Hormones and Their Actions

In Chapter 41, we learned that control and regulation require information. In multicellular animals, most of this information is transmitted as electric signals and as chemical signals. The electric signals are nerve impulses, which will be a major focus of later chapters on the nervous system. The chemical signals are hormones, which are secreted by cells, diffuse locally in the extracellular fluid, and are picked up by the blood, which distributes them throughout the body (Figure 42.1a).

Cells that secrete hormones are called endocrine cells. Target cells receive the hormonal message if they have appropriate receptors to bind the hormone. The binding of a hormone to its receptor activates mechanisms within the target cell that eventually lead to a response. The response can be developmental, physiological, or behavioral.

The secretion, diffusion, and circulation of hormones is much slower than the transmission of nerve impulses. Therefore, hormones are not useful for controlling rapid actions, such as cichlid fighting behavior. Hormones are most useful for coordinating longer-term developmental or physiological processes, such as the transformation of a nondescript, passive cichlid into a dominant, territorial male.

(a) Circulating hormones

Endocrine cell

Circulating hormones transported by the blood and bind to receptors on distant cells.

(a) Circulating hormones

Endocrine cell

Circulating hormones transported by the blood and bind to receptors on distant cells.

Blood vessel

Target cell

Blood vessel

Target cell

(b) Local hormones

Paracrine hormones bind to receptors on nearby cells.

(b) Local hormones

Paracrine hormones bind to receptors on nearby cells.

42.1 Chemical Signaling Systems (a) Most hormones are distributed throughout the body by the circulatory system. (b) An autocrine hormone influences the cell that releases it; a paracrine hormone influences nearby cells.

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Essentials of Human Physiology

Essentials of Human Physiology

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.

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