Endocrinology is the study of the endocrine system and its diseases. The endocrine system includes hormones (chemical mediators) and the organs/cells which secrete them. Endocrinology includes the study of the biosynthesis, storage, chemistry, and physiological function of hormones and the tissues that secrete them. The endocrine system consists of different parts of the body that secrete hormones directly into the blood rather than into a duct system. Hormones have many different functions and modes of action. They may act locally or away from their site of origin. They often interact with other biological systems.