Terms in this set (44)
acts as a means of internal communication, coordinating the activities of
the organ systems.
synthesize and secrete chemical substances called hormones
directly into the circulatory system.
such as the gallbladder, secrete
substances transported by ducts.
In response to stress, adrenocorticotropic hormone (ACTH), which is produced by the anterior pituitary, stimulates the adrenal cortex to produce more than two dozen different steroid hormones, collectively
known as adrenocortical steroids, or simply corticosteroids. In the bloodstream, these corticosteroids
are bound to transport proteins called transcortins. Corticosteroids exert their mechanism of action by determining which genes are transcribed in the nuclei of their target cells and at what rate. The
subsequent changes in the nature and concentration of the enzymes produced in the target cells will
affect cellular metabolism.
three major classes of corticosteroids
Glucocorticoids; Mineralocorticoids; Cortical sex hormones
Glucocorticoids, such as cortisol and cortisone, are involved in glucose
regulation and protein metabolism. Glucocorticoids raise blood glucose levels by promoting protein breakdown and gluconeogenesis and decreasing protein synthesis. Glucocorticoids
increase the plasma glucose levels and are antagonistic to the effects of insulin. Glucocorticoids
release amino acids from skeletal muscle as well as lipids from adipose tissue. They also
promote peripheral use of lipids and have anti-inflammatory effects.
Mineralocorticoids, particularly aldosterone, regulate plasma levels of
sodium and potassium and, consequently, the total extracellular water volume. Aldosterone
causes active reabsorption of sodium and passive reabsorption of water in the nephron of the kidney. This results in an increase in both blood volume and blood pressure. Excess production of
aldosterone results in excess retention of water with resulting hypertension (high blood pressure).
The mineralocorticoids are stimulated by angiotensin II and inhibited by ANP (atrial natriuretic peptide). See Chapter 24, Excretory System, for more information on the action of aldosterone.
Cortical sex hormones:
The adrenal cortex secretes small quantities of androgens (male sex
hormones) like androstenedione and dehydroepiandrosterone in both men and women. In men, most of the androgens are produced by the testes, so the physiologic effect of the adrenal androgens is quite small. In women, however, overproduction of the adrenal androgens may have masculinizing effects, such as excessive facial hair.
The adrenal medulla produces epinephrine (adrenaline) and norepinephrine (noradrenaline), both
of which belong to a class of amino acid-derived compounds called catecholamines.
Epinephrine increases the conversion of glycogen to glucose in liver and muscle tissue, causing an
increase in blood glucose levels and an increase in the basal metabolic rate. Both epinephrine and
norepinephrine increase the rate and strength of the heartbeat and dilate and constrict blood vessels in such a way as to increase the blood supply to the skeletal muscles, heart, and brain, while decreasing
the blood supply to the kidneys, skin, and digestive tract. Both epinephrine and norepinephrine will
also promote the release of lipids by adipose tissue. These effects are known as the "fight or flight response" and are elicited by sympathetic nervous stimulation in response to stress.
Epinephrine will inhibit certain vegetative functions, such as digestion, which are not immediately important for survival. Both of these hormones are also neurotransmitters, proteins used by neurons
to transmit signals. The release of these hormones is stimulated during sympathetic activation by sympathetic preganglionic fibers.
The pituitary (hypophysis) is a small, trilobed gland at the base of the brain. The two main lobes,
anterior and posterior, are functionally distinct. (In humans, the third lobe, the intermediate lobe, is rudimentary.) Specifically, the pituitary gland hangs below the hypothalamus and is connected to it by a slender cord known as the infundibulum.
The anterior pituitary synthesizes both direct hormones, which directly stimulate their target organs, and tropic hormones, which stimulate other endocrine glands to release hormones. The hormonal
secretions of the anterior pituitary are regulated by hypothalamic secretions called releasing/ inhibiting hormones or factors.
direct hormones of the anterior pituitary
Growth hormone (GH, somatotropin):
GH promotes bone and muscle growth. GH also
promotes protein synthesis and lipid mobilization and catabolism. In children, a GH deficiency can lead to stunted growth (dwarfism), while overproduction of GH results in gigantism. Overproduction of GH in adults causes acromegaly, a disorder characterized by a disproportionate overgrowth of bone, localized especially in the skull, jaw, feet, and
Prolactin stimulates milk production and secretion in female mammary glands.
The tropic hormones
Adrenocorticotropic hormone (ACTH):
ACTH stimulates the adrenal cortex to synthesize
and secrete glucocorticoids and is regulated by the releasing hormone corticotropin-releasing
Thyroid-stimulating hormone (TSH):
TSH stimulates the thyroid gland to synthesize and
release thyroid hormones, including thyroxine.
Luteinizing hormone (LH):
In women, LH stimulates ovulation and formation of the corpus luteum. LH is also responsible for regulating progesterone secretion in women. In men, LH stimulates the interstitial cells of the testes to synthesize testosterone.
