blood samples, researchers can determine how long it takes for the circulating hormone to drop to half of that maximum concentration. The fight-or-flight response to epinephrine, for example, is relatively quick in its onset and termination; the half-life of epinephrine in the blood is only 1-3 minutes. The effects of other hormones, such as cortisol and thyrox-ine, are expressed over much longer periods, and their half-lives are on the order of days or weeks.
A hormone's half-life is partially determined by processes of degradation and elimination. Hormones are typically degraded in the liver, then removed from the blood in the kidney and excreted in the urine. The presence of hormones or their breakdown products in the urine is the reason urine samples provide important information in clinical tests.
Another factor that influences the half-life of a hormone is its ability to leave the blood. The extent to which a hormone is bound to carrier proteins limits its ability to diffuse out of the blood to reach its target cells, to be degraded in the liver, or to be excreted by the kidney. For example, when the min-eralocorticoid aldosterone is released, about 15 percent of it binds to carrier proteins, and its half-life is 25 minutes. In contrast, when thyroxine is released, almost 100 percent of it binds to carrier proteins, and thyroxine has a half-life of 6 days. This variation in the time course of hormone action allows hormone signaling systems to have temporal characteristics appropriate to their functions.
Of course, the nature of the target cell's response to a hormone is also a factor in determining the time course of hormone action. For a hormone that stimulates a developmental effect, the time course of hormone action can be months, years, or even a lifetime. A very good example of a long-term process regulated primarily by hormones is animal reproduction, the topic of the next chapter.
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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.