Our Marvelous Bodies: An Introduction to the Physiology of Human Health

3. The Endocrine System and Physiological Communication

35 3 Advances in Endocrinology 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 The Endocrine System and Physiological Communication the blood to distant tissues like fat cells, muscles, and the liver. Among pancreatic exocrine functions are the synthesis, storage, and secretion of digestive enzymes and bicarbonate ions. These are released into pancreatic ducts (tubules), then pass through the common bile duct, and are finally delivered into the duodenum of the small intestine. In the duodenum, the enzymes participate in the breakdown and uptake of fats and proteins and the bicarbonate ions help buffer acids emptied from the stomach into the small intestine. We have learned in recent decades that a product released from one cell can have an effect on the behavior of an adjacent cell (a paracrine function, see figure .). Moreover, that same product might evoke a physiological or biochemical response on the same cell in which it was synthesized and released (an autocrine function). These concepts were not known in the late nineteenth and early twentieth centuries when the fields of endocrinology and neuroendocrinology were beginning to develop. In fact, though I began to study endocrinology in the late s and early s, I did not become familiar with these terms until the mid s, when I was already into my academic career as a professor of physiology. Paracrines and autocrines often reach their target cells by the principles of simple diffusion. This means the differences in their concentrations in two locations, the distance between the two locations, and the random movements of the molecules (thermodynamics), among other considerations, determine their movements between the two points. The average student probably thinks of endocrinology as the functions of the pituitary, adrenal, thyroid, and pancreas glands. However in recent decades, OUR MARVELOUS BODIES 36 Endocrine gland Bloodstream Target organ Panel A Panel B Panel C Endocrine Paracrine Autocrine cell A cell B cell A FIGURE 3.1 Contrasting mechanisms by which a hormone can be released, transported, and can act on a target cell. In panel A, the target cell is distant to the endocrine gland (endocrine function). In panel B, both cells A and B reside in the same gland (paracrine function). In panel C, the cell that releases the hormone might itself be the target tissue (autocrine function). the field has developed and shown that the endocrine system is much broader, more complex, and more dynamic than this. We now know of organs, tissues, and cells that have endocrine-like properties but were formerly not thought of as dedicated endocrine organs. Prior to the second half of the twentieth century, physiologists used the word neurohumoral to refer to chemicals that are released either as neurotransmitters or from...