BIO100 Chptr 13 Endocrine System Flashcards
1
Q
cort-
A
cort-, bark, rind: adrenal cortex—outer portion of an adrenal gland.
2
Q
-crin
A
-crin, to secrete: endocrine—internal secretion.
3
Q
secrete:
A
(of a cell, gland, or organ) produce and discharge (a substance).
4
Q
diure-
A
diure-, to pass urine: diuretic—substance that promotes urine production.
5
Q
endo-
A
endo-, inside: endocrine gland—gland that internally secretes into a body fluid.
6
Q
exo-
A
exo-, outside: exocrine gland—gland that secretes to the outside through a duct.
7
Q
horm-
A
horm-, impetus, impulse: hormone—substance that a cell secretes that affects another cell.
8
Q
impetus
A
the force or energy with which a body moves.
9
Q
hyper-
A
hyper-, above: hyperthyroidism—condition resulting from an above-normal secretion of thyroid hormone.
10
Q
hypo-
A
hypo-, below: hypothyroidism—condition resulting from a below-normal secretion of thyroid hormone.
11
Q
lact-
A
lact-, milk: prolactin—hormone that promotes milk production.
12
Q
med-
A
med-, middle: adrenal medulla—inner portion of an adrenal gland.
13
Q
para-
A
para-, beside: parathyroid glands—set of glands on the posterior surface of the thyroid gland.
14
Q
toc-
A
toc-, birth: oxytocin—hormone that stimulates the uterine muscles to contract during childbirth.
15
Q
-tropic
A
-tropic, influencing: adrenocorticotropic hormone—a hormone secreted by the anterior pituitary gland that stimulates the adrenal cortex.
16
Q
vas-
A
vas-, vessel: vasopressin—hormone that helps maintain blood pressure by constricting blood vessels.
17
Q
Endocrine means
A
Endocrine means “internal secretion.”
18
Q
The endocrine system
A
is so named because the cells, tissues, and organs that compose 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. Hormones can effect changes in target cells even in extremely low concentrations.
19
Q
exocrine secretions
A
Pertaining to the secretion of a substance out through a duct. The exocrine glands include the salivary glands, sweat glands and glands within the gastrointestinal tract. Exocrine is as opposed to endocrine which refers to the secretion of a substance (a hormone) into the bloodstream.
20
Q
exocrine glands
A
enter tubes or ducts that lead to body surfaces. Two examples of exocrine secretions are stomach acid reaching the lumen of the digestive tract and sweat released at the skin’s surface
21
Q
Other glands
A
secrete substances into the internal environment that are not hormones by the traditional definition, but they function similarly as messenger molecules and are sometimes termed “local hormones.”
These include paracrine secretions, which enter the interstitial fluid but affect only nearby cells, and autocrine secretions, which affect only the cell secreting the substance.
22
Q
paracrine secretions
A
Paracrine signaling is a form of cell signaling or cell-to-cell communication in which a cell produces a signal to induce changes in nearby cells, altering the behavior of those cells. - Cells that produce paracrine factors secrete them into the immediate extracellular environment.
23
Q
Cells of the endocrine system and the nervous system both -
A
communicate using chemical signals that bind to receptor molecules. - In contrast to the nervous system, which releases neurotransmitter molecules into synapses, the endocrine system releases hormones into the bloodstream, which carries these messenger molecules everywhere.
24
Q
Endocrine glands and their hormones help -
A
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.
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Hormones are released into -
the extracellular spaces surrounding endocrine cells. From there, they diffuse into the bloodstream and are carried to all parts of the body.
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Hormones are organic compounds. They...
- are of two major types: steroids, or steroidlike substances; and nonsteroids, which include amines, peptides, proteins, and glycoproteins. Hormones can stimulate changes in target cells even in extremely low concentrations.
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Steroid Hormones
| 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. All steroid hormones are derived from cholesterol. They include sex hormones such as testosterone and the estrogens, and secretions of the adrenal cortex (the outer portion of the adrenal gland), including aldosterone and cortisol. Vitamin D is a modified steroid and when converted to the active form in the kidneys and liver becomes a hormone
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Nonsteroid Hormones
| Hormones called amines -
including norepinephrine and epinephrine, are derived from the amino acid tyrosine. These hormones are synthesized in the adrenal medulla (the inner portion of the adrenal gland)
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Nonsteroid Hormones
| Protein hormones -
like all proteins, are composed of long chains of amino acids that are linked and folded into specific molecular structures. They include the hormone secreted by the parathyroid gland and some of those secreted by the anterior pituitary gland. Certain other hormones secreted from the anterior pituitary gland are glycoproteins, which consist of carbohydrates joined to proteins.
