Final answer:
Hormones are chemical messengers secreted into the bloodstream by glands of the endocrine system, acting on target cells across the body at low concentrations. They differ from neurotransmitters in range and duration of effect, with an example being insulin regulating blood glucose levels.
Step-by-step explanation:
For a chemical to be defined as a hormone, it must be a chemical messenger molecule that is secreted by a gland of the endocrine system into the bloodstream for transport. These hormones travel through the body to reach target cells where they bind to specific receptors and elicit a response, even at low concentrations due to their diluted nature in the blood.
Hormones like endocrine hormones differ from neurotransmitters in that they have a widespread effect, are slower to take effect, and tend to last longer. The release of a hormone is typically in response to a physiological stimulus. An example is insulin, which is secreted by the pancreas and acts on various cells throughout the body to regulate blood glucose levels.
Different hormones have various transport mechanisms. For example, lipid-derived (soluble) hormones, such as steroid hormones, diffuse across the membranes of the endocrine cell and often bind to transport proteins in the bloodstream, which maintain their solubility until they reach their target cells.