A hormone is a chemical that is made by specialist cells, usually within an endocrine gland, and it is released into the bloodstream to send a message to another part of the body. It is often referred to as a ‘chemical messenger’. Hormones are found in all multicellular organisms and their role is to provide an internal communication system between cells located in distant parts of the body.
In the human body, hormones are used for two types of communication. The first is for communication between two endocrine glands, where one gland releases a hormone which stimulates another target gland to change the levels of hormones that it is releasing. The second is between an endocrine gland and a target organ, for example when the pancreas releases insulin which causes muscle and fat cells to take up glucose from the bloodstream.
Since hormones are released into the bloodstream and can therefore be carried around the entire body, they can perform both of these actions on many different targets. The complex interplay between the glands, hormones and other target organs is referred to as the endocrine system. Hormones affect many physiological activities including growth, metabolism, appetite, puberty and fertility.