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Nonsteroid Hormones
| The peptide hormones -
are short chains of amino acids. This group includes hormones associated with the posterior pituitary gland and some produced in the hypothalamus.
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Nonsteroid Hormones
| Another group of compounds, called prostaglandins -
are paracrine substances. They regulate neighboring cells. Prostaglandins are lipids (20-carbon fatty acids that include 5-carbon rings) and are synthesized from a type of fatty acid (arachidonic acid) in cell membranes. Prostaglandins are produced in a wide variety of cells, including those of the liver, kidneys, heart, lungs, thymus, pancreas, brain, and reproductive organs.
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Actions of Hormones
Hormones exert their effects by altering metabolic processes. A hormone might change the activity of an enzyme necessary for synthesizing a particular substance or alter the rate at which particular chemicals are transported through cell membranes. A hormone delivers its message to a cell by uniting with the binding site of its receptor. The more receptors the hormone binds on its target cells, the greater the response.
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Upregulation
is an increase in the number of receptors on a target cell, which often occurs as a response to a prolonged decrease in the level of a hormone
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Downregulation
is the opposite of upregulation, it's a decrease in the number of receptors in response to a prolonged increase in hormone levels.
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Steroid Hormones and Thyroid Hormones
Steroid hormones are poorly soluble in water. They are carried in the bloodstream bound to plasma proteins in a way that they are released in sufficient quantity to affect their target cells. Unlike amine, peptide, and protein hormones, steroid hormones are soluble in the lipids that make up the bulk of cell membranes. For this reason, these hormones can diffuse into cells relatively easily and are able to enter any cell in the body, although only target cells will respond.
36
Steroid hormones and thyroid hormone are able to freely cross the cell membrane. Because -
hormone secretion raises the extracellular hormone concentration, these hormones will diffuse down their concentration gradients into cells throughout the body. Only target cells will respond.
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Once inside a target cell -
steroid and thyroid hormones combine (usually in the nucleus) with specific protein receptors. The resulting hormone-receptor complex binds to particular DNA sequences, either activating or repressing specific genes.
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Activated genes are transcribed into
messenger RNA (mRNA) molecules. The mRNAs enter the cytoplasm, where they direct the synthesis of specific proteins, which may be enzymes, transport proteins, or even hormone receptors. The activities of these hormones produce the cellular changes associated with the particular hormone
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An example of a steroid hormone is aldosterone, which is
secreted from the adrenal glands and stimulates the kidneys to retain sodium. In response to aldosterone, cells that form tubules in the kidney begin to synthesize more Na+/K+ pumps, which are the proteins that actively transport sodium ions out of the cell, in this case returning sodium to the bloodstream.
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Types of Hormones
| Amines
Type of Compound - Amines
Formed from - Amino acids
Examples - Norepinephrine, epinephrine
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Types of Hormones
| Peptides
Type of Compound - Peptides
Formed from - Amino acids
Examples - ADH, OT, TRH, SS, GnRH
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Types of Hormones
| Steroids
Type of Compound - Steroids
Formed from - Cholesterol
Examples - Estrogens, testosterone, aldosterone, cortisol
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Types of Hormones
| Glycoproteins
Type of Compound - Glycoproteins
Formed from - Protein and carbohydrate
Examples - FSH, LH, TSH
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Types of Hormones
| Proteins
Type of Compound - Proteins
Formed from - Amino acids
Examples - PTH, GH, PRL
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A nonsteroid hormone, such as -
an amine, peptide, or protein, combines with specific receptor molecules on the target cell membrane. Each receptor molecule is a protein that has a binding site and an activity site. The hormone combines with the binding site, which causes the receptor’s activity site to interact with other membrane proteins. The hormone that triggers this first step in what becomes a cascade of biochemical activity is considered a first messenger. The biochemicals in the cell that induce the changes leading to the hormone effect are called second messengers. The entire process of chemical communication, from outside cells to inside, is called signal transduction.
