The complex interplay between the glands, hormones and other target organs is referred to as the endocrine system. The endocrine system is one of two main communication systems in the body together with the nervous system. Endocrine glands are small organs that make and release hormones directly into the bloodstream. At its simplest a hormone is a chemical messenger from one cell or group of cells to another.  Hormones are released (secreted) into the bloodstream and have an effect on other parts of the body. Hormones are found in all organisms with more than one cell, and so they are found in plants and animals.  They influence or control a wide range of physiological activities such as growth, development, puberty, regulating sugar levels, bone growth and appetite.