A hormone is a chemical that is synthesized and secreted by a group of specialized cells that are called glands.  Glands are classified in two ways: exocrine or endocrine.  Exocrine glands use ducts to transport their secretions.  Endocrine glands, on the other hand, don’t use ducts to transport their hormones.  Instead they secrete into the blood stream for distribution. 

     Another key component of the endocrine system is the receptors for the hormones on the surface of the cell membrane.  Receptors are composed of proteins and imbedded in the cell membrane just like other proteins. 

     There are over 50 hormones in the human body, and they can be grouped together by their chemical structure. Steroids are produced from cholesterol.  Peptides are chains of amino acids.  Other hormones are derived from amino acids. 

     Several glands comprise the endocrine system.  The hypothalamus integrates the endocrine system and the nervous system.  It receives signals from the brain and the peripheral nerves.  It then initiates the endocrine system.  The cells that comprise the hypothalamus are called nuerosecretory cells.  The cells are able to receive a nerve impulse just like a nerve cell would, but instead of passing it on to another cell they trigger the release of hormones in the blood stream.

     The pituitary is sometimes referred to as the master gland.  However, most of the time the pituitary receives orders from the hypothalamus.  There are two lobes on the pituitary.  The posterior lobe, called the neurhypophysis, stores hormones manufactured by the hypothalamus and secretes them on command.  The anterior lobe, called the adenohypophysis, is able to produce it’s own hormones which act on other glands.