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  • 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 secretion, degradation, and excretion on plasma hormone concentrations.
  • Understand the basis of hormone measurements and their interpretation.

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, or the maintenance of a constant internal environment, is critical to ensuring appropriate cellular function.

Some of the key functions of the endocrine system include:

  • 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

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 definition of the endocrine system is that of an integrated network of multiple organs derived from different embryologic origins that release hormones ranging from small peptides to glycoproteins, which exert their effects either in neighboring or distant 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.

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.

Figure 1–1.

The endocrine system. Endocrine organs are located throughout the body, and their function is controlled by hormones delivered through the circulation or produced locally or by direct neuroendocrine stimulation. Integration of hormone production from endocrine organs is regulated by the hypothalamus. ACTH, adrenocorticotropic hormone; CRH, corticotropin-releasing hormone; FSH, follicle-stimulating hormone; GHRH, growth hormone-releasing hormone; GnRH, gonadotropin-releasing hormone; LH, luteinizing hormone; MSH, melanocyte-stimulating hormone; TRH, thyrotropin-releasing hormone; TSH, ...

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