A hormone is a chemical that is produced by a group of specialized cells that are called glands.  Two types of glands exist: exocrine or endocrine.  Exocrine glands use ducts to transport their products.  Endocrine glands don’t use ducts to transport their hormones.  Instead they secrete their hormones into the blood stream. 

     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, and then triggers the release of hormones into the bloodstream.

     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.  One stores hormones manufactured by the hypothalamus and secretes them on command.  The other lobe is able to produce it’s own hormones which act on other glands.

     The thyroid gland is located on the front surface of the trachea.  It has two lobes, and it produces two hormones.  Both hormones have the same effect on the target cell.  The thyroid also regulates metabolism.  The thyroidal secretions are controlled by both the hypothalamus and the pituitary.

     The pancreas secretes insulin, a hormone that regulates glucose in take of the cells.  A deficiency in this hormone results in a type of diabetes.  Insulin regulates the blood sugar levels by stimulating cells to take in glucose.  It also stimulates the synthesis of protein and fat storage.

     The adrenal medulla is the gland behind the fight-or-flight reaction.  It secretes adrenaline.  Adrenaline secretion is triggered by stress, and it increases the rate and the stroke volume of the heart.

     The testis and ovaries secrete hormones that are grouped into three major categories: androgens, estrogens, and progestins.  All three are found in both males and females, but each in varying amounts. 

     Androgens are primarily produces by the testis.  Testosterone is the primary hormone in this group.  Androgens stimulate the maintenance and development of the male reproductive system.

     Estrogens are the female equivalent of Androgens.  They maintain the female reproductive system and trigger the development of female second sex characteristics.  Progestins maintain and prepare the uterus for growth and development of an embryo.