The endocrine system is a series of glands that produce and secrete hormones that the body uses for a wide range of functions.Hormones are produced by glands and sent into the bloodstream to the various tissues in the body. They send signals to those tissues to tell them what they are supposed to do. When the glands do not produce the right amount of hormones, diseases develop that can affect many aspects of life.Hormones are agents of biological coordination that circulate systemically to signal diverse cells and tissues, thereby influencing nearly all aspects of the phenotype, including behavior, morphology, physiology, and life history. Hormonal phenotypes can be both heritable and subject to natural selection.