HAP II Endocrine System Review

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Chapter Review

Review

Introduction

1.Hormones regulate the activity of smooth muscle, cardiac muscle, and some glands; alter metabolism; spur growth and development; influence reproductive processes; and participate in circadian (daily) rhythms.

18.1 Comparison of Control by the Nervous and Endocrine Systems

1.The nervous system controls homeostasis through nerve impulses and neurotransmitters, which act locally and quickly. The endocrine system uses hormones, which act more slowly in distant parts of the body. (See Table 18.1.)

2.The nervous system controls neurons, muscle cells, and glandular cells; the endocrine system regulates virtually all body cells.

18.2 Endocrine Glands

1.Exocrine glands (sudoriferous, sebaceous, mucous, and digestive) secrete their products through ducts into body cavities or onto body surfaces. Endocrine glands secrete hormones into interstitial fluid. Then, the hormones diffuse into the blood.

2.The endocrine system consists of endocrine glands (pituitary, thyroid, parathyroid, adrenal, and pineal glands) and other hormone-secreting tissues (hypothalamus, thymus, pancreas, ovaries, testes, kidneys, stomach, liver, small intestine, skin, heart, adipose tissue, and placenta).

18.3 Hormone Activity

1.Hormones affect only specific target cells that have receptors to recognize (bind) a given hormone. The number of hormone receptors may decrease (down-regulation) or increase (up-regulation).

2.Circulating hormones enter the bloodstream; local hormones (paracrines and autocrines) act locally on neighboring cells.

3.Chemically, hormones are either lipid-soluble (steroids, thyroid hormones, and nitric oxide) or water-soluble (amines; peptides, proteins, and glycoproteins; and eicosanoids). (See Table 18.2.)

4.Water-soluble hormone molecules circulate in the watery blood plasma in a “free” form (not attached to plasma proteins); most lipid-soluble hormones are bound to transport proteins synthesized by the liver.

18.4 Mechanisms of Hormone Action

1.Lipid-soluble steroid hormones and thyroid hormones affect cell function by altering gene expression.

2.Water-soluble hormones alter cell function by activating plasma membrane receptors, which elicit production of a second messenger that activates various enzymes inside the cell.

3.Hormonal interactions can have three types of effects: permissive, synergistic, or antagonistic.

18.5 Homeostatic Control of Hormone Secretion

1.Hormone secretion is controlled by signals from the nervous system, chemical changes in blood, and other hormones.

2.Negative feedback systems regulate the secretion of many hormones.

18.6 Hypothalamus and Pituitary Gland

1.The hypothalamus is the major integrating link between the nervous and endocrine systems. The hypothalamus and pituitary gland regulate virtually all aspects of growth, development, metabolism, and homeostasis. The pituitary gland is located in the hypophyseal fossa and is divided into two main portions: the anterior pituitary (glandular portion) and the posterior pituitary (nervous portion).

2.Secretion of anterior pituitary hormones is stimulated by releasing hormones and suppressed by inhibiting hormones from the hypothalamus.

3.The blood supply to the anterior pituitary is from the superior hypophyseal arteries. Hypothalamic releasing and inhibiting hormones enter the primary plexus and flow to the secondary plexus in the anterior pituitary by the hypophyseal portal veins.

4.The anterior pituitary consists of somatotrophs that produce growth hormone (GH), lactotrophs that produce prolactin (PRL), corticotrophs that secrete adrenocorticotropic hormone (ACTH) and melanocyte-stimulating hormone (MSH), thyrotrophs that secrete thyroid-stimulating hormone (TSH), and gonadotrophs that synthesize follicle-stimulating hormone (FSH) and luteinizing hormone (LH). (See Tables 18.3 and 18.4.)

5.Growth hormone (GH) stimulates body growth through insulinlike growth factors (IGFs). Secretion of GH is inhibited by GHIH (growth hormone–inhibiting hormone, or somatostatin) and promoted by GHRH (growth hormone–releasing hormone).

