Unit 10.1: The Endocrine System

Lesson 1 Chapter – The Endocrine System 1 Unit 10 Study Guide The endocrine system affects bodily activities by releasing chemical messengers, called hormones, into the bloodstream. The system as a whole sends messages to cells in virtually any part of the body. It not only helps to regulate the activity of smooth and cardiac muscle and some glands, it significantly affects virtually all other tissues as well. Endocrine organs are widely scattered throughout the body. The nervous and endocrine systems coordinate their activities and together are considered the control systems of the body. The Ways Hormones Affect Targets Although the effects of hormones are many and varied, their actions can be categorized into 5 broad areas: 1. Help control the internal environment by regulating its chemical composition and volume. 2. Respond to changes in the environmental conditions to help the body cope with emergency demands such as infection, trauma, emotional stress, dehydration, starvation, hemorrhage, and temperature extremes. 3. Integration of growth and development. 4. Contribute to the basic processes of reproduction. 5. Help regulate organic metabolism and energy balance. There are two kinds of glands in the body: endocrine and exocrine 1. 2. Exocrine glands have ducts through which their non-hormonal products are routed to a membrane surface. Examples are sweat, sebaceous, mucous, and digestive glands. Endocrine glands are ductless glands which release hormones directly into the blood and lymph. They typically have a rich vascular and lymphatic drainage. Endocrine glands include the pituitary, thyroid, parathyroid, adrenal, pineal, and thymus glands. Several organs in the body, such as the pancreas and gonads, contain discrete areas of endocrine tissue within their substance and produce hormones as well. The hypothalamus is both an integral part of the nervous system and also a producer and releaser of hormones; it is considered a neuroendocrine organ. There are pockets of hormoneproducing cells elsewhere, but these are the major endocrine organs. Lesson 1 Chapter – The Endocrine System Hormone: Hypothalamus ● Hypothalamus received input from limbic system, cerebral cortex, thalamus and reticular activating system – also from internal organs, retina ● Hypothalamus has three endocrine functions: 1. Produces regulatory hormones that direct the actions of the anterior pituitary 2. Releases oxytocin and ADH (anti-diuretic hormone) 3. Autonomic control of the adrenal glands 2 Lesson 1 Chapter – The Endocrine System 3 Pituitary Gland About the size and shape of a pea, located in the sella turcica of the sphenoid bone, attached to the hypothalamus via a stalk called the infundibulum. It has two major lobes, one neural and one glandular. Lesson 1 Chapter – The Endocrine System 4 I. The pituitary gland secretes eight hormones. o The anterior pituitary secretes growth hormone, thyroid-stimulating hormone, adrenocorticotropic hormone, follicle-stimulating hormone, luteinizing hormone, and prolactin. o The posterior pituitary releases antidiuretic hormone (also called vasopressin) and oxytocin, both of which are produced in the hypothalamus and transported to the posterior pituitary by the hypothalamo-hypophyseal nerve tract. II. The release of posterior pituitary hormones is controlled by neuroendocrine reflexes. III. Secretions of the anterior pituitary are controlled by hypothalamic hormones that stimulate or inhibit these secretions. o Hypothalamic hormones include TRH, CRH, GnRH, PIH, somatostatin, and a growth hormone-releasing-hormone. o These hormones are carried to the anterior pituitary by the hypothalamohypophyseal portal system. Lesson 1 Chapter – The Endocrine System IV. Secretions of the anterior pituitary are also regulated by the feedback (usually negative feedback) exerted by target gland hormones. V. Higher brain centers, acting through the hypothalamus, can influence pituitary secretion. Posterior Lobe (Neurohypophysis) – composed of nervous tissue; releases: - neurohormones that it receives ready-made from the hypothalamus oxytocin and anti-diuretic hormone (ADH) released on demand in response to nerve impulses from hypothalamic