The nervous system quickly passes electrochemical messages from different parts of the body along the nerves to the brain and from the brain to the rest of the body, allowing for almost instantaneous responses. By contrast, the endocrine system works slowly. This system is composed of glands that secrete chemical messengers called hormones into the bloodstream and receptor sites on organ cells to receive the messages. The thyroid, pituitary, adrenal gland, testes and ovaries are part of the endocrine system. Each hormone has a unique shape, like a key.
Receptor sites have complimentary shapes, like a lock.
When a hormone contacts a receptor site, the two fit together precisely and the hormone acts as an ignition key for the organ's function. The same hormone-receptor pair can activate many different organs, just as if the same key turned on your automobile and opened your front door. Endocrine disrupters are synthetic or natural chemicals that mimic the shape of hormones enough to either fit into the receptor site and activate the organ or jam the receptor site and prevent the organ's functioning. During critical periods of development, either action could lead to long-term detrimental effects.