36. The Endocrine System




The Endocrine System


Objectives


Theory



Clinical Practice



Key Terms


adenohypophysis (ă-DĔN-ŏ-hī-PŎF-ă-sĭs, p. 823)


adrenocorticotropic hormone (ă-DRĔN-ŏ-KŎR-tĭ-kō-TRŌ-pĭk, p. 823)


endocrine (ĔN-dŏ-krĭn, p. 821)


exocrine (Ĕk-sŏ-krĭn, p. 821)


fructosamine assay (p. 827)


glucocorticoids (glū-kō-KŎR-tĭ-kŏydz, p. 821)


glucose tolerance test (p. 827)


hemoglobin A1c (A1C) (HĒ-mō-glō-bĭn, p. 827)


hormones (HŎR-mōnz, p. 822)


hypersecretion (hī-pĕr-SĔ-KRĒ-shŭn, p. 823)


hyposecretion (hī-pō-SĔ-KRĒ-shŭn, p. 823)


insulin (ĬN-sū-lĭn, p. 822)


mineralocorticoids (mĭn-ĕr-ăl-ō-KŎR-tĭ-kŏydz, p. 821)


negative feedback (p. 823)


parathormone (păr-ă-THŎR-mōn, p. 821)


pressor (p. 821)


target cells (p. 822)


target tissues (p. 822)


thyrocalcitonin (thī-rō-KĂL-sĭ-TŌ-nĭn, p. 819)


thyroid panel (THĪ-rŏyd, p. 827)


thyroxine (THĪ-rŏk-sĭn, p. 819)


triiodothyronine (trī-ī-ō-dō-THĪ-rō-nĕn, p. 819)


image http://evolve.elsevier.com/deWit/medsurg


Overview of anatomy and physiology of the endocrine system


What are the organs and structures of the endocrine system?



• The pituitary gland connects to the hypothalamus via the hypophyseal stalk. The pituitary gland has two parts: the anterior pituitary and the posterior pituitary.


• The thyroid gland has two lobes and lies below the larynx over the thyroid cartilage, in front of and on either side of the trachea.


• The parathyroid glands are four to six small glands that are located on the posterior surface of the thyroid gland.


• The adrenal glands are located on the anterior upper surface of each kidney; each is composed of the cortex and medulla.


• The pancreas sits in the upper left aspect of the abdominal cavity. Beta cells, which secrete the hormone insulin, are found in the islets of Langerhans.


• The ovaries are located in the pelvic cavity of the female.


• The testes hang suspended in the scrotum of the male.


• The pineal gland is in the midbrain, in the cranial vault.


• The thymus gland lies at the base of the neck, in the front of the thoracic cavity.


What are the functions of the endocrine system?