Follicle-stimulating hormone (FSH):
In women, FSH causes maturation of ovarian follicles that begin secreting estrogen; in men, FSH stimulates maturation of the seminiferous tubules
and sperm production.
Melanocyte-stimulating hormone (MSH):
MSH is secreted by the intermediate lobe of the
pituitary. In mammals, the function of MSH is unclear, but in frogs, MSH causes darkening
of the skin via induced dispersion of molecules of pigment in melanophore cells.
The posterior pituitary (neurohypophysis) does not synthesize hormones; it stores and releases the
peptide hormones oxytocin and antidiuretic hormone, which are produced by the neurosecretory
cells of the hypothalamus. Hormone secretion is stimulated by action potentials descending from the
Oxytocin, which is secreted during childbirth, increases the strength and frequency
of uterine muscle contractions. Oxytocin secretion is also induced by suckling; oxytocin
stimulates milk secretion in the mammary glands.
Antidiuretic hormone (ADH; vasopressin):
ADH increases the permeability of the
nephron's collecting duct to water, thereby promoting water reabsorption and increasing blood volume, which subsequently increases blood pressure. ADH is secreted when plasma osmolarity increases, as sensed by osmoreceptors in the hypothalamus, or when blood volume decreases, as sensed by baroreceptors in the circulatory system.
The hypothalamus is part of the forebrain and is located directly above the pituitary gland. The
hypothalamus receives neural transmissions from other parts of the brain and from peripheral
nerves that trigger specific responses from its neurosecretory cells. The neurosecretory cells regulate pituitary gland secretions via negative feedback mechanisms and through the actions of inhibiting
and releasing hormones.
Interactions with the anterior pituitary
Hypothalamic-releasing hormones stimulate or inhibit the secretions of the anterior pituitary. For
example, GnRH stimulates the anterior pituitary to secrete FSH and LH. Releasing hormones are secreted into the hypothalamic-hypophyseal portal system. In this circulatory pathway, blood from
the capillary bed in the hypothalamus flows through a portal vein into the anterior pituitary, where it diverges into a second capillary network. In this way, releasing hormones can immediately reach the
A complicated feedback system regulates the secretions of the endocrine system. For example, when the plasma levels of adrenal cortical hormones drop, hypothalamic cells (via a negative feedback
mechanism) release ACTH-releasing factor (ACTH-RF) into the portal system. When the plasma
concentration of corticosteroids exceeds the inhibitory effect on the hypothalamus.
Interactions with the posterior pituitary
Neurosecretory cells in the hypothalamus synthesize both oxytocin and ADH and transport them via their axons into the posterior pituitary for storage and secretion.
Thyroid hormones affect the function of nearly every organ system in the body. In children, thyroid
hormones are essential for growth and neurological development; in adults, thyroid hormones are
essential for maintenance of metabolic stability. They increase the rate of metabolism throughout
Thyroid hormones (thyroxine and triiodothyronine)
The thyroid hormones, thyroxine (T4) and triiodothyronine (T3), are formed from the glycoprotein thyroglobulin, which is synthesized in thyroid cells. Because of the specific tertiary structure of this glycoprotein, iodinated tyrosine residues present in thyroglobulin are able to bind together to form
active thyroid hormones. The thyroid hormones possess the following characteristics:
● T3 is five times more potent than T4.
● T4 and T3 are transported via plasma proteins. Approximately 99.5% of these hormones are bound to proteins, but only an unbound hormone is able to enter a cell and elicit a cellular
● All of the T4 in the body is formed and secreted by the thyroid gland; however, only 20% of
T3 is produced by the thyroid gland.
● The majority of T3 is produced by the conversion of T4 to T3 by the enzyme 5′-monodeiodase,
found primarily in the peripheral tissues.
thyroid hormones are undersecreted or not secreted at all. Common symptoms of hypothyroidism include a slowed heart rate and respiratory rate, fatigue, cold intolerance, and
weight gain. Hypothyroidism in newborn infants, called cretinism, is characterized by mental retardation and short stature.
In hyperthyroidism, the thyroid is overstimulated, resulting in the oversecretion of thyroid hormones. Symptoms often include increased metabolic rate, feelings of
excessive warmth, profuse sweating, palpitations, weight loss, and protruding eyes. In both disorders,
the thyroid often enlarges, forming a bulge in the neck called a goiter.
Hypothyroidism is often treated with
Hypothyroidism is often treated with supplementation of thyroid hormones via synthetic or animal-
Hyperthyroidism can be treated by
Hyperthyroidism can be treated by antithyroid medications that suppress the
thyroid's release of excess hormone or ablation of the thyroid with radiotherapy. After ablation the thyroid no longer produces thyroid hormone, and the patient must take thyroid supplementation for
the rest of his or her life.
Calcitonin decreases plasma Ca2+ concentration by inhibiting the release of Ca2+ from bone. Calcitonin secretion is regulated by plasma Ca2+ levels. Calcitonin is antagonistic to parathyroid hormone.
