The endocrine system is the body's network of nine glands and over 100 hormones that maintain and regulate numerous events throughout the body. The glands of the endocrine system include the pituitary, thyroid, parathyroids, thymus, pancreas, pineal, adrenals, and ovaries or testes; in addition, the hypothalamus, in the brain, regulates the release of pituitary hormones. Each of these glands secrete hormones (chemical messengers) into the blood stream. Once hormones enter the blood, they travel throughout the body and are detected by receptors that recognize specific hormones. These receptors exist on target cells and organs. Once a target site is bound by a particular hormone, a cascade of cellular events follows that culminates in the physiological response to a particular hormone.

Most endocrine hormones are maintained at specific concentrations in the plasma, the non-cellular, liquid portion of the blood. Receptors at set locations monitor plasma hormonal levels and...