The Endocrine System (Structure and Function) (Nursing) Part 1

Learning Objectives

1.    Differentiate between exocrine and endocrine glands.

2.    Describe the general functions of the endocrine system, and the actions related to each function.

3.    Describe the relationship between the hypothalamus and the pituitary gland.

4.    Identify the major hormones released by the anterior, middle, and posterior divisions of the pituitary gland, and describe the functions of each hormone.

5.    Describe the actions of hormones responsible for calcium balance.

6.    Describe the relationships between “releasing” hormones and “inhibiting” hormones.

7.    Describe the hormones involved in “fight or flight,” and give examples of their effects and body responses during an emergency.

8.    Explain the functions of the thyroid hormones.

9.    Describe the functions of mineralocorticoids and glucocorticoids secreted by the adrenal glands.

10.    Discuss the location of insulin secretion and explain how insulin and glucagon regulate blood sugar levels.

11.    Discuss the role of the thymus as an endocrine organ and its relationship to the body’s immune response.

12.    Briefly identify the male and female sex hormones and functions of each.

13.    Name the hormones secreted by nonendocrine glands or organs, and state the function of each hormone.

14.    Discuss negative and positive feedback as they relate to the endocrine system.

15.    Explain the role of prostaglandins in the body.

16.    Describe four effects of aging on the endocrine system.

IMPORTANT TERMINOLOGY
adenohypophysis	glucagon	mineralocorticoid
adrenal gland	glucocorticoid	neurohypophysis
corticosteroid	goiter	parathyroid
endocrine gland	gonadotropin	pineal gland
endocrinology	hormone	pituitary gland
erythropoietin	hypothalamus	prostaglandin
exocrine gland	insulin	thymus
gland	islets of Langerhans	thyroid gland

Acronyms
ACTH	GRH	PRH
ANf	HCG	PRL
ANP	hGH	PTH
CRH	ICSH	T3
FSH	LH	T4
GH	LT	TRH
GHIH	MIF	TSH
GHRH	MSH
GnRH	PIH/DA

The endocrine system controls body processes via chemical substances. Most of these substances are secreted in glands. Endocrine glands are located throughout the body and each contains a group of specialized cells that secrete hormones in response to body signals. (The term, hormone, means to excite or to spur on.) Endocrine glands themselves usually neither need nor use the hormones they produce.

Endocrinology is the specialty that studies the endocrine glands, their secretions, and related disorders.

Exocrine glands also secrete special substances into ducts that open onto the body’s external or internal surfaces. Exocrine glands usually secrete substances that serve a protective or functional purpose, not always 100% hormones. (These substances may contain hormones in conjunction with other materials.) Exocrine glands include sweat glands, mammary glands, mucous membranes, salivary glands, and lacrimal (tear) glands. Examples of exocrine secretions are sweat, milk, bile, tears, and pancreatic fluid. Certain glands can perform both endocrine and exocrine functions. Table 20-1 compares endocrine and exocrine glands.

TABLE 20-1. Comparison of Endocrine and Exocrine Glands

GLAND	DEFINITION	ACTION	FUNCTIONS OF SECRETIONS	EXAMPLES
Endocrine	Ductless glands; glands of internal secretion; vascular; usually contain vacuoles or granules to store hormones	Secrete hormones into circulation	Regulatory	Insulin, adrenocorticotropic hormone (ACTH)
Exocrine	Secrete into a duct; glands of external secretion; less vascular than endocrine glands	Secrete substances directly into duct or body opening	Protective, functional	Digestive juices, tears, sweat, saliva

Endocrine glands (ductless glands, glands of internal secretion) secrete hormones directly into the bloodstream, where they are transported throughout the body. These hormones act on remote tissues (called target tissues) via endocrine signaling. The major portion of this topic discusses endocrine glands.

Hormones are chemical messengers that unlock, initiate, regulate, integrate, and/or coordinate body activities. They speed up or slow down activities of entire body organs or systems. Some hormones affect the rate of various activities of individual cells. Hormones also affect one another. Too much or too little of a particular hormone interferes with or counteracts the actions of other hormones. Some glandular secretions signal each other in sequence. This is known as a hormonal axis. An example of a hormonal axis is the hypothalamic s pituitary s adrenal axis.