The pancreas is both an exocrine organ and an endocrine organ. The exocrine function is performed by the cells that secrete digestive enzymes into the small intestine via a series of ducts. The endocrine
function is performed by small glandular structures called the islets of Langerhans, which are
composed of alpha and beta cells. Alpha cells produce and secrete glucagon; beta cells produce and secrete insulin.
The endocrine hormones secreted by the pancreas include the following:
Glucagon stimulates protein and fat degradation, the conversion of glycogen to
glucose, and gluconeogenesis, all of which serve to increase blood glucose levels. Glucagon's actions are largely antagonistic to those of insulin.
Insulin is a protein hormone secreted in response to a high blood glucose concentration. It stimulates the uptake of glucose by muscle and adipose cells and the
storage of glucose as glycogen in muscle and liver cells, thus lowering blood glucose levels.
It also stimulates the synthesis of fats from glucose and the uptake of amino acids. Insulin's
actions are antagonistic to those of glucagon and the glucocorticoids. Underproduction
of insulin, or insensitivity to insulin, leads to diabetes mellitus, which is characterized by
hyperglycemia (high blood glucose levels). Diabetes is the most common endocrine disorder and, if improperly managed, is characterized by long-term complications involving the eyes,
nerves, kidneys, and blood vessels. Table 25.1 identifies the distinguishing characteristics of
Type I and Type II diabetes.
The parathyroid glands are four small, pea-shaped structures embedded in the posterior surface of the
thyroid. These glands synthesize and secrete parathyroid hormone (PTH), which regulates plasma Ca2+ concentration. PTH raises the Ca2+ concentration in the blood by stimulating Ca2+ release from
the bone and decreasing Ca2+ excretion in the kidneys. Calcium in bone is bonded to phosphate, and breakdown of the bone releases phosphate as well as calcium. Parathyroid hormone compensates for
this by stimulating excretion of phosphate by the kidneys.
When blood volume falls, the kidneys produce renin—an enzyme that converts the plasma protein angiotensinogen to angiotensin I. Angiotensin I is converted to angiotensin II, which stimulates the
adrenal cortex to secrete aldosterone. Aldosterone helps restore blood volume by increasing sodium reabsorption at the kidney, leading to an increase in water. This removes the initial stimulus for renin
production. The kidneys also produce erythropoietin (EPO). EPO is a glycoprotein that stimulates red blood cell production; it is normally produced in the kidneys. This hormone causes the following:
● stimulation of the stem cells to differentiate into rubriblasts (least mature erythrocytes)
● increased rate of mitosis
● increased release of reticulocytes from the bone marrow
● increased hemoglobin (HgB) formation, which creates the critical HgB concentration necessary for maturity to be reached at a more rapid rate
Ingested food stimulates the stomach to release the hormone gastrin. Gastrin is carried to the
gastric glands and stimulates the glands to secrete HCl in response to food in the stomach. Secretion of pancreatic juice, the exocrine product of the pancreas, is also under hormonal control; the hormone secretin is released by the small intestine when acidic food material enters from the
stomach. Secretin stimulates the secretion of an alkaline bicarbonate solution from the pancreas that
neutralizes the acidity of the chyme (partially digested food coming from the stomach). The hormone cholecystokinin is released from the small intestine in response to the presence of fats and causes the
contraction of the gallbladder and release of bile into the small intestine. Bile, which is not a hormone,
is involved in the emulsification and digestion of fats.
The pineal gland is a tiny structure at the base of the brain that secretes the hormone melatonin. The role of melatonin in humans is unclear, but it is believed to play a role in the regulation of circadian rhythms—physiological cycles lasting approximately 24 hours. Melatonin secretion is regulated by
light and dark cycles in the environment. In primitive vertebrates, melatonin lightens the skin by concentrating pigment granules in melanophores (melatonin is an antagonist to MSH).
Peptide hormones range from simple short peptides (amino acid chains), such as ADH, to complex polypeptides, such as insulin. Peptide hormones act as first messengers. When they bind to specific
receptors on the surface of their target cells, they trigger a series of enzymatic reactions within each cell, the first of which may be the conversion of ATP to cyclic adenosine monophosphate (cyclic
AMP); this reaction is catalyzed by the membrane-bound enzyme adenylate cyclase. Cyclic AMP acts
as a second messenger, relaying messages from the extracellular peptide hormone to cytoplasmic enzymes and initiating a series of successive reactions in the cell. This is an example of a cascade effect; with each step, the hormone's effects are amplified. Cyclic AMP activity is inactivated by the
cytoplasmic enzyme phosphodiesterase.
Steroid hormones, such as estrogen and aldosterone, belong to a class of lipid-derived molecules with a characteristic ring structure. They are produced by the testes, ovaries, placenta, and adrenal cortex.
Because they are lipid soluble, steroid hormones cross the phospholipid bilayer and enter their target
cells directly in order to bind to specific receptor proteins in the cytoplasm. This receptor-hormone
complex enters the nucleus and directly activates the expression of specific genes by binding to receptors on the chromatin. This induces a change in mRNA transcription and protein synthesis.
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