Technical Discussion

Endocrinology is the study of chemical communication systems that provide the means to control a huge number of physiologic processes. Like other communication networks, endocrine systems contain transmitters, signals and receivers that are called, respectively, hormone producing cells, hormones and receptors.

The first step in understanding endocrinology is to explore the meaning of such terms as hormone, receptor and target cell, and to obtain an understanding of how chemical communication is controlled.

Lay Discussion

Endocrinology is the study of the chemical command & control system of the body.

To understand this amazing and intricate system, we first need to explore a bit of the endocrine system and understand a few of its terms.

Overview of Endocrinology

How is it that humans and animals maintain quite constant blood concentrations of glucose throughout their lives despite wildly varying frequencies of meals? If your blood glucose concentration drops much below 1 mg per ml, your neurons will begin to misbehave, leading ultimately to coma and death. Yet skipping breakfast is rarely life-threatening.

The answer is that a battery of chemical messengers - hormones - are secreted into blood in response to rises and falls in blood glucose concentration and stimulate metabolic pathways that pull glucose concentrations back into the normal range.

It is through hormones, chemical messengers, that our bodies maintain balance.

Some levels are so critical (like blood sugar) that even a very minor disturbance to them can radically alter mental function.

Hormones are secreted into the blood stream by different glands and tissues, regulating and responding to all of the chemical processes and changes within us.

Two systems control all physiologic processes:

The nervous system exerts point-to-point control through nerves, similar to sending messages by conventional telephone. Nervous control is electrical in nature and fast. (right)

The endocrine system broadcasts its hormonal messages to essentially all cells by secretion into blood and extracellular fluid. Like a radio broadcast, it requires a receiver to get the message - in the case of endocrine messages, cells must bear a receptor for the hormone being broadcast in order to respond.  (left)

As will be repeatedly demonstrated, the nervous and endocrine systems often act together to regulate physiology. Indeed, some neurons function as endocrine cells.

Two systems of the body controls all of the functions within the body:

Nerves - direct communication between the brain and muscles, glands and other tissues via nerves.

Endocrine - chemical messages distributed in the majority by the blood stream.

Only cells with 'receptor sites' specific to that hormone will respond directly to that hormone. 

An example of this is a radio signal.  Whilst all frequencies are available, you can ask it to respond only to one frequency by tuning the receiver of the radio to a specific station.

Endocrinology is the study of hormones, their receptors and the intracellular signalling pathways they invoke. Distinct endocrine organs are scattered throughout the body. These are organs that are largely or at least famously devoted to secretion of hormones.

The study of the chemically based command and control systems of the body and their associated tissues

In addition to the classical endocrine organs, many other cells in the body secrete hormones. Myocytes in the atria of the heart and scattered epithelial cells in the stomach and small intestine are examples of what is sometimes called the "diffuse" endocrine system. If the term hormone is defined broadly to include all secreted chemical messengers, then virtually all cells can be considered part of the endocrine system.

Not all hormones are produced just by glands, some specialised tissues through the body produce hormones both to regulate other functions and provide feedback on their own performance.

A final introductory comment is warranted. Pursuit of an understanding of endocrinology is complicated by several of its principles:

• All pathophysiologic events are influenced by the endocrine milieu: There are no cell types, organs or processes that are not influenced - often profoundly - by hormone signalling.

Some base thoughts:

There are no cells, organs or body processes that are not influenced or controlled by hormones.

• All "large" physiologic effects are mediated by multiple hormones acting in concert: Normal growth from birth to adulthood, for example, is surely dependent on growth hormone, but thyroid hormones, insulin-like growth factor-1, glucocorticoids and several other hormones are also critically involved in this process.

For any hormonal effect to be appropriate, a balance of many hormones is required.  Even things like growth are not the sole domain of hGH, but include many other major influences; in many ways governed by that same hGH.

• There are many hormones known and little doubt that others remain to be discovered.