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signal transduction
is the process by which a chemical or physical signal is transmitted through a cell as a series of molecular events, most commonly protein phosphorylation catalyzed by protein kinases, which ultimately results in a cellular response.
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cyclic adenosine monophosphate
In this mechanism, a hormone binds to its receptor, and the resulting hormone-receptor complex activates a protein called a G protein, which then activates an enzyme called adenylate cyclase, which is an integral membrane protein with its active site facing the inside of the cell. Activated adenylate cyclase removes two phosphates from ATP and reconnects the exposed oxygen, forming cyclic AMP. Cyclic AMP activates another set of enzymes called protein kinases that transfer phosphate groups from ATP molecules to protein substrate molecules, a process called phosphorylation. Phosphorylation activates some proteins, and renders others inactive.
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Sequence of Steroid Hormone Action
1.Endocrine gland secretes steroid hormone.
2.Blood carries hormone molecules throughout the body.
3.Steroid hormone diffuses through target cell membrane and enters cytoplasm or nucleus.
.4Hormone combines with a receptor molecule in the cytoplasm or nucleus.
5.Steroid hormone-receptor complex binds to DNA and promotes transcription of messenger RNA.
6.Messenger RNA enters the cytoplasm and directs protein synthesis.
7.Newly synthesized proteins produce the steroid hormone’s specific effects.
49
Hormones whose actions require cyclic AMP include
releasing hormones from the hypothalamus; thyroid- stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH), and luteinizing hormone (LH) from the anterior pituitary gland; antidiuretic hormone (ADH) from the posterior pituitary gland; parathyroid hormone (PTH) from the parathyroid glands; norepinephrine and epinephrine from the adrenal glands; calcitonin from the thyroid gland; and glucagon from the pancreas.
50
An abnormality in cAMP-mediated signaling can lead to
symptoms from many endocrine glands. In McCune-Albright syndrome, for example, a defect in the G protein that activates adenylate cyclase results in conversion of ATP to cAMP even without hormonal stimulation. As a result, certain cells in the pituitary, thyroid, gonads, and adrenal glands secrete hormones in excess. One symptom is precocious puberty. Infant girls menstruate, and boys as young as six years produce mature sperm.
51
Certain nonsteroid hormones use second messengers other than cAMP. For example,
a second messenger called diacylglycerol (DAG), like cAMP, activates a protein kinase, leading to a cellular response.
52
Prostaglandins are
paracrine substances, acting locally. They are present in small amounts, but are potent. They are not stored in cells; instead they are synthesized just before they are released. They are rapidly inactivated.
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negative feedback
occurs when a product feeds back to decrease its own production. This type of feedback brings things back to normal whenever they start to become too extreme. The thyroid gland is a good example of this type of regulation.
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tropic hormones
Hormones that act on other glands are called tropic hormones
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The adrenal medulla
The nervous system directly stimulates some glands. --
secretes its hormones (epinephrine and norepinephrine) in response to impulses from preganglionic sympathetic neurons. The secretory cells replace the postganglionic sympathetic neurons, which would normally secrete norepinephrine alone as a neurotransmitter
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Pituitary Gland
at the base of the brain, is about one centimeter in diameter. It is attached to the hypothalamus by the pituitary stalk, or infundibulum, and lies in the sella turcica of the sphenoid bone
57
The pituitary gland consists of two distinct portions:
an anterior lobe (adenohypophysis) and a posterior lobe (neurohypophysis).
The anterior lobe secretes a number of hormones, including growth hormone (GH), thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), and prolactin (PRL). The cells that make up the posterior lobe (pituicytes) do not synthesize hormones. However, specialized neurons, whose axon endings enter the posterior lobe of the pituitary, secrete into the bloodstream two important hormones: antidiuretic hormone (ADH) and oxytocin (OT). The cell bodies of these neurons are in the hypothalamus.
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intermediate lobe
In the fetus, a narrow region develops between the anterior and posterior lobes of the pituitary gland. Called the intermediate lobe (pars intermedia), it produces melanocyte-stimulating hormone (MSH), which regulates the synthesis of melanin—the pigment in skin and in parts of the eyes and brain. In most adults this intermediate lobe is no longer a distinct structure, but its secretory cells persist in the two remaining lobes.
