Chapter_47_EndocrineSystem

Chapter_47_EndocrineSystem - Chapter 47 The Endocrine...

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Chapter 47 - The Endocrine System The theme of this chapter amplifies the theme of Chapter 45 regarding how cells, organs, and organ systems communicate with each other. The term “homeostasis” is used in this chapter. Homeostasis occurs when the body is at a relatively stable internal physiological condition (definition - your book). However, the meaning of “homeostasis,” like “integration” is a difficult term to grasp. Chapter 49 takes a closer look at homeostasis. In Chapter 47, emphasis is placed on the molecules and tissues that produce them in both the Nervous and Endocrine systems as these two organ systems interact to regulate the body’s activities. As you will see, the line between these two systems is often blurred. The Concept Outline (P. 991) and the Concept Review Outline (P. 1011) provide excellent coverage of the subject. Regulation is Accomplished by Chemical Messengers: There are three types of regulatory molecules (Fig. 47.2). Neurotransmitters are released only by neurons into the synapse, travel a very short distance (fractions of a micron), and are reabsorbed. Hormones are produced by endocrine gland and released into the bloodstream. Hormones may travel widely over the body, but interact only with target ” cells. Neurons also secrete molecules into the blood. These types of hormones are called neurohormones . Molecules that are released into, and stay within an organ, but are used to regulate functions of the organ, are termed paracrine regulators. Endocrine Glands and their Hormones: The human endocrine system is illustrated in Fig. 47.3. But keep in mind that other tissues also secrete hormones such as the endothelial lining of blood vessels and some neurons. There are four major categories of hormones secreted by the endocrine glands Table 47.1): 1. Polypeptides: short chains of amino acids (less than 100 aa units). Insulin and Antidiuretic hormone (ADH0 are examples. 2. Glycoproteins: Polypeptide chains longer than 100 amino acids to which a carbohydrate group is attached. FSH and LH are good examples of this class. 3. Amines: derived from the amino acids tyrosine and tryptophan. These are secreted by the adrenal medulla, thyroid (thyroxin), and pineal glands. 4. Steroids: These are lipid derived from cholesterol (remember this molecule from our discussion about Digestion and Nutrition???). Molecules like testosterone, estrogen, progesterone, and aldosterone. Steroid hormones can be further divided into: sex steroids, and corticosteroids. The latter is secreted by the adrenal cortex. All hormones may also be categorized as to whether they are water soluble. Lipophilic are fat-soluble hormones and include the steroid hormones and throxine. All others are lipophobic or water-soluble. This subtle distinction is important when we begin to discuss “target cells.” Paracrine Regulation: This is a new and evolving field. Many examples are given on P. 994. The most diverse group of paracrine molecules are the prostoglandins . Prostoglandins are produced in almost every organ and have a variety of regulatory functions.
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