Porth's Essentials of Pathophysiology, 4e

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Endocrine System

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endocrine hormones tend to take much longer than the response to neurotransmitters, but once initiated they tend to be much more prolonged than those induced by the nervous system. The glands of the endocrine system are widely scat- tered throughout the body (Fig. 31-1). These glands include the pituitary, thyroid, parathyroid, and adrenal glands. In addition, several organs of the body contain discrete areas of endocrine tissue and produce hor- mones as well as exocrine products. Such organs, which include the pancreas and the gonads (testes and ovaries), are also major endocrine glands. The hypothalamus also falls into this category. In addition to its neural func- tions, it releases hormones that influence the secretion of hormones by other endocrine organs. Besides the major endocrine glands, there are endo- crine cells within organs whose primary function is not endocrine. These include cells within the heart that pro- duce atrial natriuretic factor, cells within the kidney that produce erythropoietin, and numerous cell types within the gastrointestinal tract that produce hormones. Hormones The effects of hormones are many and varied. Their actions are involved in regulating water and electrolyte balance; responding to adverse conditions, such as infec- tion, trauma, and stress; sequentially integrating the pro- cesses of growth and development; contributing to the processes of reproduction, including gamete (ovum and sperm) production, fertilization, and maintenance of a pregnancy; and digesting, using, and storing nutrients.

A characteristic of endocrine hormones is that a single hormone can exert various effects in different tis- sues or, conversely, a single function can be regulated by several different hormones. For example, estradiol, which is produced by the ovary, can act on the ovarian follicles to promote their maturation, on the uterus to stimulate its growth and maintain the cyclic changes in the uterine mucosa, on the mammary gland to stimulate ductal growth, on the hypothalamic-pituitary system to regulate the secretion of gonadotropins and prolactin, on the bone to maintain skeletal integrity, and on gen- eral metabolic processes to affect adipose tissue distri- bution. Lipolysis, which is the release of free fatty acids from adipose tissue, is an example of a single function that is regulated by several hormones, including the catecholamines, insulin, and glucagon, and also by the cytokine tumor necrosis factor- α . Table 31-1 lists the major actions and sources of body hormones. Although some hormones are released into the blood- stream and transported to distant target sites, where they exert their actions, other hormones and hormone- like substances never enter the bloodstream but instead act locally in the vicinity in which they are released (Fig. 31-2). When they act locally on cells other than those that produce the hormone, the action is called paracrine. The action of sex steroids on the ovary is a paracrine action. Hormones also can exert an autocrine action on the cells from which they were produced. For example, the release of insulin from pancreatic beta cells can inhibit its release from the same cells. Structural Classification Hormones, which have diverse chemical structures rang- ing from single amino acids to complex proteins and lip- ids, can be divided into three categories: (1) amines and amino acids; (2) peptides, polypeptides, proteins, and glycoproteins; and (3) steroids (Table 31-2). The first category, the amines and amino acid hor- mones, includes norepinephrine and epinephrine, which are derived from a single amino acid (i.e., tyrosine), and the thyroid hormones, which are derived from two iodinated tyrosine amino acid residues. The second cat- egory, the peptide, polypeptide, protein, and glycopro- tein hormones, can be as small as thyrotropin-releasing hormone (TRH), which contains 3 amino acids, and as large and complex as growth hormone (GH) and follicle- stimulating hormone (FSH), which have approximately 200 amino acids. Glycoproteins are large peptide hor- mones associated with a carbohydrate (e.g., FSH). The third category consists of the steroid hormones, such as the glucocorticoids, which are derivatives of cholesterol. A group of compounds that have a hormone-like action are the eicosanoids, which are derived from polyunsaturated fatty acids in the cell membrane. Among these, arachidonic acid is the most important and abundant precursor of the various eicosanoids (see Chapter 3, Fig. 3-4). The most important of the eicosanoids are the prostaglandins, leukotrienes, and thromboxanes. These fatty acid derivatives are pro- duced by most body cells, are rapidly cleared from the

Hypothalamus Pineal gland Pituitary gland

Thyroid gland Parathyroid glands (on dorsal aspect of thyroid gland)

Thymus gland Adrenal glands Pancreas (Islets of Langerhans)

Ovaries (female) Testes (male) Gonads:

FIGURE 31-1. Location of endocrine glands.