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Chapter 17 The Endocrine System Endocrine System: Overview Acts with the nervous system to coordinate and integrate the activity of body cells Influences metabolic activities by means of hormones transported in the blood Responses occur more slowly but tend to last longer than those of the nervous s...

Chapter 17 The Endocrine System Endocrine System: Overview Acts with the nervous system to coordinate and integrate the activity of body cells Influences metabolic activities by means of hormones transported in the blood Responses occur more slowly but tend to last longer than those of the nervous system Endocrine glands: pituitary, thyroid, parathyroids, adrenals, and pineal gland Endocrine System: Overview Some organs produce both endocrine and exocrine products (e.g., pancreas and gonads) Other tissues and organs that produce hormones include adipose cells, thymus, cells in the walls of the small intestine, stomach, kidneys, and heart Endocrine Glands The Pituitary Gland and Hypothalamus The pituitary gland (hypophysis) has two major lobes 1. Neurohypophysis (posterior lobe): – Neural tissue – Pituicytes (glial-like supporting cells) and nerve fibers 2. Adenohypophysis (anterior lobe): – Simple cuboidal tissue Pituitary-Hypothalamic Relationships Posterior pituitary (Neurohypophysis) – Neural connection from the hypothalamus through the infundibulum – Oxytocin and antidiuretic hormone (ADH) are produced in the hypothalamus and transported to posterior lobe – Both hormones are stored and released from the posterior pituitary – No hormones are produced in the posterior lobe they are only stored and released Posterior Pituitary Oxytocin Positive feedback mechanism – increases stimulus until activity occurs Stimulates uterine contractions during childbirth Also triggers milk ejection (“letdown” reflex) in women producing milk Plays a role in sexual arousal and orgasm in males and females Antidiuretic Hormone (ADH) Causes retention of sodium (Na⁺) in the kidneys which and prevents urine formation ADH also increases blood pressure by the retention of water If overall solute concentration is high – ADH is synthesized and released, inhibiting urine formation If solute concentration is low – ADH is not released, allowing water loss Antidiuretic Hormone (ADH) Diabetes Insipidus – a decrease in ADH release – copious urine formation – increased thirst Alcohol inhibits ADH release and causes copious urine output Anterior Pituitary Pituitary-Hypothalamic Relationships Anterior pituitary (Adenohypophysis): – Made of simple cuboidal epithelium – Hormones are produced and released from anterior pituitary – Hypophyseal portal system – the circulatory system around the pituitary gland and infundibulum Secondary capillary plexus Carries releasing and inhibiting hormones to the anterior pituitary to regulate hormone secretion Anterior Pituitary Hormones Growth hormone (GH) Thyroid-stimulating hormone (TSH) or thyrotropin Adrenocorticotropic hormone (ACTH) Follicle-stimulating hormone (FSH) Luteinizing hormone (LH) Prolactin (PRL) Anterior Pituitary Hormones Growth Hormone – Stimulates most cells, but targets bone and skeletal muscle – Promotes protein synthesis and encourages use of fats for fuel – Hyposecretion: In children results is pituitary dwarfism Reach maximum height of 4 feet Normal body proportions – Hypersecretion: Children results in gigantism – Normal body proportions except long bones in children Adults results in acromegaly – Causes over growth of hand, feet and face Anterior Pituitary Hormones Thyroid Stimulating Hormone (Thyrotropin) Released to its target cells in the Thyroid gland Causes the release of Thyroid hormones Adrenocorticotropic Hormone (Corticotropin) – Targets cells in the adrenal cortex – Stimulates the adrenal cortex to release cortisol (glucocorticoid) – Responsible for the growth and development of the adrenal cortex Anterior Pituitary Hormones Follicle Stimulating Hormone – Gonadotropin – Targets ovaries and testes – Men Causes sperm production – Women Maturation of follicles and egg production Luteinizing Hormone – Gonadotropins – Targets ovaries and testes – Men Effects testosterone production – Women Effects ovulation (release of egg) Effects estrogen production Prolactin – – – – Targets the breasts Causes lactation (milk production) Blood levels rise toward the end of pregnancy Suckling stimulates PRH release and promotes continued milk production Thyroid Gland The largest completely endocrine gland Produces triiodothyronine, thyroxine and calcitonin Follicles: hollow openings filled with colloid Follicle cells: produce colloid (thyroglobulin) – Colloid is a precursor to thyroxine – Colloid will mature into thyroxine