The Endocrine System

THE ENDOCRINE SYSTEM

Unit 8

Overview of Endocrine System

Works with nervous system to coordinate and direct cell activity

Works more slowly than nervous system

Uses glands all over the body to secrete chemical messengers known as hormones

Chemistry of Hormones

Three main classes: Amino-acid based

Proteins, peptides, and amines Steroids

Made from cholesterol Sex hormones, hormones from adrenal

glands Prostaglandins

Produced locally Made from highly active lipids

Mechanisms of Hormone Action Hormones only affect certain tissue cells or

organs = target cells or organs Target cell has specific protein receptors in

plasma membrane that only specific hormones can bind to

Causes one or more of the following to occur: Changes in plasma membrane permeability or

electrical state Synthesis of proteins or certain regulatory

molecules (such as enzymes) in the cell Activation or inactivation of enzymes Stimulation of mitosis

Mechanisms of Hormone Action Two main mechanisms:

Steroid hormone action Nonsteroid hormone action

Steroid hormone action

1. Diffuse through plasma membrane of target cell.

2. Enter nucleus. 3. Bind to specific

receptor protein. 4. Bind to specific site

on cell’s DNA. 5. Activate certain genes

to transcribe mRNA. 6. Translate mRNA,

making new proteins.

Nonsteroid hormone action

1. Hormone binds to membrane receptor.

2. Activate enzyme. 3. Enzyme catalyzes

reaction that produces a second messenger molecule.

4. Oversees additional intracellular changes that promote the typical response of the target cell to the hormone.

Control of Hormone Release

Negative feedback mechanisms = main mechanism for regulating blood levels of most hormones, rising hormone levels inhibit further hormone release

Three major kinds of stimuli: Hormonal stimulus = endocrine organs are

activated by other hormones Humoral stimuli = endocrine organs are activated

by fluctuating levels of nutrients, ions, etc. in the blood

Neural stimuli = nerve fibers stimulate hormone release

Major Endocrine Organs

Pituitary gland Thyroid gland Parathyroid glands Adrenal glands Pineal gland Thymus gland Pancreas Gonads (ovaries and testes) Hypothalamus

Pituitary gland

Approximately the size of a grape, hangs underneath the hypothalamus in the brain

Two functional lobes – anterior pituitary (glandular tissue) and posterior pituitary (nervous tissue)

Anterior pituitary gland = “master endocrine gland” because it controls activity of so many other endocrine glands, but it is controlled by the hypothalamus

Hypothalamus also makes oxytocin and antidiuretic hormone, which are stored in the posterior pituitary gland until they are released

Hormones of the Anterior Pituitary

Growth hormone (GH) = general metabolic hormone, major effects directed toward growth of skeletal muscles and long bones Causes amino acids to be built into proteins Stimulates most target cells to grow in size and

divide Causes fats to be broken down and used for

energy Prolactin = protein hormone, very similar to

growth hormone in structure, stimulates and maintains milk production after childbirth

Hormones of the Anterior Pituitary

Tropic hormone = stimulates target organ, which is another endocrine gland, to secrete its hormone

Adrenocorticotropic hormone (ACTH) = regulates endocrine activity of cortex portion of adrenal gland

Thyroid-stimulating hormone (TSH) / thyrotropic hormone (TH) = influences the growth and activity of the thyroid gland

Hormones of the Anterior Pituitary

Gonadotropic hormone = regulates hormonal activity of gonads (ovaries and testes)

Follicle-stimulating hormone (FSH) = stimulates follicle development in ovaries (females only), mature follicles produce estrogen and prepare eggs for ovulation. In males, FSH stimulates sperm development.

Luteinizing hormone (LH) = triggers ovulation of egg from ovary and causes ruptured follicle to become a corpus luteum and then release progesterone and some estrogen. In males, LH is also called interstitial cell stimulating hormone (ICSH)

Hormones of the Posterior Pituitary

Technically not an endocrine gland because it doesn’t make the hormones, it only stores them until they are released

Oxytocin = released in significant amounts only during childbirth and nursing (women only), causes uterine wall to contract (childbirth and sex) and lactation (nursing). Sometimes, it is given to women to induce labor or stop postpartum bleeding.

