Unit 8-Endocrine System

Chapter 24

The Endocrine System in General

1. General Principles:

     All the organs of the body are linked to each other by the blood stream, as well as by the central nervous system. This linking by way of the blood stream makes it theoretically possible for any organ to influence the activity of any other organ by way of chemical substances released into the blood by the first organ. Although it is theoretically possible that any organ can send chemical messages to any other, the number which do so is quite small, and the number of specific substances which carry chemical information is also limited.

     The organs which produce chemical messengers are called the endocrine glands. Emptying their secretions directly into the blood stream, they require no duct and they are also called the ductless glands. The secretion which enters the blood stream is called a hormone, which in the blood to the organ which is to be activated. The target organ may be a single organ or a group of organs, but each target organ responds to a hormone in its own characteristic way.

     Some examples of hormone action mentioned earlier will be given here to show the nature of the endocrine response.

     Using these examples, we can discuss some general principles which apply quite regularly in the endocrine system:

     Many of the properties of hormones are illustrated by a toy with which some readers may be familiar. This toy is a black box with only a switch in front--that is all.

     If the switch is turned on, the top of the box opens and a little hand comes out. The hand reaches for the switch, turns it off, and withdraws. The lid closes and the box remains as before. The student may find the analogy depressing, since the toy seems to serve no useful purpose at all. On the other hand, if one considers that the off portion of the switch is the best one; and that many such "off" switches exist in the body, the purpose becomes evident. Most parts of the organism seem to function best when they begin from the inactive state. Stimuli-producing changes cause deviation from the inactive state. The endocrine system, among other systems, restores the initial condition.

2. Classification of Hormones:

     Consideration of hormones will be assisted by the following system of functional classification.

     The first five groups will be considered in Chapter 25, and the sex hormones will be considered in Chapter 26.

3. The Principal Endocrine Glands:

     Most hormones are derived from a few organs which will be described briefly.

     The location of these glands is shown in Figure 338. Figures 339 to 347 show the gross and microscopic structures of each gland.

4. Types of Endocrine Disorders:

     In general, the regulation of hormonal activity is so good that the hormone is produced in precisely the right quantities. This is partly due to the property of most endocrine systems of being "turned on" when needed and "turned off" when they have done the job.

     This regulating system is not always perfect, however, and sometimes the signal for hormone production is not well detected; sometimes it is too well detected. The same applies to the signals for secretion of hormone production. Thus various types of under-production and over-production of hormones may be observed. When the hormone in question is under produced, the prefix hypo- is used together with the name of the hormone (or sometimes the endocrine gland) concerned. Thus we may speak of hypothyroidism or hypo-insulinism. Conversely, an overactive endocrine system is described with the prefix hyper-. Too much thyroid activity is hyperthyroidism, while too much much insulin production is hyperinsulinism. Examples of these types of disorders will be given in the next two chapters.

Continue to Chapter 25.