Control of Hormones

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Chapter: Anatomy and Physiology for Health Professionals: Endocrine System

Increasing or decreasing secretion of a hormone results in increased or decreased blood levels of the hormone.

Control of Hormones

Increasing or decreasing secretion of a hormone results in increased or decreased blood levels of the hormone. This is because hormones are continually excreted in the urine and broken down mostly by liver enzymes. Negative feedback is the mechanism that controls hormone secretion. It is triggered by an internal or external stimulus. FIGURE 16-5 shows the steps of hormone secretion:

1.The nervous system directly stimulates certain glands

2. The hypothalamus regulates the release of hormones from the anterior pituitary gland, with its proximity allowing constant communication about the internal environment.

3. Other glands respond directly to internal changes.

As hormone levels rise, negative feedback inhibits the system and secretion decreases. As blood levels of hormones decrease, the system starts up again. Hormones in the bloodstream fluctuate resultantly, but remain relatively stable. Three types of stimuli trigger endocrine gland actions:

■■ Humoral stimuli: Changing blood levels of certain vital ions and nutrients. They are the simplest type of endocrine controls. For example, when para-thyroid gland cells or chief cells monitor bloodcalcium andsecreteparathyroid hormone(PTH) when they detect lower than normal levels Humoral stimuli are also linked to release of insulin from the pancreas and aldosterone from theadrenal cortex.

■■Neural stimuli: Nerve fiber stimulation. Thebest example is how the sympathetic nervoussystem responds to stress by stimulating theadrenal medulla to release norepinephrine andepinephrine.

■■Hormonal stimuli: Hormone release due to theproduction of other hormones. For example,when the hypothalamus regulates secretion ofmost anterior pituitary hormones via the actionsof its own releasing hormones and inhibitinghormones. Then, many anterior pituitary hormones act to stimulate other endocrine organsto cause them to release their own hormones.Increasing blood levels of the hormones fromthe final target glands then inhibit the releaseof anterior pituitary hormones and, therefore,their own release.

Interaction of Hormones

Remember that multiple hormones may act on the same target cells at the same time, often with unpre-dictable reactions. There are three basic types of hor-mone interaction:

■■Permissiveness: One hormone is not able to exertits full effects without the presence of anotherhormone. For example, when reproductive system hormones require the presence of thyroidhormone to develop reproductive structures at 0 their normal times in the life span. If the thyroidhormone is lacking, reproductive development isdelayed.

■■Synergism: One hormone produces the sameeffects as another hormone at the target cell,amplifying their combined effects. For example,when glucagon from the pancreas, along with epinephrine, signals the liver to release glucose intothe blood.

Antagonism: One hormone opposes anotherhormone’s actions. For example, when insulin is antagonized by glucagon. Insulin lowers blood glucose levels while glucagon raises them, an opposite effect.

Hormones may also produce complimentary yet different effects in certain organs and tissues described as integrative effects.

1. Explain the three types of stimuli that control hormone release.

2. Define the three basic types of hormone interaction.

3. Explain cyclic adenosine monophosphate

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