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The Endocrine System


                      You are probably very familiar with your skeletal system    or your muscular system. You can feel both your bones and your muscles and you understand a lot about them.

   But, are you as familiar with the endocrine system? Your endocrine system helps you control critical processes in your body. It does this through the release of chemicals through ductless glands.


Chemicals secreted by these glands have effects on many other organs of multiple systems and control processes of every other body system.


Read below to find out more about this important system.


Learning Objectives

By the end of this lesson, students should be able to:

  • Explain the function of the endocrine system.

  • Compare and contrast the control mechanisms of the nervous and endocrine systems.

  • Describe the negative feedback mechanism.

  • List the major glands of the endocrine system and the function of each.



adrenal gland—one of a pair of glands which produce hormones that help control heart rate, breathing and other metabolic activities.


endocrine gland—any gland that secrete hormones directly into the blood; a ductless gland.

hormone—secretions from endocrine glands that control certain body activities.


ovary—one of two glands in females that produce primary and secondary sex characteristics; produces eggs.


pancreas—an endocrine gland that secretes insulin which regulates blood sugar.


pituitary gland—a small gland at the base of the brain which produces hormones which have an effect all other hormonal functions in the body.


target tissue—tissue affected by hormones.


testes—glands in males that produce primary and secondary sex characteristics; produces sperm.


threshold amount—the level of a hormone in the blood that is necessary for a response in a target tissue to occur.


thymus—a ductless gland that helps in the production of T blood cells, important in the immune system.


thyroid gland—a gland at the base of the neck that secretes hormones that regulate growth and development.
















How the Endocrine System Works

   While the nervous and endocrine systems are both control systems, there are important differences. The nervous system is one of motor control; the endocrine system controls through chemicals. The nervous system is fast acting. It doesn’t take you very long to sense a stimuli and react to it; you pull your hand away from a hot pan very quickly! .


   The endocrine system, however, responds much more slowly. The level of chemicals released by a gland has to reach a certain level in the blood, called the threshold amount, for a response to occur. Once the level is reached, the response can occur, and it is long-lasting. 


   The response will continue until the level of the chemical decreases enough to stop the response. This mechanism is called the negative feedback mechanism because when the level decreases, the feedback stops the effects.

   Endocrine glands secrete chemicals, called hormones, directly into the blood. They do not have ducts that lead to the tissues they control.


   The endocrine system and its chemicals (hormones) help regulate many important metabolic processes. Hormones control chemical reactions and help regulate water balance, growth and development, reproductive cycles, heart rate, blood pressure, appetite, sleeping and waking cycles and digestive processes.

   Some endocrine glands are so

complex that certain sections of the

gland produce different hormones. For

example, the anterior lobe (section) of

the pituitary produces six different

hormones that regulate vital body

functions. The posterior lobe produces

different hormones—and the pituitary

gland is about the size of a pea!



Fast Fact: All body functions are influenced by the endocrine glands; there are no processes that are NOT affected by hormone signaling. Many functions are controlled by multiple hormones working together.



Major Endocrine Glands

1) Pituitary gland

       At the base of the brain

       Helps regulate growth
       Regulates other endocrine glands to produce hormones

2) Pineal gland

       In between the cerebral (section of the brain) hemispheres

       Produces melatonin which regulates sleep cycles

3) Thyroid gland

       On either side and in front of the trachea; has two lobes connected by an isthmus.
         Helps regulate digestion
       Regulates release of energy
       Stimulates breakdown of fats
       Essential for normal growth and development


4) Adrenal gland

       Two-one on top of each kidney

       Increases heart rate
       Increases breathing rate
       Increases blood pressure
       Helps maintain blood pressure


5) Parathyoid glands

       Two-one of top of each lobe of the thyroid gland

       Increases blood calcium
       Affects bones, kidneys and intestines


6) Thymus

       Behind the sternum (breastbone); large in young children, but shrinks with age

       Affects production of white blood cells (important immune function)


7) Ovaries

       In females, two ovaries, one on either side in the pelvic cavity

       Controls development of female primary and secondary sex characteristics

       Produces ova (eggs)

8) Testes

       In males, two testes, which lie outside the body cavity

       Controls development of male primary and secondary sex characteristics

       Produces sperm

 Check for Understanding:

1. Describe the general function of the endocrine system.


2. Compare and contrast the two control systems: the endocrine and the nervous system.


3. Explain the negative feedback system of the endocrine system.





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diagram showing glands of the endocrine system on an outline of the human body
diagram of the location of teh pituitary and hypothymus glands on a cross section of a human head
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