Table 36-1


The Principal Endocrine Glands and Their Hormones




















































































































Gland Hormone Target Tissue Principal Actions
Hypothalamus Releasing and inhibiting hormones Anterior lobe of pituitary gland Stimulates or inhibits secretion of specific hormones
Anterior lobe of pituitary Growth hormone (GH) Most tissues in the body Stimulates growth by promoting protein synthesis
  Thyroid-stimulating hormone (TSH) Thyroid gland Increases secretion of thyroid hormone; increases the size of the thyroid gland
Adrenocorticotropic hormone (ACTH) Adrenal cortex Increases secretion of adrenocortical hormones, especially glucocorticoids, such as cortisol
Follicle-stimulating hormone (FSH) Ovarian follicles in the female; seminiferous tubules in male Follicle maturation and estrogen secretion in the female; spermatogenesis in the male
Luteinizing hormone (LH); called interstitial cell–stimulating hormone (ICSH) in males Ovary in females, testis in males Ovulation; progesterone production in female; testosterone production in male
Prolactin Mammary gland Stimulates milk production
Posterior lobe of pituitary (storage only: ADH and oxytocin are synthesized in the hypothalamus) Antidiuretic hormone (ADH) Kidney Increases water reabsorption (decreases water lost in urine)
Oxytocin Uterus; mammary gland Increases uterine contractions; stimulates ejection of milk from mammary gland
Thyroid gland Thyroxine and triiodothyronine Most body cells Increases metabolic rate; essential for normal growth and development
Calcitonin Primarily bone Decreases blood calcium by inhibiting bone breakdown and release of calcium; antagonistic to parathyroid hormone
Parathyroid gland Parathyroid hormone (PTH) or parathormone Bone, kidney, digestive tract Increases blood calcium by stimulating bone breakdown and release of calcium; increases calcium absorption in the digestive tract; decreases calcium lost in urine
Adrenal cortex Mineralocorticoids (aldosterone) Kidney Increases sodium reabsorption and potassium excretion in kidney tubules; increases water retention
Glucocorticoids (cortisol) Most body tissues Increases blood glucose levels; inhibits inflammation and immune response
  Androgens and estrogens Most body tissues Secreted in small amounts; effect is generally masked by the hormones from the ovaries and testes
Adrenal medulla Epinephrine, norepinephrine Heart, blood vessels, liver, adipose tissue Helps cope with stress; increases heart rate and blood pressure; increases blood flow to skeletal muscle; increases blood glucose
Pancreas (islets of Langerhans) Glucagon Liver Increases breakdown of glycogen to increase blood glucose levels
Insulin General, but especially liver, skeletal muscle, adipose tissue Decreases blood glucose levels by facilitating uptake and utilization of glucose by cells; stimulates glucose storage as glycogen and production of adipose tissue
Testes Testosterone Most body cells Maturation and maintenance of male reproductive organs and secondary sex characteristics
Ovaries Estrogens Most body cells Maturation and maintenance of female reproductive organs and secondary sex characteristics; menstrual cycle
Progesterone Uterus and breast Prepares uterus for pregnancy; stimulates development of mammary gland; menstrual cycle
Pineal gland Melatonin Hypothalamus Inhibits gonadotropin-releasing hormone, which consequently inhibits reproductive functions; regulates daily rhythms, such as sleep and wakefulness
Thymus Thymosin Tissues involved in immune response Immune system development and function


Image


What are the effects of the pituitary hormones?




What are the effects of the thyroid hormones?



What are the functions of the parathyroid glands?




What are the functions of the hormones secreted by the adrenal glands?



• The adrenal medulla (middle portion) secretes two hormones, epinephrine and norepinephrine, in response to stimulation from the sympathetic nervous system.


• Epinephrine prepares the body to meet stress or emergency situations and prevents hypoglycemia (Figure 36-2). Norepinephrine functions as a pressor (causing blood vessel constriction) hormone to maintain blood pressure.


• The hormones secreted by the adrenal cortex are called adrenal corticosteroids. (The word steroid is sometimes used to designate an adrenal corticosteroid or a synthetic compound with similar properties.)


• The two major types of hormones secreted by the adrenal cortex are the mineralocorticoids and the glucocorticoids (Figure 36-3).


• The adrenal glands also secrete small amounts of androgenic hormones, which have effects similar to those of the male and female sex hormones.


• The mineralocorticoids affect the electrolytes, particularly sodium, potassium, and chloride. The chief mineralocorticoid is aldosterone, which promotes conservation of water by acting on the kidney to retain sodium in exchange for potassium, which is excreted in the urine.


• Without the mineralocorticoids, a person would die within 3 to 7 days, because these hormones directly control fluid balance, blood volume, cardiac output, exchange of nutrients, and wastes in each cell; mineralocorticoids affect all chemical processes and glandular functions within the body.


• The glucocorticoids are essential to the metabolic systems for proper utilization of carbohydrates, proteins, and fats.


• The primary glucocorticoid is cortisol, or hydrocortisone. Cortisol acts to increase glucose levels in the blood. Cortisol also helps counteract the inflammatory response.


• Both aldosterone and cortisol are controlled by adrenocorticotropic hormone (ACTH)–releasing hormone from the hypothalamus and ACTH secreted by the anterior pituitary (see Figure 36-3).




What is the hormonal function of the pancreas?





What are the effects of aging on the endocrine system?



• The pituitary gland becomes smaller.


• The thyroid becomes more lumpy or nodular; beginning around age 20, metabolism gradually declines.


