Endocrine glands and hormones

The physiological functions of the human body are regulated and coordinated by two communication systems working together: the nervous system, which signals instantly along nerve fibres, and the endocrine system, which signals more slowly and diffusely through chemicals released into the bloodstream. The word 'hormone' literally means 'to excite', and hormones are chemical messengers released directly into the blood by endocrine glands. Once in circulation, a hormone is carried everywhere in the body, but only cells carrying the matching receptor for it — its target organs or target tissues — actually respond, even though many different hormones come into contact with many different cells. A hormone's action is always one of three kinds: it can speed up, slow down, or otherwise alter the normal activity of its target, and a single hormone can even produce several different effects, whether on one target tissue or across several different ones. …