The endocrine system is one of two major regulatory systems that release chemicals in the body (the other one is the nervous system; see below). Hormones, or chemicals made and secreted by endocrine glands or neurosecretory cells, are the main messengers of the endocrine system. Hormones are transported in the blood to all parts of the body and interact with target cells (cells that contain hormone receptors), and they regulate metabolic rate, growth, maturation, and reproduction.