How Much Blood Vessels Are Found In Exocrine Gland

Describe some of the changes associated with aging of the endocrine system.

Understanding ^Vo rds cort-, bark, rind: adrenal cortex— outer portion of an adrenal gland.

-crin, to secrete: endocrine— pertaining to internal secretions. diuret-, to pass urine: diuretic— substance that promotes the production of urine. endo-, within: endocrine gland— gland that releases its secretion internally into a body fluid. exo-, outside: exocrine gland— gland that releases its secretion to the outside through a duct. horm-, impetus, impulse:

hormone—substance that a cell secretes that affects another cell. hyper-, above:

hyper thyroidism— condition resulting from an above-normal secretion of thyroid hormone. hypo-, below: hypothyroidism— condition resulting from a below-normal secretion of thyroid hormone. lact-, milk: pro7aciin—hormone that promotes milk production. med-, middle: adrenal medulla— middle section of an adrenal gland. para-, beside: parathyroid glands—set of glands located near the surface of the thyroid gland. toc-, birth: oxytocin—hormone that stimulates the uterine muscles to contract during childbirth. -tropic, influencing:

adrenocorticotropic hormone—a hormone secreted by the anterior pituitary gland that stimulates the adrenal cortex.

vas-, vessel: vasopressin— substance that causes blood vessel walls to contract.

he sweet-smelling urine that is the hallmark of type I

T (insulin-dependent) diabetes mellitus was noted as far back as an Egyptian papyrus from 1500 b.c. In a.d. 96 in Greece, Aretaeus of Cappadocia described the condition as a "melting down of limbs and flesh into urine." A look at the three-year-old boy in the photo on the chapter opening page illustrates how apt a description that was. In December 1922, the boy weighed only 15 pounds. But he was far luckier than people who had suffered with this form of diabetes before him. He was among the first to receive as a drug insulin, a hormone that his body could not produce. As the inset shows, the boy rapidly improved, doubling his weight in just two months of insulin treatment.

Insulin and the gland that produces it—the pancreas—are familiar components of the endocrine system. Understanding type I diabetes mellitus provides a fascinating glimpse into the evolution of medical technology that continues today.

In 1921, Canadian physiologists Sir Frederick Grant Banting and Charles Herbert Best discovered the link between lack of insulin and diabetes. They induced diabetes symptoms in a dog by removing its pancreas, then cured it by administering insulin from another dog's healthy pancreas. Just a year later, people with diabetes—such as the starving three-year-old—began to receive insulin extracted from pigs or cattle.

And so it went until 1982, when pure human insulin became available by genetically engineering bacteria to produce the human protein. Human insulin helped those who were allergic to the product from pigs or cows. Today, people receive insulin in a variety of ways, discussed in Clinical Application 13.4. Although a person with type I diabetes mellitus today is considerably healthier than the boy on the brink of the discovery of insulin, implants, injections, and aerosols to deliver insulin cannot exactly duplicate the function of the pancreas. Better understanding of the endocrine system will lead to better treatment of this and other hormonal disorders.

General Characteristics of the Endocrine System

The endocrine system is so named because the cells, tissues, and organs that comprise it, collectively called endocrine glands, secrete substances into the internal environment. The secreted substances, called hormones, diffuse from the interstitial fluid into the bloodstream, and eventually act on cells, called target cells, some distance away.

Other glands secrete substances into the internal environment, and although they are not hormones by the traditional definition, they function in similar fashion as messenger molecules and are sometimes referred to as "local hormones." These include paracrine secretions, which enter the interstitial fluid but affect only neighboring cells, and autocrine secretions, which affect only the secreting cell itself.

Another category of substances, secreted by exocrine glands, enter tubes or ducts that lead to body surfaces. In contrast to endocrine secretions, exocrine secretions are released externally. Two examples are stomach acid reaching the lumen of the digestive tract and sweat being released at the skin's surface (fig. 13.1).

The interrelationships of the glands of the endocrine system, although not well understood, are vividly obvious in families that have an inherited cancer syndrome called multiple endocrine neoplasia (MEN). Different glands are affected in different individuals within a family, although the genetic cause is the same. One family member might have a tumor of the adrenal glands called pheochromocytoma; another might have thyroid cancer; yet a third relative might have parathyroid hyperplasia, a precancerous condition.

Endocrine glands and their hormones help regulate metabolic processes. They control the rates of certain chemical reactions, aid in transporting substances through membranes, and help regulate water balance, electrolyte balance, and blood pressure. Endocrine hormones also play vital roles in reproduction, development, and growth.

Specialized small groups of cells produce some hormones. However, the larger endocrine glands—the pituitary gland, thyroid gland, parathyroid glands, adrenal glands, and pancreas—are the subject of this chapter (fig. 13.2). Subsequent chapters discuss several other hormone-secreting glands and tissues.

Hormone Action

Hormones are released into the extracellular spaces surrounding endocrine cells, from where they diffuse into the bloodstream and are carried to all parts of the body. Although some hormones may have widespread effects, in all cases, hormones affect only their target cells. This is because only target cells have receptors for any particular hormone. The other chemical messengers, paracrine and autocrine substances, also bind to specific receptors, and some examples of these are included in the chapter.

Chemistry of Hormones

Chemically, most hormones are either steroids (or steroidlike substances) that are synthesized from cholesterol (see chapter 2, p. 52), or they are amines, peptides, proteins, or glycoproteins that are synthesized from amino acids. Hormones are organic compounds. They can stimulate changes in target cells even if present in extremely low concentrations.

Endocrine gland

Endocrine gland

Exocrine — cells

Exocrine Glands

Figure 13.1

Exocrine gland (sweat gland)

Exocrine — cells

Figure 13.1

Exocrine gland (sweat gland)

(a) Endocrine glands release hormones into the internal environment (body fluids). (b) Exocrine glands secrete to the outside environment, through ducts that lead to body surfaces.

Steroids (ste'roidz) are lipids that include complex rings of carbon and hydrogen atoms. Steroids differ by the types and numbers of atoms attached to these rings and the ways they are joined (see fig. 2.15). All steroid hormones are derived from cholesterol, including sex hormones (such as testosterone and the estrogens) and those the adrenal cortex secretes (including aldosterone and cortisol). Vitamin D is a modified steroid and can be converted into a hormone, as is discussed later in this chapter, in the section titled "Parathyroid Hormone" (see also chapter 18, pp. 751-752).

Pineal gland

Parathyroid gland

Kidney

Testis (in male)

Pineal gland

Parathyroid gland

Kidney

Testis (in male)

Blood Vessels Pancreas

Hypothalamus Pituitary gland

Thyroid gland Thymus

Adrenal gland Pancreas

Ovary (in female)

Figure

Hypothalamus Pituitary gland

Thyroid gland Thymus

Adrenal gland Pancreas

Ovary (in female)

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  • How much blood vessels are found in exocrine gland?
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