Antidiuretic hormone (ADH) = inhibits urine production, causes kidneys to reabsorb more water from urine, which decreases urine volume and increases blood volume, which increases blood pressure. This is why this hormone is sometimes called vasopressin.

Thyroid gland

Located at base of throat below the Adam’s apple

Consists of two lobes joined by a central mass (isthmus)

Produces two hormones: Thyroid hormone = actually two hormones,

thyroxine (T4) and triiodothyronine (T3). Thyroxine is the major hormone secreted by the thyroid, formed when T4 is converted to T3. Controls rate at which glucose is oxidized and converted to energy targets all cells in body, also helps control growth and development of reproductive and nervous systems

Thyroid gland

Calcitonin = also called thyrocalcitonin, decreases blood calcium levels by causing calcium to be deposited in bones, works against parathyroid hormone, released directly into blood in response to rising calcium levels, decreases in the elderly

Thyroid gland

Goiter = enlargement of the thyroid gland due to a lack of iodine in the diet, which leads to an under production of TSH, no longer common due to adding iodine to our salt

Cretinism = dwarfism in which adult body proportions remain childlike, due to lack of TH stimulation

Grave’s disease = form of hyperthyroidism, high metabolism, intolerance of heat, inability to relax, bulging eyes

Parathyroid gland

Tiny masses of glandular tissue on the posterior surface of thyroid gland

Usually, there are only four lobes (two on each side), but there can be up to eight

Parathyroid hormone (PTH) = also called parathormone, regulates calcium levels of blood. If blood calcium levels drop too low, PTH stimulates osteoclasts to break down the bone matrix and release calcium back into the blood.

Parathyroid gland

PTH increases blood levels of calcuim, calcitonin decreases blood levels of calcium

Parathyroid gland

Previously, surgeons would remove the parathyroid glands of a hyperthyroid patient completely when they removed some of the thyroid.

This resulted in tetany, uncontrollable, potentially fatal muscle spasms that result from neurons becoming extremely irritable and overactive due to low calcium levels

Hyperparathyroidism can cause massive bone destruction

Adrenal glands

Bean-shaped, sit on top of kidneys Two endocrine glands in one, cortex

(glandular tissue) and medulla (neural tissue)

Hormones of the Adrenal Cortex

Three layers of adrenal cortex Corticosteroids = three major groups of

steroid hormones Mineralocorticoids (produced by outer layer) –

regulate mineral/salt concentrations of blood Glucocorticoids (produced by middle layer) –

regulate cellular metabolism Sex hormones (produced by inner layer) – play a

role in sex characteristics, both androgens (male hormones) and estrogens (female hormones) are produced regardless of one’s gender

Hormones of the Adrenal Cortex

Aldosterone = mineralocorticoid, regulates mineral (or salt) content of blood. When aldosterone levels rise, kidneys absorb more sodium from blood and release more potassium ions into urine

Renin = enzyme produced by kidneys when blood pressure drops, causes aldosterone to be released

Hormones of the Adrenal Cortex

Cortisone – glucocorticoid, suppresses immune system and reduces inflammation, increases blood glucose levels

Cortisol – glucocorticoid, suppresses immune system, reduces inflammation, increases blood glucose levels, aids in metabolism, promotes the production of surfactant in fetal lungs

Hormones of the Adrenal Cortex

Addison’s disease = caused by low aldosterone levels, sodium and water are rapidly lost from the body, characterized by bronze skin, weak muscles, decreased ability to cope with stress, and a suppressed immune system

Cushing’s syndrome = excessive output of glucocorticoids, results in “moon face” and “buffalo hump” fat deposits, high blood pressure, hyperglycemia, weakened bones, and sometimes diabetes