Hormones may be produced in response to nervous stimulation, the level of specific substances in the blood, or other hormones. This topic discusses specific hormones and the glands or structures that secrete them.

STRUCTURE AND FUNCTION

The endocrine system provides a mechanism for the regulation, integration, and coordination of all body cells, organs, and systems. The main functions of the endocrine system are regulation of growth, maturation, metabolism, and reproduction. Box 20-1 summarizes these functions.

The glands of the endocrine system include the pituitary, thyroid, parathyroid, adrenal, and pineal. In addition, several organs that are not exclusively endocrine glands also contain cells that secrete hormones. Four such organs are the hypothalamus, gonads, pancreas, and thymus. Figure 20-1 illustrates the shapes and locations of specific endocrine structures. In addition, specialized hormones are secreted in such diverse organs of the body as the gastrointestinal tract, the kidneys, and the heart. The following sections describe the more specific locations of these glands and organs, the hormones they produce, and hormonal actions.

Key Concept Functions of the endocrine system include regulation of growth and maturation, metabolism, and reproduction. Genetics determine the basis of a person’s body structure and function. Hormones carry out these genetic instructions.

Pituitary Gland

The pituitary gland, also called the hypophysis, is about the size of a pea. It is located in a saddle-shaped hollow in the sphenoid bone called the sella turcica. (The sphenoid bone is located at the base of the brain’s frontal lobe.) Two parts make up the pituitary gland: the anterior and posterior lobes. These lobes are sometimes classified as two separate glands because their functions and embryonic development are very different.

Role of the Hypothalamus

In the past, theorists believed that the pituitary itself secreted hormones needed for all vital body functions. However, research has shown that “pituitary hormones” are either secreted by—or their secretion is directly controlled by— the hypothalamus.

The hypothalamus is a tiny but complex portion of the brain, attached to the pituitary by means of the infundibular (hypophyseal) stalk. The hypothalamus is considered the “master controller” or the “master gland.” Specialized cells in the hypothalamus release hormones that either inhibit release or promote release of other hormones from the anterior lobe of the pituitary. These hypothalamic hormones are described as releasing hormones or inhibiting hormones (Table 20-2). Figure 20-2 illustrates the role of the hypothalamus in regard to the pituitary gland; the secretion of pituitary hormones; and the organs affected by pituitary hormones.

BOX 20-1.

Functions of the Endocrine System

Growth and Maturation

♦    Regulates growth and maturation

♦    Regulates body’s response to stress

Metabolism

♦    Regulates metabolism

♦    Regulates absorption of nutrients

♦    Regulates use of glucose in cellular respiration

♦    Maintains body pH by maintaining fluid and electrolyte concentrations

Reproduction

♦    Produces sexual characteristics

♦    Controls reproductive and birth processes

♦    Activates lactation

♦    Influences sexual response

FIGURE 20-1 ● Location of the major endocrine glands in the body.

Anterior Lobe

The anterior lobe of the pituitary, also called the adenohypophysis,releases several hormones (Table 20-3). Many of these hormones are called glycoproteins because they are made of carbohydrates and proteins. The hypothalamus controls the adenohypophysis; therefore, neural commands release these hormones. Five of these hormones (the tropic hormones) control the growth, development, and proper functioning of other endocrine glands.

Tropic Hormones. Corticotropin-releasing hormone (CRH) from the parvocellular neurosecretory neurons (corticotropes) of the hypothalamus causes the release of adrenocorticotropic hormone (ACTH or corticotropin) from the anterior lobe of the pituitary gland. ACTH stimulates the adrenal cortex to produce glucocorticoids and androgens (corticosteroids)— such as cortisol—which are vital in metabolizing carbohydrates. ACTH also has melanocyte-stimulating properties that can increase skin pigmentation.