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hypothalamic neurons secrete hormones called releasing hormones
These releasing hormones are carried in the blood via a capillary bed associated with the hypothalamus.
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The vessels merge to form the hypophyseal portal veins
that pass downward along the pituitary stalk and give rise to a second capillary bed in the anterior lobe.
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portal system
The arrangement of two capillaries in series is unusual and is called a portal system. It exists in three places in the body: the hepatic portal vein connects intestinal capillaries to special liver capillaries called sinusoids, the efferent arteriole of kidney nephrons connects two sets of capillaries, and the hypophyseal portal vein gives rise to capillaries in the anterior lobe of the pituitary gland.
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The epithelial tissue has five types of secretory cells.
They are somatotropes that secrete GH, mammatropes that secrete PRL, thyrotropes that secrete TSH, corticotropes that secrete ACTH, and gonadotropes that secrete FSH and LH. In males, LH (luteinizing hormone) is also referred to as ICSH (interstitial cell-stimulating hormone) because it affects the interstitial cells of the testes
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Growth hormone, also called somatotropin (or somatotrophic hormone, STH)
is a protein that stimulates cells to enlarge and more rapidly divide. It enhances the movement of amino acids through cell membranes and increases the rate of protein synthesis. GH also decreases the rate at which cells use carbohydrates and increases the rate at which they use fats.
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Prolactin
is a protein, and as its name suggests, it promotes milk production.
Prolactin secretion is mostly under inhibitory control by prolactin-inhibiting hormone (PIH) from the hypothalamus. PIH is now known to be dopamine. A number of substances can stimulate prolactin secretion, but a specific hypothalamic prolactinreleasing factor (PRF) has yet to be identified.
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Thyroid-stimulating hormone, also called thyrotropin
is a glycoprotein. It controls secretion of certain hormones from the thyroid gland. TSH can also stimulate growth of the gland, and abnormally high TSH levels may lead to an enlarged thyroid gland, or goiter.
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Adrenocorticotropic hormone also called “corticotropin,”
is a peptide that controls the manufacture and secretion of certain hormones from the outer layer (cortex) of the adrenal gland. The secretion of ACTH is stimulated by corticotropin-releasing hormone (CRH), which the hypothalamus releases in response to decreased concentrations of adrenal cortical hormones. Stress can increase secretion of ACTH by stimulating release of CRH.
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Both follicle-stimulating hormone FSH and luteinizing hormone LH are
glycoproteins and are called gonadotropins, which means they act on the gonads or reproductive organs. FSH controls growth and development of follicles that house developing oocytes in the ovaries. It also stimulates the follicular cells to secrete a group of female sex hormones, collectively called estrogen (or estrogens).
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Posterior Pituitary Hormones
The posterior lobe of the pituitary largely consists of neuroglia (pituicytes) and the axons of hypothalamic neurons. This is different from the anterior lobe, which is primarily glandular epithelium. The neuroglia support the axons. The secretions of these neurons function not as neurotransmitters but as hormones, because they enter the bloodstream
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The hormones associated with the posterior pituitary are
antidiuretic hormone (also known as vasopressin) and oxytocin. These hormones are transported down axons through the pituitary stalk to the posterior pituitary and are stored in vesicles (secretory granules) near the ends of the axons. The hormones are released into the blood in response to action potentials conducted on the axons of the neurosecretory cells. Therefore, posterior pituitary hormones are synthesized in the hypothalamus, and their secretion is controlled by the hypothalamus, but they are named for where they enter the bloodstream.
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ADH’s two effects
| ADH Antidiuretic hormone (ADH): / Vasopressin
vasoconstriction and water retention—are possible because the hormone binds two different receptors on target cells. The binding of ADH to V1 receptors increases the concentration of the second messenger inositol triphosphate, which increases the intracellular calcium ion concentration in the smooth muscle of blood vessel walls, leading to vasoconstriction. The second receptor, V2, is on cells that make up the kidneys’ microscopic structures called collecting ducts. ADH binding there activates the cAMP second messenger system, which ultimately causes the collecting ducts to reabsorb water that would otherwise be excreted in the urine.
