McKenna's Pharmacology for Nursing, 2e

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C H A P T E R 3 4 Introduction to the endocrine system

KEY POINTS

Hypothalamus

■■ The pituitary gland has three lobes: the anterior lobe produces stimulating hormones in response to hypothalamic stimulation. ■■ The posterior lobe of the pituitary stores ADH and oxytocin, which are two hormones produced by the hypothalamus. ■■ The intermediate lobe of the pituitary produces endorphins and encephalins to modulate pain perception. ENDOCRINE REGULATION The production and release of hormones needs to be tightly regulated within the body. Hormones are released in small amounts to accomplish what needs to be done to maintain homeostasis within the body. The fine tuning and regulation of hormone release through the hypothalamus is often regulated by a series of negative feedback systems. Other hormones are not controlled in this fashion but respond to other direct stimuli. Hypothalamic–pituitary axis Because of its position in the brain, the hypothalamus is stimulated by many things, such as light, emotion, cerebral cortex activity and a variety of chemical and hormonal stimuli. Together, the hypothalamus and the pituitary function closely to maintain endocrine activity along what is called the hypothalamic–pituitary axis (HPA) using a series of negative feedback systems. A negative feedback system works much like the law of supply and demand in business. In business, when there is an adequate supply of a product, production of that product will slow down because there is an adequate supply and no current demand for it. When the supply is used up, demand will increase, and so production will pick up. Production continues until the supply is adequate and demand is reduced. When the hypothalamus senses a need for a particular hormone, for example, thyroid hormone, it secretes the releasing factor TRH directly into the anterior pituitary. In response to the TRH, the anterior pituitary secretes TSH, which in turn stimu­ lates the thyroid gland to produce thyroid hormone. When the hypothalamus senses the rising levels of thyroid hormone, it stops secreting TRH, resulting in decreased TSH production and subsequent reduced thyroid hormone levels. The hypothalamus, sensing the falling thyroid hormone levels, secretes TRH again. The negative feedback system continues in this fashion, maintaining the levels of thyroid hormone within a rela­ tively narrow range of normal (see Figure 34.2). It is thought that this feedback system is more complex than once believed. The hypothalamus probably also senses TRH and TSH levels and regulates

Levels of TRH influence hypothalamus activity

TRH

Anterior pituitary

TSH

Thyroid gland

Levels of TSH influence anterior pituitary and

hypothalamus activity

pituitary activity

Thyroid hormones

Levels of hormone regulate hypothalamus and anterior

TRH secretion within a narrow range, even if thyroid hormone is not produced. The anterior pituitary may also be sensitive to TSH levels and thyroid hormone, regulating its own production of TSH. This complex system provides backup controls and regulation if any part of the HPA fails. This system can also create com­ plications, especially when there is a need to override or interact with the total system, as is the case with hormone replacement therapy or the treatment of endo­ crine disorders. Supplying an exogenous hormone, for example, may increase the hormone levels in the body, but then may affect the HPA to stop production of releas­ ing and stimulating hormones, leading to a decrease in the body’s normal production of the hormone. Two of the anterior pituitary hormones (i.e. growth hormone and prolactin) do not have a target organ to produce hormones and so cannot be regulated by the same type of feedback mechanism. The hypothalamus in this case responds directly to rising levels of growth hormone and prolactin. When levels rise, the hypothal­ amus releases the inhibiting factors somatostatin and PIF directly to inhibit the pituitary’s release of growth hormone and prolactin, respectively. The HPA func­ tions through negative feedback loops or the direct use of inhibiting factors to constantly keep these hormones regulated. Other forms of regulation Hormones other than stimulating hormones are also released in response to stimuli. For example, the pancreas produces and releases insulin, glucagon and somatostatin from different cells in response to varying blood glucose levels. The parathyroid glands release parathyroid hormone, or parathormone, in response to local calcium levels. The juxtaglomerular cells in the kidney release erythropoietin and renin in response to decreased pressure or decreased oxygenation of the blood flowing into the glomerulus. GI hormones are released in response to local stimuli in areas of the GI tract, such as acid, proteins or calcium. The thyroid gland produces and secretes another hormone, called FIGURE 34.2  Negative feedback system. Thyroid hormone levels are regulated by a series of negative feedback systems influencing thyrotropin-releasing hormone (TRH), thyrotropin (TSH) and thyroid hormone levels.