Thyrotropin-releasing hormone (TRH), also from the parvocellular neurosecretory neurons (thyrotropes) of the hypothalamus, causes release of thyroid-stimulating hormone (TSH or thyrotropin) from the pituitary, as well as the release of prolactin. TSH stimulates the thyroid gland to produce and to secrete thyroxine (T4) and triiodothyronine (T3) (discussed later in this topic). The hypothalamus also functions to inhibit TSH and GH, by releasing growth hormone inhibiting hormone (GHIH).

TABLE 20-2. Major Hypothalamus Hormones Affecting Hormone Secretion From the Pituitary Gland

RELEASING HORMONES (STIMULATING HORMONES) FROM HYPOTHALAMUS	INHIBITING HORMONES FROM HYPOTHALAMUS	PITUITARY HORMONES STIMULATED OR INHIBITED
Corticotropin-releasing hormone (CRH)	Melanocyte-inhibiting factor (MIF)	Adrenocorticotropic hormone (ACTH) Melanocyte-stimulating hormone (MSH)
Growth hormone-releasing hormone (GRH or GHRH)	Growth hormone-inhibiting hormone (GHIH) or somatostatin	Growth hormone (GH) or human growth hormone (hGH) or somatotropin
Thyrotropin-releasing hormone (TRH)	Growth hormone-inhibiting hormone (GHIH) or somatostatin (SS)	Growth hormone (GH), thyroid-stimulating hormone (TSH) or thyrotropin; (minor effect—stimulates prolactin release)
Prolactin-releasing hormone (PRH)	Prolactin-inhibiting hormone (PIH) or dopamine (DA)	Inhibits release of prolactin (PRL) and thyroid-stimulating hormone (TSH)
Gonadotropin-releasing hormone (GnRH)		Interstitial cell-stimulating hormone (ICSH) in men or luteinizing hormone (LH) in women Follicle-stimulating hormone (FSH)

FIGURE 20-2 · The pituitary gland, the relationship of the hypothalamus to pituitary action, and the hormones secreted by the anterior; middle, and posterior pituitary lobes.

The hormone known as growth hormone (GH), human growth hormone (hGH), or somatotropin is produced by somatotropic cells and released from the anterior pituitary. This hormone is stimulated by the release of growth hormone-releasing hormone (GRH or GHRH) by the hypothalamus. GH stimulates growth and cell reproduction in all body tissues. It assists with the movement of amino acids into tissue cells and the transformation of amino acids into proteins that the body needs. It aids in the release of fatty acids from adipose (fat) tissue so that they can be used for energy. GH helps to regulate blood nutrient levels after eating and during periods of fasting. GH also stimulates the release of insulin-like growth factor 1 from the liver. When sufficient amounts of GH and TSH have been released, the hypothalamus secretes GHIH (discussed in the previous paragraph) to inhibit further release of growth hormone.

Gonadotropin-releasing hormone (GnRH) secreted in the neuroendocrine cells of the preoptic area causes the anterior pituitary to secrete two hormones called gonadotropins that stimulate the sex glands (gonads) in the body. These two gonadotropic hormones are follicle-stimulating hormone and luteinizing hormone:

•    Follicle-stimulating hormone (FSH), produced in gonadotropic cells, stimulates both the growth and secretion of ovarian follicles in women and the production of sperm in men.

•    Luteinizing hormone (LH), also produced in gonadotropic cells, in women stimulates ovulation and the formulation of the corpus luteum. In men, LH stimulates the production of testosterone and may be called interstitial cell-stimulating hormone (ICSH). ICSH influences the secretion of testosterone and other sex hormones from specialized areas in the testes. Luteinizing hormone/ICSH and FSH are known as gonadotropic hormones because they influence the gonads (the reproductive organs).

Prolactin. Prolactin (PRL), stimulated by prolactin-releasing hormone (PRH), and secreted by lactotropic cells, is a hormone secreted by the anterior lobe of the pituitary that stimulates milk production in women following pregnancy. Men secrete PRL as well; however, its function in men is not yet understood. Inhibition of prolactin and TSH occurs because of the hypothalamus’ secretion in its arcuate nucleus of prolactin-inhibiting hormone or dopamine (PIH/DA).