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neurons in this part of the brain, called osmoreceptors
sense changes in the concentration of body fluids. For example, if a person is dehydrating due to a lack of water intake, the solutes in blood become more concentrated. The osmoreceptors, sensing the resulting increase in osmotic pressure, signal the posterior pituitary to release ADH, which acts on target cells in the kidneys, causing the kidneys to retain water. On the other hand, if one drinks water in excess of one’s needs, body fluids become more dilute, which inhibits the release of ADH. In response, the kidneys excrete a more dilute urine until the concentration of body fluids returns to normal.
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Diabetes insipidus, which is unrelated to diabetes mellitus (sugar diabetes)
impairs ADH regulation of water balance. In one form of diabetes insipidus, the kidneys are unable to retain water because ADH V2 receptors on the kidney collecting ducts are defective. The hormone can bind, but the receptor fails to trigger cAMP formation. Tumors and injuries affecting the hypothalamus and posterior pituitary can also cause diabetes insipidus.
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Oxytocin has an antidiuretic action, but
less so than ADH. In addition, oxytocin can contract smooth muscle in the uterine wall, playing a role in the later stages of childbirth. The uterus becomes more sensitive to oxytocin’s effects during pregnancy. Stretching of uterine and vaginal tissues late in pregnancy, as the fetus grows, initiates sensory impulses to the hypothalamus, which then signals the posterior pituitary to release oxytocin, which, in turn, stimulates the uterine contractions of labor.
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thyroid gland
is a vascular structure that consists of two large lateral lobes connected by a broad isthmus (is′mus). The thyroid lies just inferior to the larynx (voicebox) on either side and anterior to the trachea (windpipe). The gland is specialized to remove iodide from the blood.
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Structure of the Thyroid Gland
The thyroid gland produces three important hormones. The follicular cells synthesize two of these, which have marked effects on the metabolic rates of body cells. The extrafollicular cells produce the third type of hormone, which influences blood concentrations of calcium and phosphate ions.
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The two thyroid hormones that affect cellular metabolic rates are
thyroxine (thi-rok′sin) and triiodothyronine (tri″io″do-thi′ro-nēn), collectively referred to as thyroid hormone. Thyroxine, or tetraiodothyronine, is also called T4 because it includes four atoms of iodine. Triiodothyronine is also called T3 because it includes three atoms of iodine (fig. 13.20)
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The importance of the "two Thyroid hormones"
Thyroid hormone helps regulate the metabolism of carbohydrates, lipids, and proteins. It increases the rate at which cells release energy from carbohydrates, enhances the rate of protein synthesis, and stimulates breakdown and mobilization of lipids. Thyroid hormone is the major factor determining how many calories the body must consume at rest to maintain life, which is measured as the basal metabolic rate (BMR). It is essential for normal growth and development and for maturation of the nervous system. TSH from the anterior pituitary gland controls the level of thyroid hormone.
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hypothyroidism
One form of hypothyroidism, infantile hypothyroidism, results from insufficient thyroid hormone during infancy and early childhood. In the absence of treatment, symptoms may include stunted growth, abnormal bone formation, slowed mental development, low body temperature, and sluggishness. Hypothyroidism is also common among older adults, producing fatigue and weight gain.
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hyperthyroidism
Hyperthyroidism produces an elevated metabolic rate, restlessness, and overeating. In Graves’ disease, the most common cause of hyperthyroidism, the eyes may protrude (exophthalmia) because of swelling in the tissues behind them
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The thyroid gland produces
calcitonin, which is not referred to as “thyroid hormone” because it is synthesized by the C cells, distinct from the gland’s follicles. Calcitonin plays a role in the control of blood calcium and phosphate ion concentrations. It helps lower concentrations of calcium and phosphate ions by decreasing the rate at which they leave the bones and enter extracellular fluids by inhibiting the bone-destroying activity of osteoclasts.
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The 4 parathyroid glands
are on the posterior surface of the thyroid gland
Most individuals have four of them—a superior and an inferior gland associated with each of the thyroid’s bilateral lobes. The parathyroid glands secrete a hormone that regulates the concentrations of calcium and phosphate ions in the blood.
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Structure of the 4 Parathyroid Glands
Each parathyroid gland is a small, yellowish brown structure covered by a thin capsule of connective tissue. The body of the gland consists of many tightly packed secretory cells closely associated with capillary networks
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Parathyroid Hormone
parathyroid glands secrete a protein, parathyroid hormone (PTH), or parathormone
This hormone increases the blood calcium ion concentration and decreases the blood phosphate ion concentration through actions in the bones, kidneys, and intestines.