• Hormones that usually decrease with age include aldosterone, renin, calcitonin, and growth hormone; specific hormones decrease in older women (estrogen and prolactin) and older men (testosterone).


• Hormones that may increase with age include follicle-stimulating hormone (FSH), luteinizing hormone (LH), norepinephrine, and antidiuretic hormone (ADH).


• Hormones that remain unchanged or are only slightly decreased with age include thyroid hormones (T3 and T4), cortisol, insulin, epinephrine, parathyroid hormone, and 25-hydroxyvitamin D.


• Blood glucose levels rise with age, with fasting levels climbing about 1 mg/dL for each decade and postprandial levels increasing 6 to 13 mg/dL.


• Although insulin levels remain unchanged with age, decreased glucose tolerance may occur due to changes in the cell receptor sites: the older adult experiences hypoglycemia more quickly than a younger person and may progress to dangerously low levels of blood glucose before signs and symptoms are obvious. This decreased glucose tolerance because of cell receptor change can place the older adult at risk for hyperglycemia and the onset of type 2 diabetes.


• Although thyroid hormone levels may decrease with aging, the body makes up for it by decreasing the rate at which thyroid hormone is broken down; therefore resting levels of thyroid hormone are usually normal in the elderly. Thyroid disorders are, however, twice as common in the older adult. Hypothyroidism is the most common thyroid disorder, especially in older women.


• The amount of hormones secreted by the older adult changes, decreasing the individual’s ability to adapt to stress and respond to environmental changes.


• Because of decreasing liver and kidney function in the elderly, hormone replacement therapy must be done very cautiously, to prevent hormone overdosage.


The endocrine system


The endocrine system regulates metabolism, growth and development, and sexual function and reproductive processes. A primary function of the endocrine system is to synthesize and release hormones directly into the bloodstream and the body fluids. The cells and tissues that are affected by a specific hormone are called its target cells or target tissues.


Some of the endocrine hormones, such as the thyroid hormones, affect practically every cell in the body. Others, such as the sex hormones, exert their special effects on only one kind of organ. Moreover, hormones from one endocrine gland can affect another endocrine gland. The pituitary, for example, secretes several different kinds of hormones that affect other endocrine glands. For this reason, the pituitary gland is often referred to as the “master gland” of the body.


The endocrine system and the nervous system are the two major control systems of the body, and their regulatory functions are interrelated. However, the endocrine system typically controls body processes that occur slowly, such as cell growth, whereas the nervous system controls body processes that occur more rapidly, such as breathing and body movement.


The secretion of a particular hormone normally depends on the need. If an endocrine gland receives a message that its particular hormone is in short supply, it will synthesize and release more of that hormone. If, on the other hand, the hormonal need of a target tissue is being satisfied, production or secretion of the hormone will be inhibited—a concept known as negative feedback.


Some glands, such as the adrenal medulla and posterior pituitary, receive their information about hormone levels in the body directly, and respond only to stimulation of nerve endings within the glands themselves. However, the posterior pituitary gland indirectly receives notice to either release or inhibit hormones: stimulation comes by way of the hypothalamus and the anterior lobe of the pituitary (the adenohypophysis). The hypothalamus contains special nerve endings that produce releasing and inhibiting hormones; these hormones are then absorbed into capillaries of a portal system that transports the hormones to the adenohypophysis (the anterior lobe of the pituitary). Thus the hypothalamus controls the secretion of hormones from the pituitary. The pituitary, in turn, controls the release or inhibition of hormones from other glands. Many of the hormones of the anterior pituitary are “tropic” hormones; that is, they tend to cause a change in the endocrine gland that is the target of the specific pituitary hormone. An example is adrenocorticotropic hormone (ACTH), which acts on the adrenal cortex. (If you break down this term, you can easily see that the components of adrenal–cortex–tropic tell you exactly where or what type of hormone this is and where it comes from.) The major endocrine glands can be found in Figure 36-6; see Table 36-1 for the various tropic hormones and target tissues.


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Nov 17, 2016 | Posted by in NURSING | Comments Off on 36. The Endocrine System

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