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OBJECTIVES
Contrast the terms endocrine, paracrine, and autocrine.
Define the terms hormone, target cell, and receptor.
Understand the major differences in mechanisms of action of peptides, steroids, and thyroid hormones.
Compare and contrast hormone actions exerted via plasma membrane receptors with those mediated via intracellular receptors.
Understand the role of hormone-binding proteins.
Understand the feedback control mechanisms of hormone secretion.
Explain the effects of hormone secretion, degradation, and excretion on plasma hormone concentrations.
Understand the basis of hormone measurements and their interpretation.
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The function of the endocrine system is to coordinate and integrate cellular activity within the whole body by regulating cellular and organ function throughout life and maintaining homeostasis. Homeostasis, defined as the dynamic maintenance of an internal environment balance or equilibrium despite changing internal (ie, glucose levels, blood pH) and external (ie, light/dark, ambient temperature) conditions. Maintaining homeostasis is critical to ensuring appropriate cellular function.
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THE ENDOCRINE SYSTEM: PHYSIOLOGIC FUNCTIONS AND COMPONENTS
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Some of the key functions of the endocrine system include:
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Regulation of sodium and water balance and control of blood volume and pressure
Regulation of calcium and phosphate balance to preserve extracellular fluid concentrations required for cell membrane integrity and intracellular signaling
Regulation of energy balance and control of fuel mobilization, utilization, and storage to ensure that cellular metabolic demands are met
Coordination of the host hemodynamic and metabolic counterregulatory responses to stress
Regulation of reproduction, development, growth, and senescence
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In the classic description of the endocrine system, a chemical messenger or hormone produced by an organ is released into the circulation to produce an effect on a distant target organ. Currently, the understanding of the endocrine system is that of an integrated network of multiple organs of diverse embryologic origins that release hormones ranging from small peptides to large glycoproteins, which exert their effects at specific target cells. This endocrine network of organs and mediators does not work in isolation and is closely integrated with the central and peripheral nervous systems as well as with the immune systems, leading to currently used terminology such as “neuroendocrine” or “neuroendocrine-immune” systems for describing their interactions. Three basic components make up the core of the endocrine system.
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Endocrine glands—The classic endocrine glands are ductless and secrete their chemical products (hormones) into the interstitial space from where they reach the circulation. Unlike the cardiovascular, renal, and digestive systems, the endocrine glands are not anatomically connected and are scattered throughout the body (Figure 1–1). Communication among the different organs is ensured through the release of hormones or neurotransmitters.
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