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The liver changes vitamin D to
hydroxycholecalciferol, which is carried in the bloodstream or stored in tissues. When PTH is present, hydroxycholecalciferol can be changed in the kidneys into the active form of vitamin D (dihydroxycholecalciferol), which stimulates absorption of calcium ions from the intestine
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The adrenal glands (suprarenal glands)
are closely associated with the kidneys. A gland sits atop each kidney like a cap and is embedded in the mass of adipose tissue that encloses the kidney. Adrenal hormones play roles in maintaining the blood sodium level and responding to stress. They also include certain sex hormones.
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Structure of the Adrenal Glands
The adrenal glands are shaped like pyramids. Each adrenal gland is vascular and consists of two parts. The central portion is the adrenal medulla, and the outer part is the adrenal cortex. These regions are not sharply divided, but they are distinct in that they secrete different hormones.
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The adrenal medulla
consists of irregularly shaped cells grouped around blood vessels. These cells are intimately connected with the sympathetic division of the autonomic nervous system. The adrenal medullary cells (chromaffin cells) are modified sympathetic postganglionic neurons, and preganglionic sympathetic nerve fibers control their secretions
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The adrenal cortex
makes up the bulk of the adrenal gland. It is composed of closely packed masses of epithelial layers that form outer, middle, and inner zones—the zona glomerulosa, the zona fasciculata, and the zona reticularis, respectively
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Hormones of the Adrenal Medulla
The chromaffin cells of the adrenal medulla produce, store, and secrete two closely related hormones, epinephrine, also called adrenalin, and norepinephrine, also called noradrenalin. Both of these substances are a type of amine hormone called a catecholamine, and they have similar molecular structures and physiological functions. Epinephrine is synthesized from norepinephrine.
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Hormones of the Adrenal Cortex
The cells of the adrenal cortex produce more than thirty different steroids, including several hormones (corticosteroids). Unlike the adrenal medullary hormones, without which a person can survive, some of those released by the cortex are vital. In the absence of these adrenal cortical secretions, without extensive electrolyte therapy a person usually dies within a week. The most important adrenal cortical hormones are aldosterone, cortisol, and certain sex hormones.
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Aldosterone
Cells in the outer zone (zona glomerulosa) of the adrenal cortex synthesize aldosterone. This hormone is called a mineralocorticoid because it helps regulate the concentration of mineral electrolytes, such as sodium and potassium ions. Specifically, aldosterone causes the kidney to conserve sodium ions and to excrete potassium ions. The cells that secrete aldosterone respond directly to changes in the composition of blood plasma.
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renin-angiotensin system
or renin–angiotensin–aldosterone system, is a hormone system that regulates blood pressure and fluid and electrolyte balance, as well as systemic vascular resistance.
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Groups of specialized kidney cells (juxtaglomerular cells)
are able to respond to changes in blood pressure and the plasma sodium ion concentration. If the level of either of these factors decreases, these kidney cells release an enzyme called renin (re′nin). Renin reacts with a blood protein called angiotensinogen (an″je-o-ten-sin′o-jen) to catalyze the partial breakdown of angiotensinogen into a peptide called angiotensin I. Another enzyme (angiotensin-converting enzyme, or ACE), found primarily in lung blood vessels, catalyzes a reaction that converts angiotensin I into another form, angiotensin II, which is carried in the bloodstream. When angiotensin II reaches the adrenal cortex, it stimulates the release of aldosterone. ACTH is necessary for the adrenal gland to respond to this and other stimuli.
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Cortisol (hydrocortisone)
is a glucocorticoid, which means it affects glucose metabolism. It is produced primarily in the middle zone (zona fasciculata) of the adrenal cortex and has a molecular structure similar to aldosterone. In addition to affecting glucose, cortisol influences protein and fat metabolism.
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Among the more important actions of cortisol are the following
It inhibits the synthesis of protein in various tissues, increasing the blood concentration of amino acids.
It stimulates liver cells to synthesize glucose from noncarbohydrates (gluconeogenesis), such as circulating amino acids and glycerol, thus increasing the blood glucose concentration.
It promotes the release of fatty acids from adipose tissue, increasing the use of fatty acids and decreasing the use of glucose as energy sources, thus preserving glucose availability for the brain.
