You are going to see in this unit that our bodies have many endocrine glands, each of which typically secretes more than one hormone into the blood.   The hormones they secrete are chemicals.  Therefore, endocrine gland action is to cause chemicals to circulate in our blood.  What can these chemicals do?  How do they work?

    You may already have an image of these chemicals working on tissues of our bodies.  Like you may think that growth hormone works on bone tissue.   But in actuality, any hormone works on the cells within certain tissues.  It is the cells that accomplish the tasks of the tissue.  Right?  To be certain you understand this, think back to the nervous system-- remember, neurons are the cells that fire action potentials and glial cells assist these neurons.  Or, think back to muscle-- muscle fibers (a.k.a., muscle cells) contract... and the contraction of these muscle fibers lead to overall muscle contraction.

    These hormones (chemicals) therefore have to work on the cells of our bodies.  Maybe one hormone, like insulin, will work on the cells of the liver, but not on the cells of the gonads.  Another hormone, like estrogen might work on the cells of the gonads, but not the cells of the liver.  Either way, in order for a hormone to have an effect on any tissue of the body, it has to affect individual cells within that tissue.  The cells that a hormone actually works on are called target cells.

    The next burning question should then be, how do hormones know which cells to work on and which cells to leave alone?

    The answer to this question may be a little unexpected... the hormones do not know where to work.  Instead, the cells that receive the hormone know that they are supposed to receive it!

    Let me try to explain.  Let's step through this process in sequence:

1. An endocrine gland secretes a hormone
2. The hormone travels in the blood
3. The blood circulates throughout the entire body... no preference is given to any one tissue within the body by the blood.  So, any hormone in the blood could visit any cells throughout the entire body.
4. The cells (target cells) that can be affected by this hormone have receptors on (or in) them for this hormone.
5. The hormone binds to the receptor and is removed from general blood circulation to cause its effect on the cell
6. The cell is affected

Receptors are made of protein.  We discussed that a long time ago in chapter 2.   We also revisited the notion of receptors when we discussed "postsynaptic receptors" at the synapse.  Keep in mind that receptors are very specific-- each receptor type binds to only one type of hormone.

    In order to understand this process in any more detail we have to understand what type of hormone we are dealing with.  Is it hydrophobic (like steroids) or hydrophilic?  Depending on your answer, there is a slightly different way that the hormone would work.

    However, in the end, the idea of hormone action is the same.   If a hormone is going to affect a cell, and a cell is made up of chemicals, the way that the hormone must work is to cause chemical reactions to happen.  I describe this in a little more detail at the end of the "Steroid Action" web page.

© 2011 STCC Foundation Press
written by Dawn A. Tamarkin, Ph.D.