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In previous chapters of this book, we have discussed the structure and function of the cells, tissues, and organs that comprise the human body. Two organ systems have evolved that integrate and coordinate the activities of these components—the nervous system and the endocrine system. The key activity of both systems is signaling. Signaling by the nervous system is rapid and specifically targeted, whereas signaling by the endocrine system is usually slower and more wide ranging. The activities of the two systems are closely linked and complementary.
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The endocrine system consists of all the cells, tissues, and organs that secrete signaling molecules called hormones, a diverse array of peptides, lipids, and amino acid derivatives. Hormones are released into the circulation, bind to receptors on or in target cells, and function to regulate their activity. Hormone secretion itself is highly regulated by various mechanisms, including hormonal signals from the hypothalamus and the pituitary gland, and by negative feedback (either direct or indirect) from target cells. The physiologic processes regulated by hormones include blood pressure and composition, metabolism and energy balance, and growth, development, and reproduction.
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Endocrine cells synthesize and secrete signaling molecules called hormones, which function to regulate the activity of other cells and organs to achieve a state of metabolic or physiologic balance called homeostasis. Most hormones are released into blood and lymphatic channels and circulate throughout the body to affect distant target cells, a process that is known as classic endocrine signaling. In other instances, hormones only affect neighboring cells (paracrine signaling) or act on the secretory cells themselves (autocrine signaling). Hormones exert their effects by binding to and activating hormone receptors on or in specific populations of target cells. Thus, although hormones may circulate widely, their effects are restricted to cells that express the appropriate receptors.
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DISTRIBUTION OF ENDOCRINE CELLS
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Endocrine cells are found in the following sites (Figure 14-1A):
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Endocrine glands. Endocrine cells comprise the entire parenchyma of specialized organs, such as the pituitary (hypophysis), adrenal, thyroid, parathyroid, and pineal glands, whose sole function is hormone production.
Discrete clusters of cells. Groups of hormone-producing cells are found in organs that have other functions, such as the pancreas, ovary, placenta, and testis (see Chapters 12, 15, and 16).
Isolated individual cells. Single hormone-producing cells of the diffuse neuroendocrine system (DNES) are interspersed among cells in the epithelial lining of other organs, primarily those in the digestive system and respiratory tract.
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