Steroids belong to a class of naturally occurring organic substances and their derivatives, important in biology, chemistry, and medicine. Theses compounds includes all the sex hormones, adrenal cortical hormones, bile acids, and sterols of vertebrates, the molting hormones of insects, and many other physiologically active substances of animals and plants. Synthetic steroids of therapeutic value include a large number of anti-inflammatory agents, anabolic (growth-stimulating) agents, and oral contraceptives. Different categories of steroids are often distinguished from one another by names that relate to their biological source—e.g., phytosterols (found in plants), adrenal steroids, and bile acids—or to some important physiological function—e.g., gestogens (promoting gestation), androgens (promoting development of masculine characteristics), and cardiotonic steroids (facilitating proper heart function). Steroids of similar function and origin tend to fall into families of closely related structure. Biological categorization therefore underlies an internationally accepted systematic chemical nomenclature. Steroids are characterized by a molecular structure of 17 carbon atoms arranged in four rings. In the parent structure (named gonane and referred to as the steroid nucleus), the carbon atoms are bonded to 28 hydrogen atoms.