I. General Characteristics of the Endocrine System
A. The endocrine glands secrete hormones.
B. Hormones diffuse from interstitial fluids into the blood stream and eventually act on target cells.
C. Paracrine secretions are secretions that do not travel in the blood stream to their targets.
D. Autocrine secretions are secretions that affect the secreting cell itself.
E. Exocrine glands secrete substances into ducts.
F. Endocrine glands and their hormones control metabolic processes.
G. Endocrine hormones also play vital roles in reproduction, development, and growth.
H. The larger endocrine glands are the pituitary, thyroid, parathyroids, adrenals, and pancreas.
II. Hormone Action
A. Introduction
1. Hormones are released into the extracellular spaces surrounding endocrine cells.
2. They diffuse into the bloodstream and are carried to all parts of the body.
B. Chemistry of Hormones
1. Introduction
a. Most hormones are either steroids or steroid-like substances or nonsteroids.
b. Nonsteroid hormones include amines, peptides, proteins, and glycoprotiens.
2. Steroid Hormones
a. Steroids are lipids that include complex rings of carbon and hydrogen atoms.
b. Examples of steroid hormones are testosterone, estrogen, aldosterone, and cortisol.
3. Nonsteroid Hormones
a. Examples of hormones called amines are norepinephrine and epinephrine.
b. Protein hormones are composed of long chains of amino acids.
c. Examples of protein hormones are those secreted by the anterior pituitary and parathyroid glands.
d. Hormones called glycoproteins are produced by the anterior pituitary.
e. Peptide hormones are short chains of amino acids.
f. Peptide hormones come from the posterior pituitary and hypothalamus.
g. Prostaglandins are paracrine substances and are produced in a wide variety of cells.
C. Actions of Hormones
1. Introduction
a. Hormones exert their effects by altering metabolic processes.
b. The more receptors the hormone binds on its target cell, the greater the response.
c. Up-regulation is an increase in the number of receptors on a target cell.
2. Steroid Hormones and Thyroid Hormones
a. Steroid and thyroid hormones are insoluble in water but are soluble in lipids.
b. Steroid and thyroid hormones can diffuse into cells relatively easily.
c. Once steroid and thyroid hormones are inside a cell, they combine with specific protein receptors located usually in the nucleus.
d. The binding of these hormones to the receptor usually activates or inhibits a gene.
e. Activated genes code for specific proteins.
f. The new proteins may be enzymes, transport proteins, or hormone receptors and they bring about cellular changes.
3. Nonsteroid Hormones
a. A nonsteroid hormone usually binds with receptors located on the cell membrane.
b. When a nonsteroid hormone binds to a membrane receptor, this causes the receptor’s activity site to interact with other membrane proteins.
c. Receptor binding may alter the function of enzymes or membrane transport mechanisms, changing the concentrations of still other cellular components.
d. A first messenger is the hormone that triggers a cascade of biochemical activity.
e. Second messengers are the chemicals in the cell that induce the changes that are recognized as responses to the hormone.
f. Many hormones use cyclic AMP as a second messenger.
g. G proteins are activated by the binding of a hormone to a membrane receptor.
h. Adenylate cyclase is activated by G proteins.
i. Adenylate cyclase functions to form cyclic AMP from ATP.
j. Cyclic AMP activates another set of enzymes called protein kinases.
k. Protein kinases function to transfer phosphate groups from ATP to proteins substrate molecules.
l. Phosphorylated substrates may be converted from inactive to active forms.
m. Activated proteins then alter various cellular processes to bring about the effect of that particular hormone.
n. Cellular responses to second messenger activation include altering membrane permeabilities, activating enzymes, promoting synthesis of certain proteins, stimulating or inhibiting metabolic pathways, promoting cellular movements, and initiating secretion of hormones and other substances.
o. Hormones whose actions require cyclic AMP include releasing-hormones from the hypothalamus, TSH, ACTH, FSH, LH, ADH, PTH, norepinephrine, epinephrine, glucagon, and calcitonin.
p. An example of another second messenger is DAG.
q. In another mechanism, a hormone binding its receptor increases calcium, ion concentration within the target cell.
r. Calcium ions bind to the protein calmodulin to activate it.
s. Activated calmodulin functions to interact with enzymes, altering their activities.
t. Cells are highly sensitive to changes in concentration of nonsteroid hormones because responses to them is greatly amplified through second messengers.
D. Prostaglandins
1. Prostaglandins are paracrine substances that act locally.
2. Some prostaglandins regulate cellular responses to hormones.
3. The variety of effects prostaglandins can produce include relaxation of smooth muscle in airway and blood vessels, contraction of smooth muscle in the uterus, stimulation of secretion of various hormones, and promotion of inflammation.
