Hormones are chemical messengers that are secreted by endocrine glands and transported through the bloodstream to target cells or organs, where they produce specific physiological responses. Hormones can affect a wide range of physiological processes, including growth and development, metabolism, reproduction, and behavior. Examples of hormones include insulin, which regulates glucose metabolism, and testosterone, which is involved in the development of male secondary sexual characteristics. Hormones are typically produced in response to specific stimuli, such as changes in blood glucose levels or the presence of a particular chemical or hormone in the body.