The Endocrine System and
Both the nervous and endocrine systems were designed for communications. The nervous systems rely on the principles of conduction and transmission using electrical and chemical signals associated with individual neurons. Most commonly, these neurons are arranged in series whether they be on the sensory or motor sides of the central nervous system. They are physical structures (biological current-conducting cables) in contact with other physical structures (other neurons and activators such as skeletal muscle cells). Parallel arrangements of neurons and their effectors, however, are not uncommon.
The endocrine system is arranged differently. Historically endocrinology refers to selected organs called glands, for example, the thyroid gland or the adrenal gland, that produce hormones that are released into the circulation and carried to distant targets (cells, tissues, organs) to produce physiologic actions. The details of any single component of the endocrine system have been the topics of dozens of textbooks. In today's world of modern molecular biology and with the continuing revolution in the life sciences, endocrinology is much more than its classic definition. Today's student must also understand the concepts of paracrines and autocrines in addition to the classic concepts of endocrines.
An endocrine hormone is one that gets released from its mother cell into the blood; blood becomes the vehicle of transport for that hormone. Those hormones acting as autocrines and paracrines get released into the interstitial spaces and must rely on mechanisms other than the circulation for transport to their target tissues (for example, diffusion).
An exocrine gland is one that releases its products into tubules and ducts (for example, the acinar cells of the salivary and pancreatic glands). Consider the pancreas gland. It has both endocrine and exocrine functions. It synthesizes and releases the endocrine hormones insulin and glucagon. These are transported by