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Word Roots and Origins antagonistic from the Greek anti, meaning "against" and agonizesthai, meaning "to contend for a prize"

Positive Feedback

When hormones are regulated by positive feedback, release of an initial hormone stimulates release or production of other hormones or substances, which stimulate further release of the initial hormone. For example, increased estrogen concentrations stimulate a surge in luteinizing hormone secretion prior to ovulation. Figure 50-11 illustrates the difference between negative and positive feedback systems.

Antagonistic Hormones

A number of hormones work together in pairs to regulate the levels of critical substances. These hormones are referred to as antagonistic hormones because their actions have opposite effects.

Glucagon and insulin are examples of antagonistic hormones. They maintain a specific level of blood glucose in the blood. When the level of glucose in the blood is high, such as after eating a meal, insulin triggers the transfer of glucose from the blood into the body cells for storage or immediate use. In contrast, when the level of glucose in the blood is low, such as between meals, glucagon promotes the release of glucose into the blood from storage sites in the liver and elsewhere. Together, insulin and glucagon ensure that the level of glucose in the blood is maintained. Calcitonin and parathyroid hormone are other examples of antagonistic hormones.

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