6.TSH regulates thyroid gland activities. Its secretion is stimulated by TRH (thyrotropin-releasing hormone) and suppressed by GHIH.

7.FSH and LH regulate the activities of the gonads—ovaries and testes. Their secretion is controlled by GnRH (gonadotropin-releasing hormone).

8.Prolactin (PRL) helps initiate milk secretion. Prolactin-inhibiting hormone (PIH) suppresses secretion of PRL; prolactin-releasing hormone (PRH) stimulates PRL secretion.

9.ACTH regulates the activities of the adrenal cortex and is controlled by CRH (corticotropin-releasing hormone). Dopamine inhibits secretion of MSH.

10.The posterior pituitary contains axon terminals of neurosecretory cells whose cell bodies are in the hypothalamus. Hormones made by the hypothalamus and stored in the posterior pituitary are oxytocin (OT), which stimulates contraction of the uterus and ejection of milk from the breasts, and antidiuretic hormone (ADH), which stimulates water reabsorption by the kidneys and constriction of arterioles. (See Table 18.5.) Oxytocin secretion is stimulated by uterine stretching and suckling during nursing; ADH secretion is controlled by osmotic pressure of the blood and blood volume.

18.7 Thyroid Gland

1.The thyroid gland is located inferior to the larynx.

2.It consists of thyroid follicles composed of follicular cells, which secrete the thyroid hormones thyroxine (T4) and triiodothyronine (T3), and parafollicular cells, which secrete calcitonin (CT).

3.Thyroid hormones are synthesized from iodine and tyrosine within thyroglobulin (TGB). They are transported in the blood bound to plasma proteins, mostly thyroxine-binding globulin (TBG).

4.Secretion is controlled by TRH from the hypothalamus and thyroidstimulating hormone (TSH) from the anterior pituitary.

5.Thyroid hormones regulate oxygen use and metabolic rate, cellular metabolism, and growth and development.

6.Calcitonin (CT) can lower the blood level of calcium ions (Ca2+) and promote deposition of Ca2+ into bone matrix. Secretion of CT is controlled by the Ca2+ level in the blood. (See Table 18.6.)

18.8 Parathyroid Glands

1.The parathyroid glands are embedded in the posterior surfaces of the lateral lobes of the thyroid gland. They consist of chief cells and oxyphil cells.

2.Parathyroid hormone (PTH) regulates the homeostasis of calcium, magnesium, and phosphate ions by increasing blood calcium and magnesium levels and decreasing blood phosphate levels. PTH secretion is controlled by the level of calcium in the blood. (See Table 18.7.)

18.9 Adrenal Glands

1.The adrenal glands are located superior to the kidneys. They consist of an outer adrenal cortex and inner adrenal medulla.

2.The adrenal cortex is divided into a zona glomerulosa, a zona fasciculata, and a zona reticularis; the adrenal medulla consists of chromaffin cells and large blood vessels.

3.Cortical secretions include mineralocorticoids, glucocorticoids, and androgens.

4.Mineralocorticoids (mainly aldosterone) increase sodium and water reabsorption and decrease potassium reabsorption. Secretion is controlled by the renin–angiotensin–aldosterone (RAA) pathway and by K+ level in the blood.

5.Glucocorticoids (mainly cortisol) promote protein breakdown, gluconeogenesis, and lipolysis; help resist stress; and serve as anti-inflammatory substances. Their secretion is controlled by ACTH.

6.Androgens secreted by the adrenal cortex stimulate growth of axillary and pubic hair, aid the prepubertal growth spurt, and contribute to libido.

7.The adrenal medulla secretes epinephrine and norepinephrine (NE), which are released during stress and produce effects similar to sympathetic responses. (See Table 18.8.)

18.10 Pancreatic Islets

1.The pancreas lies in the curve of the duodenum. It has both endocrine and exocrine functions.

