THE ENDOCRINE SYSTEM
The endocrine system is made up of glands that produce and secrete hormones,
chemical substances produced in the body that regulate the activity of cells or organs. These
hormones regulate the body's growth, metabolism (the physical and chemical processes of the
body), and sexual development and function. The hormones are released into the bloodstream
and may affect one or several organs throughout the body.
Hormones are chemical messengers created by the body. They transfer information from one
set of cells to another to coordinate the functions of different parts of the body.
The major glands of the endocrine system are the hypothalamus, pituitary, thyroid,
parathyroids, adrenals, pineal body, and the reproductive organs (ovaries and testes). The
pancreas is also a part of this system; it has a role in hormone production as well as in
digestion.
HYPOTHALAMUS
The hypothalamus is located in the lower central part of the brain. This part of the brain is
important in regulation of satiety, metabolism, and body temperature. In addition, it secretes
hormones that stimulate or suppress the release of hormones in the pituitary gland. Many of
these hormones are releasing hormones, which are secreted into an artery (the hypophyseal
portal system) that carries them directly to the pituitary gland. In the pituitary gland, these
releasing hormones signal secretion of stimulating hormones. The hypothalamus also secretes
a hormone called somatostatin, which causes the pituitary gland to stop the release of growth
hormone.
PITUITARY GLAND
The pituitary gland is located at the base of the brain beneath the hypothalamus and is no
larger than a pea. It is often considered the most important part of the endocrine system
because it produces hormones that control many functions of other endocrine glands. When
the pituitary gland does not produce one or more of its hormones or not enough of them, it is
called hypopituitarism.
The pituitary gland is divided into two parts: the anterior lobe and the posterior lobe. The
anterior lobe produces the following hormones, which are regulated by the hypothalamus:
Growth hormone: Stimulates growth of bone and tissue (Growth hormone
deficiency results in growth failure. Growth hormone deficiency in adults results in
problems in maintaining proper amounts of body fat and muscle and bone mass. It is
also involved in emotional well-being.)
Thyroid-stimulating hormone (TSH): Stimulates the thyroid gland to produce
thyroid hormones (A lack of thyroid hormones either because of a defect in the
pituitary or the thyroid itself is called hypothyroidism.)
Adrenocorticotropin hormone (ACTH): Stimulates the adrenal gland to produce
several related steroid hormones
Luteinizing hormone (LH) and follicle-stimulating hormone (FSH): Hormones
that control sexual function and production of the sex steroids, estrogen
and progesterone in females or testosterone in males
Prolactin: Hormone that stimulates milk production in females
The posterior lobe produces the following hormones, which are not regulated by the
hypothalamus:
Antidiuretic hormone (vasopressin): Controls water loss by the kidneys
Oxytocin: Contracts the uterus during childbirth and stimulates milk production
The hormones secreted by the posterior pituitary are actually produced in the brain and
carried to the pituitary gland through nerves. They are stored in the pituitary gland.