III. Control of Hormonal Secretions
A. Introduction
1. Hormones with short half-lives control body functions that turn on and off quickly.
2. Hormones are continually excreted in urine and broken down by enzymes in the liver.
3. Increasing or decreasing blood levels of hormones requires increased or decreased secretion.
B. Control Sources
1. Control of hormone secretion is essential to maintaining the internal environment.
2. The hypothalamus controls the anterior pituitary gland’s release of tropic hormones.
3. Tropic hormones stimulate other endocrine glands to release hormones.
4. An example of an endocrine organ directly stimulated by the nervous system is the adrenal medulla.
5. Some endocrine glands respond to changes in the composition of the internal environment.
5. As a result of negative feedback mechanisms, hormone levels remain relatively stable.
IV. Pituitary Gland
A. Introduction
1. The pituitary gland is located at the base of the brain.
2. The infundibulum is a stalk that attaches the pituitary gland to the hypothalamus.
3. The two portions of the pituitary are anterior and posterior.
4. The anterior lobe secretes the following hormones: GH, TSH, ACTH, FSH, LH, PRL.
5. The posterior pituitary secretes the following hormones: OT and ADH
6. The hypothalamus controls most of the pituitary gland’s activities.
7. The posterior pituitary receives impulses from the hypothalamus.
8. Releasing hormones from the hypothalamus control the anterior pituitary.
9. The hypophyseal portal veins are vessels that pass downward along the pituitary stalk from the hypothalamus and give rise to a capillary bed in the anterior lobe of the pituitary.
B. Anterior Pituitary Hormones
1. Somatotropes secrete GH.
2. Mammotropes secrete PRL.
3. Thyrotropes secrete TSH.
4. Corticotropes secrete ACTH.
5. Gonadotropes secrete FSH and LH.
6. Actions of growth hormone are stimulation of cells to enlarge and more rapidly divide, enhance movement of amino acids through the cell membranes, and increases the rate of protein synthesis. GH also decreases the rate as which cells utilize carbohydrates and increases the rate at which cells use fats.
7. The secretion of GH is controlled by somatostatin and GHRH.
8. Actions of prolactin are to sustain mild production after birth and to amplify effect of LH in males.
9. The secretion of PRL is under inhibitory control by PIH and PRF.
10. Actions of thyroid-stimulating hormone are to stimulate the thyroid gland to release its hormones.
11. The secretion of TSH is controlled by TRH.
12. The actions of adrenocorticotropic hormone are to control secretion of certain hormone from the adrenal cortex.
13. The secretion of ACTH is controlled by CRH.
14. Gonadotropins are LH and FSH.
15. The actions of follicle-stimulating hormone are to promote development of egg-containing follicles in ovaries, to stimulate follicular cells to release estrogen, and in males, to stimulate production of sperm cells.
16. The actions of luteinizing hormone are to promote secretion of sex hormones and to promote the release egg cells in females.
17. The secretion of FSH and LH is controlled by GnRH.
C. Posterior Pituitary Hormones
1. The posterior pituitary consists of nerve fibers and neuroglial cells.
2. Specialized neurons in the hypothalamus produce two hormones called OT and ADH.
3. The hormones produced in the hypothalamus travel down axons through the pituitary stalk to the posterior pituitary.
4. The actions of antidiuretic hormone are to cause a reduction in water excretion, and to raise blood pressure.
5. The secretion of ADH is controlled by blood water concentration and blood volume.
6. The actions of oxytocin are to contact muscles in uterine wall and to contract muscles associated with milk-secreting cells.
7. The secretion of oxytocin is controlled by uterine stretch and stimulation of breasts.
V. Thyroid Gland
A. Introduction
1. The thyroid gland consists of two lobes.
2. The thyroid gland is located just below the larynx on either side and anterior to the trachea.
B. Structure of the Gland
1. Follicles are secretory parts of the thyroid gland.
2. Colloid is a viscous fluid that fills follicles and contains thyroglobulin.
3. Thyroglobulin is a glycoprotein.
4. Extrafollicular cells are located outside of follicles.
5. The follicular cells produce hormones.
C. Thyroid Hormones
1. The three hormones produced by the thyroid gland are T4, T3, and calcitonin.
2. The actions of thyroxine and triiodothyronine are to regulate metabolism of carbohydrates, lipids, and proteins.
3. The secretion of T3 and T4 are controlled by TSH.
4. Follicular cells require iodine to produce T3 and T4.
5. The actions of calcitonin are to lower blood calcium levels.
6. The secretion of calcitonin is controlled by blood calcium levels. It is released in response to high blood calcium levels.
VII. Parathyroid Glands
A. Introduction
1. Parathyroid glands are located embedded in the thyroid gland.
2. Usually a person has four parathyroid glands.
B. Structure of the Glands
1. Each parathyroid gland is covered by a thin capsule.
2. The body of a parathyroid gland consists of many tightly packed secretory cells.
C. Parathyroid Hormone
1. The actions of PTH are to raise blood calcium levels.
2. The secretion of PTH is controlled by blood calcium levels. It is released in response to low blood calcium levels.
VIII. Adrenal Glands
A. Structure of the Glands
1. The adrenal glands are shaped like pyramids.
2. The two parts of an adrenal gland are the cortex and medulla.
3. The adrenal medulla consists of irregularly shaped cells grouped around blood vessels.
4. The adrenal cortex is composed of closely packed masses of epithelial layers.
5. The three layers of the adrenal cortex are the outer zona glomerulosa, the middle zona fasciculata, and the inner zona reticularis.
