The Endocrine System


Hormones, which are produced by endocrine glands, are the chemical messengers in the body, targeting various tissues to regulate their functions.

This complex system consists of bodies of glandular tissue, including the thyroid, as well as glands which are located within organs such as the testes, heart and ovaries.  These glands and tissues secrete various hormones which coordinate and help to control certain functions of the body such as the breakdown of chemical substances involved in metabolism, urine production and the balance of fluids, sexual production and development and growth. 

As the hormones make their way through the body’s blood stream, each hormone eventually finds its way to each part of the body.  The specific molecular shape of each hormone only fits into the receptors of its own target organs or tissues, however.

Hormone production takes place in the organs of the endocrine system which include the thyroid gland, the pineal gland, the pituitary gland, the hypothalamus, the thymus gland, the adrenal gland, the heart, the kidneys, the stomach, the intestines, the pancreas and the ovaries or testes.

The chemical data that helps to control the rate that the organs and glands work are carried by hormones.  The cells which produce hormones can be found all around the body.  Many are found in glands which have specific and specialized functions within the body.

The Pituitary Gland

The pituitary gland is also known as the hypothalamus, and is the “master gland” of the human body and the endocrine system in general.  The most influential of the endocrine glands, the pituitary gland is comprised of two separate glands in one. 

The anterior or frontal lobe of the pituitary gland is known as the adenohypophysis, and forms the largest part of the gland.  The posterior lobe is known as the neurohyphysis.  Manufacturing eight major hormones, the anterior pituitary gland then releases its hormones into the blood stream.  The posterior pituitary gland does not produce hormones, but receives two different ones from the hypothalamus, which is found directly above it.  Naurosecretory cells produce these two hormones, while other such cells produce regulatory hormones.  These hormones travel to the anterior lobe via capillaries and control the release of hormones when they arrive.

The Skin

Melanocyte (the stimulating hormone known as MSH) is produced between the two pituitary lobes in a thin layer.  This hormone causes skin tissue to produce melanin pigment, which makes the skin darker in appearance.

The Thyroid Gland

The hypothalamus controls the release of the thyroid-stimulating hormone which is known as TSH.  Affecting the metabolism, TSH causes the thyroid to become more active.

The Adrenal Gland

Triggering the adrenal glands to produce steroid hormones is the adrenocorticotropic hormone.  Steroid hormones control the body’s stress response as well as the body’s use of minerals, proteins, fats and carbohydrates.

Other Glands

Acting on the whole body to promote the manufacture of protein is the growth hormone, or GH.  This hormone also stimulates bone growth and the building of new tissues through the body.  It is especially important for early development in children.

Sex glands produce their own hormones, triggered by luteinizing hormone (LH) and follicle-stimulating hormone (FSH).  These glands also produce the mature sperm cells in men and the ripe egg cells in women.

Vasopressin, also known as antidiuretic hormone (ADH), controls the microfilters, or nephrons, in the kidneys which regulate the water being removed from the blood.

Actions of Hormones

Regulating the function of their target cells, hormones can adjust the rate at which the biochemical reactions in a cell occur.  A variety of hormones can be released, each controlling its own “trigger” mechanism.

The glands in the endocrine system control the release of hormones based on various stimuli to the body.  Sometimes this is the response to levels of certain substances in the human blood stream, based on a feedback “loop.”  Other times, the hypothalamus in the pituitary system acts as an intermediary sensor and releases hormones. 

The pea-sized pineal gland releases melatonin, a sleep hormone, based on darkness.  Activity in the pineal gland is inhibited by the presence of any light detected by the eye.  Nerve impulses are sent to the pineal gland to alert it that light is nearby and to suppress the release of the hormone.

Hormones consist of two distinct types: those made up of steroids and those which are made of protein and amine molecules.  However, both groups function in similar ways, through the biochemical alteration of the production rates of various substances.  They do, however, operate using different mechanisms for release.  Steroid hormones are known to act based on receptors located inside a cell, while protein and amine hormones are regulated by receptor sites on a cell’s surface.

Feedback mechanisms, also known as “loops,” control hormone levels in the body.  This process detects the level of a hormone in the blood stream and passes the information on to a regulating unit, often the hypothalamus-pituitary system.  This control unit reduces the production of hormone if the detected levels are too high; if the levels are too low, stimulation occurs to promote hormone production.

For instance, thyroptin-releasing hormone (TRH) from the hypothalamus stimulates the pituitary gland to create thyroid-stimulating hormone, or TSH.

When the body detects high levels of hormones in the blood stream, negative feedback is sent to the hypothalamus, causing the production of less TRH.  True to typical feedback nature, this event reduces TSH levels and the thyroid gland produces less hormones.

Some hormones in the body have widespread effects, creating hormonal disorders that eventually lead to problems in many areas of the body.  When an individual is diagnosed with an endocrine disorder, it is helpful to know that the prefix “hyper” refers to an excess of hormones, meaning the glands are too active.  The prefix “hypo” implies the opposite, that too little hormone is being produced and the receptors being targeted are suffering from under activity.



  1. Marieb, E.N. & Hoehn, K. Human Anatomy & Physiology (8 edition), Pearson Education, Inc (2004)
  2. McCracken, T. & Walker, R. New atlas of human anatomy, London : Constable (2001)
  3. Parker, S. The Concise Human Body Book, Dorling Kindersley Limited (2009)
  4. Winston, R. et al. Human: The Definitive Visual Guide, DK Publishing, Inc (2004)
  5. Ullmann, H.F. Atlas of Anatomy, Elsevier GmbH, Munich (2009)


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