The endocrine system controls and regulates body metabolism and other activities by the production of hormones. Hormones are a class of molecules, varying in actual biochemical composition, that act as messenger molecules. Hormones may be steroids, protein peptides, or lipids. Regardless of their unique chemical structure, hormones, are secreted by endocrine glands and in some circumstances, specialized tissues.

Just as there are agonist muscle groups that work in opposition to one another to coordinate movements, a number of hormones exist as pairs of hormones that stimulate opposite responses in target tissues. Antagonistic hormones include such chemicals as glucagons and insulin that act to respectively increase or decrease blood levels of glucose, and thus affect the availability of glucose to cells to use in cellular metabolism.

Endocrine glands release their homes into the blood stream for distribution to target tissues. Target cells have...