neurons. Anterior Lobe (Adenohypophysis) – composed of glandular tissue; manufactures and releases its own hormones as a result of stimulation by releasing hormones from the hypothalamus; called the master endocrine gland; releases 6 hormone products which all function via second messenger systems. - - Four of the six are hormones that regulate the functioning of other endocrine glands: thyroid stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), follicle stimulating hormone (FSH), and luteinizing hormone (LH) Remaining two affect non-endocrine targets: growth hormone (GH) and prolactin. Anterior Pituitary Hormones 1. Thyroid Stimulating Hormone (TSH) – Stimulates the thyroid gland to secrete thyroid hormone. Produced by thyrotropic cells; synthesis and release of T3 and T4, and metabolic rate increased. 2. Adrenocorticotropic Hormone (ACTH) – Stimulates the adrenal cortex to release corticosteroid hormones (glucocorticoids, gonadocorticoids, and mineralocorticoids). Produced by corticotropic cells. * 3. Gonadotropins – Follicle Stimulating Hormone (FSH) and 4. Luteinizing Hormone (LH) Regulate the function of the gonads (ovaries and testes). In both sexes, FSH stimulates gamete production, while LH promotes the production of gonadal hormones (estrogen and testosterone). 5 Lesson 1 Chapter – The Endocrine System 5. Growth Hormone (GH) – Stimulates most body cells to increase in size and divide. Its major targets are the bones and skeletal muscles. Induces protein synthesis and encourages the use of fats for fuel, thus conserving glucose. GH release is controlled by hormones from the hypothalamus which stimulates or inhibits its release. Abnormalities result from hypersecretion (acromegaly) and hyposecretion (dwarfism). 6. Prolactin (PRL) – Stimulates milk production by the breast. Produced by lactotropic cells. * Melanocyte-Stimulating Hormone (MSH) Stimulates melanocytes (melanin-producing cells). Produced by same corticotropic cells of ACTH. MSH produced/released from the pars intermedia of the anterior pituitary. Posterior Pituitary Hormones 1. Antidiuretic Hormone (ADH) – aka vasopressin Influences body water balance. Targets kidney tubules, which respond by reabsorbing more water from the forming urine and returning it to the bloodstream; less urine is produced and blood volume is increased. Ingestion of alcohol inhibits ADH secretion and causes copious urine output. Under certain conditions, such as severe blood loss, exceptionally large amounts of ADH are released. At high levels, ADH has a vasoconstrictor effect to try to maintain blood pressure. 2. Oxytocin – Stimulates contraction of smooth muscle of uterus during childbirth. [PTH, Calcitonin –thyroid, Pancreas ARE NOT UNDER pituitary gland control.] Thyroid and Parathyroid Glands The thyroid is a butterfly-shaped gland located in the anterior neck, overlying the trachea. Internally, the gland is composed of hollow spherical structures called follicles with other cells located between the follicles. I. The thyroid follicles secrete tetraiodothyronine (T4, or thyroxine) and lesser amounts of triiodothyronine (T3). 6 Lesson 1 Chapter – The Endocrine System o These hormones are formed within the colloid of the thyroid follicles. o The parafollicular cells of the thyroid secrete the hormone calcitonin, which may act to lower blood calcium levels. II. The parathyroids are small structures embedded within the thyroid gland. They secrete a parathyroid hormone (PTH) which promotes a rise in blood calcium levels. III. Regulation of thyroid hormone secretion depends upon a complex thyroid gland– pituitary gland negative feedback process. Thyroid Gland 1. Thyroid Hormone – referred to as the body’s major metabolic hormone. It comes from a precursor molecule called thyroglobulin which is produced by the cells making up the walls of the follicles. Thyroglubulin is stored inside the follicle and requires iodine atoms to function properly. When thyroid hormone is needed, it is produced by breaking down thyroglobulin into smaller pieces. Thyroid hormone is actually two active iodine-containing hormones, thyroxine (T4) and triiodothyronine (T3). T4 is secreted from the thyroid in greater amounts than T3. T3 is the most active and is formed at target tissues by conversion of T4 to T3. Except for the adult brain, spleen, testes, uterus, and the thyroid gland itself, thryoid hormone affects virtually every cell in the body. It stimulates enzymes concerned with glucose oxidation, increases basal metabolic rate and body heat production, increases adrenergic receptor in blood vessels, and helps in tissue growth. The thyroid gland is unique in its ability to make, store, and release its hormones. 