when iodine is attached – Iodine must be ingested for colloid maturation Parafollicular cells: – produce calcitonin Thyroid Gland Thyroid Hormone Thyroid hormones – T₃ - Triiodothyronine – T₄ - Thyroxine – Major metabolic hormones – increases cellular metabolism Hyposecretion: – Cretinism (children) – Myxedema (adults) Also called Hypothyroidism Goiter formation due to over production of TSH when there is a lack of dietary iodine Hypersecretion: – Grave’s disease Exophthalmos – bulging eyeballs Goiter Calcitonin Produced by parafollicular cells Antagonist to parathyroid hormone (PTH) Lowers blood calcium levels When blood calcium levels are too high it inhibits osteoclast activity and release of calcium from bone matrix Stimulates calcium uptake and incorporation into bone matrix Regulated by a humoral (calcium concentration in the blood) negative feedback mechanism Parathyroid Glands Tiny glands usually embedded in the posterior aspect of the thyroid Produces parathyroid hormone (PTH) – Raises blood calcium levels – Most important hormone in calcium homeostasis – Stimulates osteoclast activity to remove calcium for the bone to release it back to the blood (negative feedback) Target cells for PTH: – Osseous tissue – Kidneys – Intestines Parathyroid Glands Parathyroid Glands Hypoparathyroidism – low blood calcium levels – – – – Makes neurons more excitable causing tingling sensations Tetany – muscle twitching Convulsions Can progress to respiratory paralysis and death Hyperparathyroidism – high blood calcium levels – Causes a weakening of bones due to calcium loss – Causes the release of too many neurotransmitters causing a decrease in neuro function – Elevated levels of calcium depresses the nervous system – Causes the formation of kidney stones Adrenal (Suprarenal) Glands Adrenal glands – paired, pyramid-shaped organs atop the kidneys Structurally and functionally, they are two glands in one – Surrounded by a fibrous capsule and fat Adrenal cortex: outer region is glandular tissue Adrenal medulla: inner region of nervous tissue, part of the sympathetic nervous system Adrenal Cortex Synthesizes and releases steroid hormones called corticosteroids Cortical cells are arranged in layers – Zona glomerulosa: produce mineralocorticoids- (aldosterone) Control electrolytes and water balance (mineral balances in the blood) Aldosterone – – – – Stimulates sodium reabsorption and water retention in the kidneys Eliminates potassium Inhibited by Atrial natriuretic Hormone (ANP) Hyposecretion - Addison’s disease – Zona fasciculata: Produce glucocorticoids - (cortisol) Metabolic hormones and keeps blood sugar levels constant Hypersecretion – Cushing’s syndrome – Zona reticularis: Produce gonadocorticoids – (androgens) Adrenal sex hormones converted to testosterone and estrogen Leads to increased sex drive Adrenal Medulla Chromaffin cells: secrete epinephrine (80%) and norepinephrine (20%) These hormones cause – Blood glucose levels to rise – Blood vessels to constrict/dilate – The heart to beat faster – Blood to be diverted to the brain, heart, and skeletal muscle Adrenal Gland Pancreas A triangular gland, which has both exocrine and endocrine cells, located behind the stomach Acinar cells (exocrine): produce enzyme-rich juice for digestion Pancreatic islets (islets of Langerhans) (endocrine): – Glucagon: produced by Alpha cells Increases blood glucose levels by causing the liver to breakdown glycogen Effects of glucagon – Glycogenolysis- breakdown of glycogen to glucose – Gluconeogenesis- synthesis of glucose from lactic acid and non-carbohydrates. Takes place in the liver. – Beta cells: produce insulin Lowers blood glucose levels Targets liver and most cells Inhibits glycogen breakdown Pancreas Effects of Insulin: – Diabetes Mellitus (DM) Either Hyposecretion or hypoactivity of insulin Signs of DM – Polyuria – huge urine output – Polydipsia – excessive thirst – Polyphagia – excessive hunger and food consumption – Hyperinsulinism – causes hypoglycemia Pancreas Pineal Gland Small gland hanging from the roof of the third ventricle of the brain Pinealocytes: produce melatonin derived from serotonin Melatonin: – – – – Responsible for sleep/wake cycles Increases in the evening hours making you sleepy Release inhibited by sunlight Causes Seasonal Affective Disorder (SAD) Gonads: Female - Ovaries Paired ovaries in the abdominopelvic cavity produce estrogen and progesterone Estrogen – Responsible for maturation of the reproductive organs – Appearance of secondary sex characteristics such as adipose tissue in the breasts and buttocks Progesterone – Causes breast development – Cyclic changes in uterine mucosa Gonads: Male Testes located in an extra-abdominal sac (scrotum) produce testosterone Testosterone: – Produced by interstitial cells – Initiates maturation of male reproductive organs – Causes appearance of secondary sexual characteristics and sex drive – Is necessary for sperm production Placenta Human Chorionic Gonadotropin (HCG) – Produced by the placenta – Maintains uterine thickness – Maintains corpus luteum – Elevates progesterone levels Thymus Lobulated gland located deep to the sternum Large in infants and children Small in adults – diminishes in size throughout adulthood Produces thymosin – Involved in the normal development of Tlymphocytes Chemical Messengers Hormones: are long-distance chemical signals that travel in the blood or lymph Autocrines: hormones that exert effects on the same cells that secrete them Paracrines: locally acting hormones that affect cells other than those that secrete them Chemistry of Hormones Two main classes 1. Non-steroid: amino acid-based hormones Water soluble Amines, thyroxine, peptides, and protein 2. Steroids Lipid soluble Synthesized from cholesterol Gonadal and adrenocortical hormones (testosterone, progesterone, and estrogen) Plasma Membrane Receptors and Second-Messenger Systems Water soluble hormones use a second messenger system for activation 1. Hormone (first messenger) binds to receptor 2. ATP is used to activate cAMP inside the cell 3. cAMP (second messenger) activates protein kinases Kinases – enzymes that can activate or deactivate other enzymes. When activated, profound effects are made in the body Second Messenger Mechanism Intracellular Receptors and Direct Gene Activation Lipid soluble hormones cause direct gene activation 1. Diffuse into their target cells and bind with intracellular receptors 2. Receptor-hormone complex enters the nucleus 3. Receptor-hormone complex binds to a specific region of DNA 4. This prompts DNA transcription of gene to produce mRNA 5. The mRNA directs protein synthesis Direct Activation Mechanism Mechanisms of Hormone Action Hormone act on target cells Target cells must have specific receptors to which the hormone binds Ex. - ACTH receptors are only found on certain cells of the adrenal cortex Ex. - Thyroxine receptors are found on nearly all cells of the body Target Cell Activation Target cell activation depends on three factors 1. Blood levels of the hormone 2. Relative number of receptors on or in the target cell 3. Affinity of binding between receptor and hormone Target Cell Activation Hormones influence the number of their receptors – Up-regulation—target cells form more receptors in response to the hormone – Down-regulation—target cells lose receptors in response to the hormone Control of Hormone Release Blood levels of hormones – Are controlled by negative feedback systems – Vary only within a narrow desirable range Hormones are synthesized and released in response to 1. Humoral stimuli 2. Hormonal stimuli 3. Neural stimuli Humoral Stimuli 1. Humoral Stimuli: changing blood levels of ions and nutrients directly stimulates secretion of hormones Example: Calcium in the blood – Declining blood calcium concentration stimulates the parathyroid glands to secrete PTH (parathyroid hormone) – PTH causes blood calcium concentrations to rise and the stimulus is removed Hormonal Stimuli 2. Hormonal stimuli - Hormones stimulate other endocrine organs to release their hormones – Hypothalamic hormones stimulate the release of most anterior pituitary hormones – Anterior pituitary hormones stimulate targets to secrete still more hormones – Hypothalamic-pituitary-target endocrine organ feedback loop: hormones from the final target organs inhibit the release of the anterior pituitary hormones Neural Stimuli 3. Neural stimuli - Nerve fibers stimulate hormone release – Sympathetic nervous system fibers within the adrenal medulla stimulate the secretion of epinephrine and norepinephrine Hormones in the Blood Hormones circulate in the blood either free or bound – Steroids and thyroid hormone are attached to plasma proteins – All others circulate without carriers Water soluble hormones – (amino Acid based) levels spike in the blood but they quickly get degraded in the body Lipid based hormones – (steroid) levels remain constant Hormones in the Blood Hormones are removed from the blood by – Half-life—the time required for a hormone’s blood level to decrease by half – Degraded by enzymes in the liver – Liver detoxifies and then flushed out by the kidneys Interaction of Hormones at Target Cells Multiple hormones may interact in several ways – Permissiveness: one hormone cannot exert its effects without another hormone being present – Synergism: more than one hormone produces the same effects on a target cell (amplification) – Antagonism: one or more hormones opposes the action of another hormone (calcitonin and parathyroid)