2.The endocrine portion consists of pancreatic islets or islets of Langerhans, made up of four types of cells: alpha, beta, delta, and F cells.

3.Alpha cells secrete glucagon, beta cells secrete insulin, delta cells secrete somatostatin, and F cells secrete pancreatic polypeptide.

4.Glucagon increases blood glucose level; insulin decreases blood glucose level. Secretion of both hormones is controlled by the level of glucose in the blood. (See Table 18.9.)

18.11 Ovaries and Testes

1.The ovaries are located in the pelvic cavity and produce estrogens, progesterone, and inhibin. These sex hormones govern the development and maintenance of female secondary sex characteristics, reproductive cycles, pregnancy, lactation, and normal female reproductive functions. (See Table 18.10.)

2.The testes lie inside the scrotum and produce testosterone and inhibin. These sex hormones govern the development and maintenance of male secondary sex characteristics and normal male reproductive functions. (See Table 18.10.)

18.12 Pineal Gland and Thymus

1.The pineal gland is attached to the roof of the third ventricle of the brain. It consists of secretory cells called pinealocytes, neuroglia, and endings of sympathetic postganglionic axons.

2.The pineal gland secretes melatonin, which contributes to setting the body’s biological clock (controlled in the suprachiasmatic nucleus). During sleep, plasma levels of melatonin increase.

3.The thymus secretes several hormones related to immunity.

4.Thymosin, thymic humoral factor (THF), thymic factor (TF), and thymopoietin promote the maturation of T cells.

18.13 Other Endocrine Tissues and Organs, Eicosanoids, and Growth Factors

1.Body tissues other than those normally classified as endocrine glands contain endocrine tissue and secrete hormones; they include the gastrointestinal tract, placenta, kidneys, skin, and heart. (See Table 18.11.)

2.Prostaglandins and leukotrienes are eicosanoids that act as local hormones in most body tissues.

3.Growth factors are local hormones that stimulate cell growth and division. (See Table 18.12.)

18.14 The Stress Response

1.Productive stress is termed eustress, and harmful stress is termed distress.

2.If stress is extreme, it triggers the stress response (general adaptation syndrome), which occurs in three stages: the fight-or-flight response, resistance reaction, and exhaustion.

3.The stimuli that produce the stress response are called stressors. Stressors include surgery, poisons, infections, fever, and strong emotional responses.

4.The fight-or-flight response is initiated by nerve impulses from the hypothalamus to the sympathetic division of the autonomic nervous system and the adrenal medulla. This response rapidly increases circulation, promotes ATP production, and decreases nonessential activities.

5.The resistance reaction is initiated by releasing hormones secreted by the hypothalamus, most importantly CRH, TRH, and GHRH. Resistance reactions are longer lasting and accelerate breakdown reactions to provide ATP for counteracting stress.

6.Exhaustion results from depletion of body resources during the resistance stage.

7.Stress may trigger certain diseases by inhibiting the immune system. An important link between stress and immunity is interleukin-l, produced by macrophages; it stimulates secretion of ACTH.

18.15 Development of the Endocrine System

1.The development of the endocrine system is not as localized as in other systems because endocrine organs develop in widely separated parts of the embryo.

2.The pituitary gland, adrenal medulla, and pineal gland develop from ectoderm; the adrenal cortex develops from mesoderm; and the thyroid gland, parathyroid glands, pancreas, and thymus develop from endoderm.

18.16 Aging and the Endocrine System

1.Although some endocrine glands shrink as we get older, their performance may or may not be compromised.

2.Production of human growth hormone, thyroid hormones, cortisol, aldosterone, and estrogens decreases with advancing age.

3.With aging, the blood levels of TSH, LH, FSH, and PTH rise.

4.The pancreas releases insulin more slowly with age, and receptor sensitivity to glucose declines.

5.After puberty, thymus size begins to decrease, and thymic tissue is replaced by adipose and areolar connective tissue.

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