B. Hormones of the Adrenal Medulla
1. The two hormones released by the adrenal medulla are epinephrine and norepinephrine.
2. The actions of epinephrine and norepinephrine are increased heart rate, increased force of cardiac muscle contraction, elevated blood pressure, increased breathing rate and decreased activity of the digestive system
3. The secretion of epinephrine and norepinephrine are controlled by the sympathetic nervous system.
C. Hormones of the Adrenal Cortex
1. Introduction
a. The adrenal cortex produces more than 30 different steroids.
b. The most important adrenal cortical hormones are aldosterone, cortisol, and certain sex hormones.
2. Aldosterone
a. Aldosterone is secreted by the zona glomerulosa and is called a mineralocorticoid because it helps regulate the concentration of mineral electrolytes.
b. The actions of aldosterone are regulation of concentration of extracellular electrolytes by conserving sodium ions and excreting potassium ions.
c. The secretion of aldosterone is controlled by electrolyte concentrations in body fluids and the renin-angiotensin mechanism.
3. Cortisol
a. Cortisol is secreted by the zona fasciculata and is called a glucocorticoid because it affects glucose metabolism
b. The actions of cortisol are to decrease protein synthesis, increase fatty acid release, and simulate glucose synthesis from noncarbohydrates.
c. The secretion of cortisol is controlled by CRH.
4. Sex Hormones
a. The sex hormones are secreted by the zona reticularis.
b. The actions of the sex hormones are to supplement sex hormones from the gonads.
c. Examples of sex hormones are androgens such as testosterone.
IX. Pancreas
A. Structure of the Gland
1. The pancreas is located posterior to the stomach.
2. The endocrine portion of the pancreas consists of islets of Langerhans which are also called pancreatic islets.
3. Three cell types of the pancreatic islets are alpha, beta, and delta.
4. Alpha cells secrete glucagon.
5. Beta cells secrete insulin.
6. Delta cells secrete somatostatin.
B. Hormones of the Pancreatic Islets
1. The actions of glucagon are to stimulate the liver to break down glycogen and to convert noncarbohydrates into glucose. It also simulates the breakdown of fats.
2. The secretion of glucagon is controlled by blood glucose concentrations.
3. The actions of insulin are to promote the formation of glycogen from glucose, to inhibit conversion of noncarbohydrates into glucose, and to enhance movement of glucose through adipose and muscle cell membranes. It also decreases blood glucose concentrations, promotes transport of amino acids into cells, and enhances synthesis of proteins and fats.
4. The secretion of insulin is controlled by blood glucose concentrations.
5. The function of somatostatin is to help regulate carbohydrates.
X. Other Endocrine Glands
A. The pineal gland is located near the roof of the third ventricle deep in the cerebral hemispheres.
B. The pineal gland produces the hormone melatonin.
C. The functions of melatonin are to help regulate circadian rhythms and to inhibit secretion of gonadotropins.
D. The thymus gland is located between the lungs.
E. The thymus gland secretes a group of hormones called thymosins.
F. The function of thymosin is to promote the maturation of T lymphocytes.
G. Reproductive organs that secrete hormones are ovaries and testes.
H. Examples of hormones produced by reproductive organs are estrogen, progesterone, and testosterone.
I. The hormone produced by the heart is ANP.
J. The hormone produced by the kidneys is erythropoietin.
XI. Stress and Its Effects
A. Introduction
1. A stressor is a factor capable of producing stress.
2. Stress is a protective response produced by the body in response to stress factors.
B. Types of Stress
1. Examples of physical stress include extreme cold or heat, decreases oxygen concentrations, infection, injuries, heavy exercise and loud sounds.
2. Examples of psychological stress are imagined dangers, personal losses, unpleasant social interaction or any factor that threatens a person.
C. Responses to Stress
1. The general stress syndrome is a group of symptoms produced by the hypothalamus in response to stress.
2. Major events of the general stress syndrome are increased blood glucose levels, increased heart rate and breathing rate, dilation of airways, and shunting of blood into muscles. See table 13.13.
XII. Life-Span Changes
A. General changes in the glands of the endocrine system are a decrease in size and increase in the proportion of each gland that is fibrous in nature.
B. Treatments for endocrine disorders include supplements of hormones or removing part of an overactive gland or using drugs to block the action of an overabundant hormone.
C. Levels of ADH increase with age and as a result, the kidneys reabsorb more water.
D. The decrease of calcitonin levels with age increases the risk of osteoporosis.
E. The most obvious changes in endocrine function involve blood glucose regulation.
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