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Adrenal Sex Hormones
Cells in the inner zone (zona reticularis) of the adrenal cortex mostly produce sex hormones. These hormones are male (adrenal androgens), but some of them are converted into female hormones (estrogens) by the skin, liver, and adipose tissues. The amounts of these adrenal hormones are very small compared to the supply of sex hormones from the gonads, but they may contribute to early development of the reproductive organs. Adrenal androgens may also play a role in the female sex drive.
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The pancreas
consists of two major types of secretory tissues. This organization reflects the dual function of the pancreas as an exocrine gland that secretes digestive juice through a duct, and an endocrine gland that releases hormones that control the blood glucose level.
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Structure of the Pancreas Gland
The pancreas is an elongated, somewhat flattened organ located posterior to the stomach and partly between the parietal peritoneum and the posterior abdominal wall (retroperitoneal) A duct that attaches the pancreas to the first section of the small intestine (duodenum) transports its digestive juice into the intestine. The endocrine portion of the pancreas consists of cells grouped around blood vessels. These groups, called pancreatic islets (islets of Langerhans), include three distinct types of hormone-secreting cells—alpha cells, which secrete glucagon; beta cells, which secrete insulin; and delta cells, which secrete somatostatin
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pancreatic islets (islets of Langerhans)
pancreatic islets (islets of Langerhans), include three distinct types of hormone-secreting cells—alpha cells, which secrete glucagon; beta cells, which secrete insulin; and delta cells, which secrete somatostatin
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The hormone insulin
is also a protein, and its main effect is exactly opposite that of glucagon. Insulin stimulates the liver to form glycogen from glucose and inhibits conversion of noncarbohydrates into glucose. Insulin also has the special effect of promoting the facilitated diffusionof glucose through the membranes of cells bearing insulin receptors. These cells include those of adipose tissue, liver, and resting skeletal muscle (glucose uptake by active muscle is not dependent on insulin). Insulin action decreases the concentration of blood glucose toward normal, promotes transport of glucose and amino acids into cells, and increases glycogen and protein synthesis. It also stimulates adipose cells to synthesize and store fat.
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Somatostatin (similar to the hypothalamic hormone)
which the delta cells release, may help regulate glucose metabolism by inhibiting secretion of glucagon and insulin, but its inhibitory effect in the pancreas is not strong.
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Other Endocrine Glands
Other organs that produce hormones are part of the endocrine system, too. They include the pineal gland; the thymus; reproductive organs; and certain cells of the digestive tract, the heart, and the kidneys.
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pineal gland
is a small, oval structure deep between the cerebral hemispheres, where it is attached to the upper portion of the thalamus near the roof of the third ventricle. It largely consists of specialized pineal cells and supportive neuroglia (see fig. 11.11b).
The pineal gland secretes a hormone, melatonin, that is synthesized from serotonin.
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circadian rhythms
is a natural, internal process that regulates the sleep-wake cycle and repeats roughly every 24 hours. It can refer to any biological process that displays an endogenous, entrainable oscillation of about 24 hours.
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thymus
lies in the mediastinum posterior to the sternum and between the lungs. It is large in young children but shrinks with age. The thymus secretes a group of hormones, called thymosins, that affect production and differentiation of certain white blood cells (T lymphocytes). The thymus plays an important role in immunity
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The reproductive organs that secrete important hormones
include the testes, which produce testosterone; the ovaries, which produce estrogens and progesterone; and the placenta, which produces estrogens, progesterone, and a gonadotropin.
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The digestive glands that secrete hormones
are generally associated with the linings of the stomach and small intestine. The small intestine alone produces dozens of hormones, many of which have not been well studied.
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Other organs that produce hormones include the heart
which secretes two natriuretic peptides - the kidneys, which secrete the active form of vitamin D, and the liver, which, along with the kidneys, secretes erythropoietin (EPO), a hormone that stimulates red blood cell production
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Stress and Its Effects
Factors that change the body’s internal or external environment are potentially life threatening. Sensory receptors detecting such changes trigger impulses that reach the hypothalamus, initiating physiological responses that resist a loss of homeostasis. These responses include increased activity in the sympathetic division of the autonomic nervous system and increased secretion of adrenal hormones. A factor capable of stimulating such a response is called a stressor, and the condition it produces in the body is called stress.
