Chapter 38 Introduction

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CHAPTER 38

Animal Endocrine Systems

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FLPA/Chris Mattison/age fotostock.

Core Concepts

  1. Animal endocrine systems release chemical signals called hormones into the bloodstream that respond to environmental cues, regulate growth and development, and maintain homeostasis.
  2. Hormones bind to receptors on or inside target cells, and their signals are amplified to exert strong effects on target cells.
  3. In vertebrate animals, the hypothalamus and the pituitary gland control and integrate diverse bodily functions and behaviors.
  4. Chemical communication can also occur locally between neighboring cells or, through the release of pheromones, between individuals, coordinating social interactions.

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We have seen how the nervous system allows an animal to sense and respond to its environment, coordinating the animal’s movement and behavior (Chapters 35–37). The nervous system also works closely with the endocrine system to regulate an animal’s internal physiological functions. This coordination is accomplished by signals sent from the nervous system to the endocrine system, which distributes chemical signals throughout the body. The endocrine system is made up of an interacting set of secretory glands and organs, located in different regions of the body, which respond to nervous system signals as well as to blood-borne signals from other organs to regulate internal bodily functions.

The signals communicated by the nervous and endocrine systems differ greatly in their modes of transmission and the times over which they act. The nervous system sends signals rapidly by action potentials running along nerve axons, and communication occurs between adjacent nerve cells by means of neurotransmitters. The endocrine system, by contrast, relies on cells and glands that secrete chemical signals called hormones, which influence the actions of other cells in the body. Whereas some hormones act locally, many others are released into the bloodstream, allowing them to circulate and influence distant target cells throughout the body. As a result, endocrine communication is generally slower and more prolonged than the rapid and brief signals transmitted by nerve cells. Hormones exert specific effects on particular target cells within the body by binding to receptors on or in target cells. The downstream influence of hormones is amplified in a series of steps so that a small amount of hormone can have dramatic effects in the body (Chapter 9).

In this chapter, we explore the properties and actions of hormones that help to coordinate and maintain a broad set of physiological functions of the organism. In particular, we examine the mechanisms by which hormones trigger cellular responses in their target organs. Because most hormones are evolutionarily conserved, their chemical structure is similar across a diverse array of animals. However, their functions often evolve rapidly, enabling hormones to serve new and broader roles.