Endocrinology is the study of the system of glands which secrete hormones, chemical substances that regulate the activity of cells or organs.
Endocrine hormones are secreted into the bloodstream and can affect a wide range of functions among which sexuality, metabolism, development and sleep.
The endocrine system is made up of the pituitary gland, thyroid gland, parathyroid glands, adrenal glands, pancreas, ovaries and testicles. Additional, other organs such as the kidney, liver, and the heart have secondary endocrine functions. Each type of hormones targets a specific set of organs or tissues.
Feedback plays a major role in the regulation of the endocrine system. For example, through the production of releasing hormones and stimulating hormones, the hypothalamus and the pituitary gland can affect the secretion of hormones by a specific set of target glands. This regulation results in stable blood concentrations of the targeted hormones.