2. Calcitonin – produced by parafollicular cells of the thyroid gland. It is released in response to high blood levels of calcium. Its function is to lower blood calcium levels by stimulating calcium uptake by bone. It is generally more important in childhood. 7 Lesson 1 Chapter – The Endocrine System Parathyroid has 4 tiny pea glands on the posterior surface of thyroid gland. Two different types of cells in the parathyroid gland: ⚫ chief cells (parathyroid cells) and oxyphil cells (function unknown) Parathyroid Gland 1. PTH – (Parathyroid hormone). It is the most important hormone controlling calcium balance. Calcium is critically important for nerve impulse conduction, muscle contraction, and clotting. PTH is released by the parathyroid gland in response to low blood calcium levels. Its function is to increase blood calcium levels by stimulating three target organs: Skeleton – causes digestion of bone matrix and release of calcium Kidneys – causes increased reabsorption of calcium from the forming urine and returns it to the blood Small intestine – causes increased absorption of calcium by the intestinal mucosal cells. Adrenal Gland The paired adrenal glands are located on top of the kidneys where they are enclosed in a fibrous capsule and a cushion of fat. Each gland is structurally and functionally two endocrine glands in one: [They look like rice hats!] 1. The inner adrenal medulla is made up of nervous tissue; part of the sympathetic nervous system [derived from endoderm] 2. The outer adrenal cortex surrounds the medulla, forms the bulk of the gland, and is organized into three layers [derived from mesoderm] Adrenal is made up of 3 cortex. - Zona glomerulosa secretes mineralocorticoids – salty Zona fasciculata secretes glucocorticoids sticky 8 Lesson 1 - Chapter – The Endocrine System 9 Zona reticularis secretes androgens – sexy Use salty, sticky, sexy to remember what is released from the outside of the cortex in. I. The adrenal cortex secretes mineralocorticoids (mainly aldosterone), glucocorticoids (mainly cortisol), and sex steroids (primarily weak androgens). o The glucocorticoids help to regulate energy balance. They also can inhibit inflammation and suppress immune function. o The pituitary-adrenal axis is stimulated by stress as a part of the general adaptation syndrome. All these hormones are steroid hormones. The cortex is organized into the following layers and major products: Adrenal Cortex 1. zona glomerulosa (outermost) – produces mineralocorticoids which help control the balance of minerals and water in the blood 2. zona fasciculata (middle) – secretes the metabolic hormones called glucocorticoids 3. zona reticularis (innermost) – secretes the glucocorticoids and small amounts of adrenal sex hormones or gonadocorticoids II. The adrenal medulla secretes epinephrine and lesser amounts of norepinephrine. These hormones complement the action of the sympathetic nervous system. III. The adrenal medulla is stimulated by the sympathetic division of the autonomic nervous system. Two powerful hormones are secreted: epinephrine and norepinephrine, collectively known as catecholamines. When the body is activated by a short term stressor or emergency the sympathetic nervous system is mobilized resulting in blood sugar level increases, vasoconstriction, and a faster heart rate. Adrenal Medulla Epinephrine Norepinephrine Pancreas Lesson 1 Chapter – The Endocrine System 10 Located behind the stomach; between the duodenum of the small intestine and the spleen. It is a mixed gland composed of both endocrine and exocrine gland cells. The bulk of the gland is exocrine, producing digestive enzymes that are carried by a duct to the small intestine. Scattered among the exocrine part are minute