Hormones are regulatory molecules secreted into the blood by endocrine glands. Chemical categories of hormones include steroids, amines, polypeptides, and glycoproteins. Interactions between the various hormones produce effects that may be synergistic, permissive, or antagonistic.
Endocrine glands lack the ducts that are present in exocrine glands (chapter 1). The endocrine glands secrete their products, which are biologically active molecules called hormones, into the blood. The blood carries the hormones to target cells that contain specific receptor proteins for the hormones, and which therefore can respond in a specific fashion to them. Many endocrine glands are discrete organs (fig. 11.1a) whose primary functions are the production and secretion of hormones. The pancreas functions as both an exocrine and an endocrine gland; the endocrine portion of the pancreas is composed of clusters of cells called the pancreatic islets (islets of Langerhans) (fig. 11.1ft). The concept of the endocrine system, however, must be extended beyond these organs. In recent years, it has been discovered that many other organs in the body secrete hormones. When these hormones can be demonstrated to have significant physiological functions, the organs that produce them may be categorized as endocrine glands, although they serve other functions as well. It is appropriate, then, that a partial list of the endocrine glands (table 11.1) should include the heart, liver, adipose tissue, and kidneys.
Some specialized neurons, particularly in the hypothalamus, secrete chemical messengers into the blood rather than into a narrow synaptic cleft. In these cases, the chemical that the neurons secrete is sometimes called a neurohormone. In addition, a number of chemicals—norepinephrine, for example—are secreted
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both as a neurotransmitter and a hormone. Thus, a sharp distinction between the nervous and endocrine systems cannot always be drawn on the basis of the chemicals they release.
Hormones affect the metabolism of their target organs and, by this means, help to regulate total body metabolism, growth, and reproduction. The effects of hormones on body metabolism and growth are discussed in chapter 19; the regulation of reproductive functions by hormones is considered in chapter 20.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.