Communication by the endocrine system is mediated by chemical messengers (known as hormones) released from secretory organs (endocrine glands) into the blood stream. These hormones usually travel through the systemic circulation and bind to receptors associated with their target tissues (usually some distance from the endocrine organ). The binding of the hormone to the target tissue receptors mediates the effects. These basic principles are summarised in the figure on the right.
Two important differences between the nervous system and endocrine system should be apparent from this overview:
1. Because of the time it takes for hormone synthesis, secretion, transport through the blood stream, receptor binding and production of effects by the target tissues, the response time of the endocrine system is comparatively slow. Whilst the nervous system can produce responses in a few milliseconds, the endocrine system takes seconds (in the special case of adrenaline secretion by the adrenal medulla) to minutes (and sometimes days) to produce an effect on target tissues.
2. As a consequence of the endocrine system utilising the blood stream as a means of transporting its chemical messengers, virtually all cells in the body are exposed to the hormone. This is very different from the very localised effects that neurotransmitters have when they are released during synaptic transmission. However some specificity is enabled by the endocrine system because not all cells have the receptors for all hormones.
Although most cells in the body are exposed to a particular hormone, only those cells that express the receptor for that hormone are affected by it. This concept is illustrated in the animation on the right.
In the human body there are seven classical endocrine glands. These are true endocrine glands in the sense that they are structurally discrete organs that can be physically identified at the macroscopic level. As a result these were the first endocrine organs to be thoroughly investigated.
As we will see, many classical endocrine glands secrete more than one hormone. However histochemical analysis has revealed that, in most instances, each of these hormones is secreted by a different cell type. Sometimes the cells secreting different hormones are found in different compartments from each other, whilst in others they are distributed throughout the organ mixed together.
The location of the classical endocrine organs along with some basic information on their whereabouts is summarised in the diagram below. You will learn a lot more about these endocrine organs and the hormones that they secrete when we cover with the systems that they control. For now the important information that you need to know about these organs can be found by clicking on the labels for each structure.
It has become increasingly apparent that a number of organs (other than the classical endocrine glands) contain cells that synthesise and secrete hormones into the systemic circulation. These include the brain, the digestive tract, heart and kidney. Clearly these endocrine cells constitute an important part of the endocrine system.