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14.4: Hormones

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    Control of Secretion

    The body has three ways to control hormone secretion. First, it can be controlled by nervous impulses that result from internal or external stimuli. For example, the secretion of norepinephrine can be controlled by sympathetic nerve impulses initiated by fear. Second, secretion can be controlled by other hormones. For example, the secretion of anterior pituitary hormones is controlled by hormones from the hypothalamus.

    Third, secretion can be controlled by the substance or condition being regulated by a hormone. For example, it was noted earlier in this chapter that the rate of parathormone secretion is controlled by calcium in the blood, which in turn is regulated by parathormone. Control of hormone secretion by a substance or condition acted on by the hormone is called substrate control.

    Hormone Elimination

    The concentration of a hormone in the blood is determined by the balance between the rate at which the hormone is secreted and the rate at which it is eliminated. Hormones are removed from the blood by being chemically broken down, converted to other materials, or excreted. The liver and kidneys are very active in these processes. Elimination of substantial amounts of some hormones requires only minutes, while elimination of significant amounts of others may require several hours.

    Receptors and Responses

    Since hormones are secreted into the blood, which transports them to virtually all parts of the body, each hormone contacts many cell types, yet most hormones affect only certain cells and organs. The affected structures, called the hormone's targets, respond because their cells contain receptor molecules to which the hormone molecules bind. Receptors for some hormones are on the target cell membranes, while receptors for others are in the target cell cytoplasm.

    Different targets exposed to the same amount of a particular hormone respond to different degrees because they have fewer or more receptors or because their receptors bind the hormone more weakly or strongly. In addition, the strength of each target's response can be changed by modulating the number or binding strength of its receptors. Finally, the effectiveness of a hormone can be influenced by conditions in the target that affect its response mechanism.

    Hormone Effectiveness

    We have seen that hormone effectiveness can be influenced by the rate of hormone secretion, the rate of hormone elimination, target receptors, and conditions within the target cells. However, many other factors, such as substances that bind hormones, changes in rhythms of secretion, and interference by nerve impulses or other hormones, can influence the effectiveness of a hormone.

    With so many factors influencing hormonal effectiveness, determining the rates of hormone secretion or measuring the concentrations of hormones in the blood at any one time provides only a small portion of the information needed to evaluate endocrine system performance and the ability of the aging endocrine system to continue contributing effectively to homeostasis.

    This page titled 14.4: Hormones is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Augustine G. DiGiovanna via source content that was edited to the style and standards of the LibreTexts platform.