McKenna's Pharmacology for Nursing, 2e

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

T he endocrine system, in conjunction with the nervous system, works to maintain internal homeostasis and to integrate the body’s response to the external environ­ ment. Their activities and functions are so closely related that it is probably more correct to refer to them as the neuroendocrine system . However, this section deals with drugs affecting the “traditional” endocrine system, which includes glands —organised groups of special­ ised cells that produce and secrete chemical messengers called hormones directly into the bloodstream to com­ municate within the body. Some organs function like endocrine glands, but they are not considered part of the traditional endocrine system. In addition, certain hormones that influence body functioning are not secreted by endocrine glands. For example, prostaglandins are tissue hormones produced in various tissues; they do not enter the bloodstream, but exert their effects just in the area where they are released. Moreover, neurotransmitters, such as noradrenaline and dopamine, can be classified as hormones because they are secreted directly into the bloodstream for dispersion throughout the body. There also are many gastrointes­ tinal (GI) hormones that are produced in GI cells and act locally. All of these hormones are addressed in the chapters most related to their effects.* STRUCTURE AND FUNCTION OF THE ENDOCRINE SYSTEM The endocrine system provides communication within the body and helps to regulate growth and develop­ ment, reproduction, energy use and electrolyte balance. The endocrine system is closely interconnected with the nervous system, and the two systems work to maintain homeostasis within the body to ensure maximum function and adequate response to various internal and external stressors. Glands The endocrine glands are collections of specialised cells that produce hormones that cause an effect at hormone- receptor sites. These glands do not have ducts, so they secrete their hormones directly into the bloodstream. There are many endocrine glands in the body. Table 34.1 *Gastrointestinal hormones are discussed in Part 11: Drugs acting on the gastrointestinal system. Neurotransmitters acting like hormones are discussed in Chapter 29: Introduction to the autonomic nervous system. The reproductive hormones are discussed in Chapter 39: Introduction to the reproductive system. Hormones active in the inflammatory and immune response are discussed in Part 3: Drugs acting on the immune system. Specific traditional endocrine glands and hormones are discussed in Chapter 35 (hypothalamic and pituitary hormones), Chapter 36 (adrenocortical hormones), Chapter 37 (thyroid and parathyroid hormones) and Chapter 38 (pancreatic hormones).

lists the endocrine glands, the hormones that they produce and the clinical effects that the hormones cause. Hormones Hormones are chemicals that are produced in the body and that meet specific criteria. All hormones: • are produced in very small amounts. • are secreted directly into the bloodstream. • travel through the blood to specific receptor sites throughout the body. • act to increase or decrease the normal metabolic cellular processes when they react with their specific receptor sites. • are immediately broken down. Hormones may act in two different ways. Some hormones react with specific receptor sites on a cell membrane to stimulate the nucleotide cyclic adeno­ sine monophosphate (cAMP) within the cell to cause an effect. For example, when insulin interacts with an insulin-receptor site, it activates intracellular enzymes that cause many effects, including changing the cell membrane’s permeability to glucose. Hormones such as insulin that do not enter the cell but react with specific receptor sites on the cell membrane act very quickly— often within seconds—to produce an effect. Other hormones, such as oestrogen, actually enter the cell and interact with a receptor site inside the cell to change messenger RNA, which enters the cell nucleus to affect cellular DNA and thereby alters the cell’s function. These hormones that enter the cell before they can cause an effect take quite a while to produce an effect. The full effects of oestrogen may not be seen for months to years, as evidenced by the changes that occur at puberty. Because the neuroendocrine system tightly regulates the body’s processes within a narrow range of normal limits, overproduction or underproduction of any hormone can affect the body’s activities and other hormones within the system. ■■ The endocrine system and the nervous system regulate body functions and maintain homeostasis largely with the help of hormones. Hormones are chemicals produced within the body that increase or decrease cellular activity. ■■ The endocrine system regulates growth and development, reproduction, energy use in the body and electrolyte balance. ■■ Hormones can react with receptors on the cell membrane to cause an immediate effect on a cell by altering enzyme systems near the cell membrane or they may enter the cell and react with receptor sites on messenger RNA, which then enters the nucleus and alters cell function. KEY POINTS