The human body is very complex and made up of trillions of cells. It is important that activities of cells, tissues and organs are coordinated in order for the body to function normally. Control andcommunication within the body occurs via the nervous system and the endocrine system. The nervous system acts through nerve impulses and neurotransmitters; the endocrine system produces hormones.The activities of these two systems must be integrated in order to allow for coordinated functioning of the body.
Messages via nerves are conducted very rapidly (in milliseconds) but the duration of the action is very short e.g. the response to a painful prick to a finger is a very rapid withdrawal of the hand. Hormonesgenerally act more slowly and it can take hours or days before a response occurs, but this response can be sustained for a longer time e.g. the hormones that regulate growth or pregnancy can be effective foryears or months