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 • Humoralstimuli = 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 antidiuretichormone, 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 luteumand 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 T4is 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, calcitonindecreases 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)