Hormones of the Adrenal Medulla

Epinephrine = also called adrenaline Norepinephrine = also called noradrenaline Together, epinephrine and norepinephrine

are called catecholamines Catecholamines are released after stimulus

from sympathetic nervous system “fight-or-flight” response, increase in heart rate, blood pressure, and blood glucose levels, also, lung passageways are dilated more oxygen in blood

Pancreas

Located close to stomach Mixed gland Pancreatic islets = formerly called islets

of Langerhans, scattered throughout pancreas, produce hormones

Pancreas

Insulin = released by beta cells of islets, acts on a cell’s ability to transport glucose across their plasma membrane, helps speed up cellular respiration, reduces blood glucose levels

Glucagon = released by alpha cells of islets, antagonist of insulin, targets the liver, which it stimulates to break down stored glycogen into glucose to release into the blood, raising blood glucose levels

Pancreas

Diabetes mellitus = caused by insufficient insulin production, blood glucose levels get too high. Glucose is flushed out in the urine, resulting in frequent urination and dehydration. Body uses fats and proteins for energy instead of glucose, resulting in loss of body weight. Can result in coma or death. Type I diabetes = “juvenile” diabetes, pancreas is

not producing any insulin, requires continuous injections of insulin

Type II diabetes = adult-onset diabetes, can be treated by special diet or oral medication

http://www.youtube.com/watch?v=jHRfDTqPzj4

Pineal gland

Small, cone-shaped gland in the roof of the third ventricle of the brain

Produces many chemical substances, we still don’t know what all of them are or what all the ones we have discovered do

Melatonin = levels of secretion rise and fall over the course of the day and night, peak production at night, believed to be the “sleep trigger,” helps coordinate fertility hormones and inhibits reproductive system until puberty, especially in females

Thymus

Located in upper thorax behind the sternum

Large in infants and children, decreases in size throughout adulthood

By old age, it is mostly fibrous connective tissue and fat

Thymosin = stimulates development of T cells (immune system)

Gonads

Produce sex hormones identical to those produced by adrenal cortex cells

Difference in source and amounts produced

Male = testes, female = ovaries

Hormones of the Ovaries

Pair of almond-sized organs in pelvic cavity, also produce female sex cells (ova, or eggs)

Estrogens = primarily estrone and estradiol, steroid hormones, produced in Graafian follicles of ovaries, stimulate development of female secondary sex characteristics, prepare uterus to receive fertilized eggs, helps control menstrual cycle

Progesterone = steroid hormone, works with estrogen to control menstrual cycle, helps keep uterine muscles from contracting during pregnancy, prepares breast tissue for lactation, produced by corpus luteum

Hormones of the Testes

Pair of oval organs suspended outside the pelvic cavity in the scrotum, also produces male sex cells (sperm)

Testosterone = made by interstitial cells of testes, causes the development of male secondary sex characteristics, promotes growth and maturation of male reproductive system, stimulates male sex drive, production begins at puberty and is stimulated by luteinizing hormone

Other Hormone-Producing Tissues and Organs

Pockets of hormone-producing cells are found in the walls of the small intestine, stomach, kidneys, and heart

Placenta = organ formed temporarily in the uterus of a pregnant woman, acts as respiratory, excretory, and nutrition-delivery systems for fetus, also produces human chorionic gonadotropin (hCG), which stimulates the continued production of estrogen and progesterone so that the uterine wall is not shed. In the third month, the placenta will start to produce the estrogen and progesterone itself. It also produces human placental lactogen (hPL), which prepares breasts for lactation, and relaxin, which causes the mother’s pelvic ligaments and pubic symphysis to relax and become more flexible.

Developmental aspects of endocrine system

Embryonic development varies between the glands

All glands are present at birth, even if they are not yet active

Endocrine organs usually function smoothly until old age, except for thymus and ovaries, which decline during menopause (middle-age)