clusters of cells (islets of Langerhans) that produce pancreatic hormones. The islets contain 2 major populations of hormone-producing cells: 1. alpha – make glucagon 2. beta – make insulin ⮚ Mostly composed of cells called pancreatic acini. Produce an alkaline pancreatic juice (exocrine function) ⮚ Scattered among the pancreatic acini are small clusters of endocrine cells called pancreatic islets (islets of Langerhans) ⮚ Islets of Langerhans are composed of four types of cells (endocrine function) I. Beta cells in the islets secrete insulin and alpha cells secrete glucagon. o Insulin lowers blood glucose and stimulates the production of glycogen, fat, and protein. o Glucagon raises blood glucose by stimulating the breakdown of liver glycogen. It also promotes lipolysis and the formation of ketone bodies. o The secretion of insulin is stimulated by a rise in blood glucose following meals. The secretion of glucagon is stimulated by a fall in blood glucose during periods of fasting. Lesson 1 Chapter – The Endocrine System Pancreas 1. Glucagon (alpha) - Released in response to low blood sugar levels. Its function is to raise blood sugar to the proper level. Its major target is the liver where it promotes: a) glycogenolysis – breakdown of glycogen to glucose b) gluconeogenesis – formation of glucose from non-carbohydrate precursors such as fatty acids and amino acids. 2. Insulin (beta) - Released in response to high blood sugar levels. Its function is to lower blood sugar to the proper level by the following mechanisms: a) inhibits breakdown of glycogen to glucose b) enhances transport of glucose into cells c) promotes breakdown of glucose inside cells for ATP production d) joins glucose together to form glycogen e) converts leftover glucose to fat after the body’s immediate energy needs are met 3. Somatostatin (delta) – Targets alpha and beta cells, and digestive epithelium. Inhibition secretion of glucagon and insulin. 4. Pancreatic polypeptide (f cell) – Targets gallbladder and pancreas, possibly GI tract. Inhibits gallbladder contractions; regulates some pancreatic enzymes, may control nutrient reabsorption. Gonads 11 Lesson 1 Chapter – The Endocrine System 12 Produce gonadal sex hormones, identical to those produced by adrenal cortex. Sex hormones are steroid hormones. - Ovaries produce estrogen and progesterone. - Estrogen is responsible for maturation of female reproductive organs and secondary sex characteristics of females at puberty. During pregnancy the levels of both hormones rise. The testes produce testosterone which is responsible for maturation of male reproductive organs, secondary sex characteristics of males at puberty, and the sex drive. I. The gonads secrete sex steroid hormones. o Leydig cells in the interstitial tissue of the testes secrete testosterone and other androgens. o Granulosa cells of the ovarian follicles secrete estrogen. o The corpus luteum of the ovaries secretes progesterone, as well as estrogen. Pineal Gland The pineal gland, located on the roof of the third ventricle of the brain, secretes melatonin. o Melatonin secretion is regulated by the suprachiasmatic nucleus of the hypothalamus, which is the major center for the control of circadian rhythms. o Melatonin secretion is the highest at night, and this hormone has a sleep-promoting effect. In many species, it also has an antigonadotropic effect and may play a role in timing the onset of puberty in humans, although this is as yet unproven. Lesson 1 Chapter – The Endocrine System 13 Thymus Gland The thymus is the site of T cell lymphocyte production and secretes a number of hormones that may play a role in regulating the immune system. - Organ of immune system - Produces thymosin - Assists in maturation of T cells Outer adrenal cortex layer, T cells aren’t mature yet. Once exposed to thymosin, they will move inside the thymus gland which will transfer to bone marrow. Clinical Application Lesson 1 Chapter – The Endocrine System � Many conditions arise out of endocrine damage or dysfunction ⚫ Type I and II diabetes ⚫ Hypothyroidism, hyperthyroidism ⚫ Some forms of infertility ⚫ Cushing’s syndrome, Addison’s disease � Pharmacology – many drugs we use act directly on endocrine receptors 14