (FSH) and luteinizing hormone (LH) (from the pituitary gland), which regulate the menstrual cycle in females; and adrenaline (released from the adrenal glands), which prepares the body for increased activity.