TABLE 20-3. Important Secretions of the Endocrine System

GLAND/ORGAN	HORMONE(S) SECRETED/RELEASED	ACTIONS
Pituitary Anterior lobe (adenohypophysis)	Adrenocorticotropic hormone (ACTH—corticotropin)—stimulated by CRH from hypothalamus	Stimulates adrenal cortex to produce cortisol, corticosteroids, and androgens; can stimulate melanocytes
	Growth hormone (GH or hGH), somatotropic hormone—stimulated by GRH, GHRH from hypothalamus	Controls bone and tissue growth and regulates metabolism (influences secretion of insulin-like growth factor I from liver)
	Thyroid-stimulating hormone (TSH), thyrotropin— stimulated by TRH from hypothalamus	Regulates thyroid hormone (via secretion of thyroxine [T4] and triiodothyronine [T3])
	Follicle-stimulating hormone (FSH)—stimulated by GnRH	Stimulates growth and secretion of eggs in ovaries (female) and sperm in testes (male)
	Luteinizing hormone (LH) (in females)	Helps control ovulation and menstruation; important in sustaining pregnancy
	Prolactin, lactogenic hormone (PRL)—stimulated by PRH from hypothalamus	Stimulates mammary glands to produce milk (after pregnancy); i nfluences sexual gratification
	LH (in males) Interstitial cell-stimulating hormone (ICSH) (males) Lipotropin	Stimulates Leydig cells to produce testosterone; stimulates secretion of male hormones (androgens) Influences breakdown of lipids (fats), production of steroids, and melanin production
Posterior lobe (neurohypophysis)	Oxytocin	Causes uterine contractions; contractions of cervix and vagina; i nfluences orgasm; stimulates milk production;
Stores hormones only—no production	Vasopressin (antidiuretic hormone—ADH)	Raises blood pressure (some vasoconstriction); promotes water reabsorption in kidney tubules; influences uterus
Middle lobe (intermediate pituitary lobe)	Melanocyte-stimulating hormone (MSH)—stimu-lated by CRH from hypothalamus	Increases skin and hair pigmentation
Thyroid	Thyroxine (tetraiodothyronine—T4) Triiodothyronine (T3)	Regulates body metabolism (requires iodine) and growth and development; affects protein synthesis; affects sensitivity to catecholamines
	Calcitonin (CT—thyrocalcitonin)	Stimulates calcium to leave plasma and allows it to enter bones Speeds calcium absorption from blood; promotes calcium deposit in bone; inhibits osteoclasts, thereby promoting bone formation Stimulates bone to release calcium into blood; regulates phosphorus balance; assists in reabsorption of magnesium
Parathyroid	Parathormone/Parathyroid hormone (PTH)	Promotes formation of calcitriol and assists in release of calcium, magnesium, and phosphorus into blood; activates vitamin D
Adrenals (Suprarenafs) Adrenal medulla	Catecholamines Epinephrine (adrenaline) Norepinephrine	Mimics actions of sympathetic nervous system, adaptation to stress; causes many body processes to speed up, especially in an emergency ("fight or flight” response); suppresses nonemergency functions; suppresses immune system
Adrenal cortex	Corticosteroids/corticoids Mineralocorticoids (mainly aldosterone)	Increases blood volume by reabsorption of sodium and secretion of potassium and hydrogen by kidneys; regulates electrolyte levels in extracellular fluid
	Glucocorticoids (mostly cortisol)	Influence glucose, amino acid, and fat synthesis in metabolism; decrease inflammatory responses and promote immunosuppression
	Male sex hormones Androgens (males)—including dehy-droepiandrosterone (DHEA) and testosterone Female sex hormones (estrogens)—very small amount	Produce male sex characteristics (anabolic steroids—develop muscle mass and strength, increase bone mass and strength) Produce female sex characteristics
	Dopamine Enkephalins	Increases heart rate and blood pressure Regulate pain, mood, behavior; serve as neurotransmitters
Gonads Testes (male)	Testosterone	Develops male sex characteristics (also influenced by androgens)

TABLE 20-3. Important Secretions of the Endocrine System Continued

GLAND/ORGAN	HORMONE(S) SECRETED/RELEASED	ACTIONS
Ovaries (female)— from ovarian follicle of corpus luteum	Estrogen and progestins (progesterone is the primary progestin)	Regulate female sex characteristics, functions, menstruation, allow sperm penetration, maintain pregnancy (inhibit premature onset of labor, suppress lactation, inhibit immune response toward embryo, anti-inflammatory)
	Progesterone	Reduces gall bladder activity, regulates levels of certain minerals, assists thyroid function, promotes healing, promotes nerve functioning, prevents endometrial cancer in women
	Estradiol	Prevents apoptosis (destruction) of germ cells, increases liver function, promotes blood coagulation, assists in fluid and electrolyte balance
	Inhibin	Inhibits FSH production
Pancreas Alpha cells (islets)	Glucagon	Speeds glycogenolysis; raises blood sugar; stimulates breakdown of fats and proteins
Beta cells (islets)	Insulin	Regulator of carbohydrate, protein, and fat metabolism. Enables cells to use glucose; lowers blood sugar; facilitates synthesis of triglycerides; suppresses exocrine secretions of pancreas
	Amylin	Helps regulate glucose balance; sends satiety signals to brain
Delta cells (islets)	Somatostatin	Inhibits release of insulin and glucagon; lowers rate of gastric emptying, reduces smooth muscle contractions and intestinal blood flow
F cells (islets—“PP cells")	Pancreatic polypeptide	Inhibits secretion of somatostatin and pancreatic digestive enzymes
Thymus	Thymosin (thymic hormone)	Stimulates production ofT cells for cellular immunity
Pineal body	Melatonin (an antioxidant)	Regulates sleep-wake cycles; may play a role in influencing reproductive processes
Heart	Atrial-natriuretic peptide (ANP), atrial-natriuretic factor (ANf) Brain-natriuretic peptide (BNP)	Reduces blood pressure by decreasing vascular resistance and fluid volume; influences balance of sodium and fats in blood Influences lowering of blood pressure
Liver	Thrombopoietin Insulin-like growth factor (somatomedin) Angiotensin and angiotensinogen	Stimulates platelet production Regulates cell growth and development; also has insulin-like effects Vasoconstriction; influence release of aldosterone from adrenal cortex
Kidney	Renin Erythropoietin (EPO) Calcitriol Thrombopoietin	Activates renin-angiotensin system by stimulating production of angiotensin I and angiotensinogen Stimulates production of erythrocytes (red blood cells [RBCs]) Increases calcium and phosphate absorption, inhibits release of parathyroid hormone (PTH) Stimulates platelet production by the megakaryocytes
Stomach and Small Intestine	Gastrin and histamine	Stimulate secretion of gastric acid
	Ghrelin	Stimulates appetite; stimulates secretion of GH
	Neuropeptide Y (NPY)	Increases food intake; decreases physical activity; decreases secretion of bicarbonate
	Secretin and pancreozymin	Enhance effects of cholecystokinin (CCK); stop production of gastric juice; stimulate pancreas to release pancreatic juice. Stimulate secretion of bicarbonate from liver; pancreas, and duodenum (Brunner’s glands)
	Somatostatin	Suppresses release of gastrin, cholecystokinin (CCK), secretin, and other substances; reduces rate of gastric emptying; reduces smooth muscle contractions and intestinal blood flow
	Histamine Endothelin	Stimulates gastric acid secretion Influences smooth muscle contractions in stomach
Duodenum	Cholecystokinin (CCK)	Stimulates release of digestive enzymes from pancreas, release of bile from gall bladder; suppresses hunger
Ilium and colon	Human incretin hormone (glucagon-like peptide-I)	Influences secretion of insulin by pancreas
Striated Muscle	Thrombopoietin	Stimulates megakarocytes to produce platelets
Adipose Tissue (fat)	Leptin	Decreases appetite; increases metabolism

The placenta serves as an endocrine gland during pregnancy.

Lipotropin. Lipotropin (also produced by corticotropes) influences lipolysis (breaking down of fats) and steroidogenesis (production of steroids, as in the adrenals), and stimulates the melanocytes to produce melanin.