Endocrine System

The Endocrine System Produces Hormones Endocrine system: collection of specialized cells, and tissues that secrete hormones Endocrine glands: ductless organs that secrete hormones into blood, interstitial fluid, lymph Hormones Chemical messengers secreted by endocrine glands Circulate in the bloodstream Act on specific cells in the body (target cells) that have the appropriate hormone receptor

The Endocrine System Produces Hormones Hormones have access to every cell Each hormone acts only on specific cells (target cells) Only specific cells have receptors for specific hormones Endocrine control is slower than nervous system Endocrine and nervous systems interact

Hormones Are Classified as Steroid and Nonsteroid Steroid hormones Structurally related to cholesterol Lipid soluble Peptide hormones Structurally related to proteins Lipid insoluble

Peptide vs Steroid Hormone Steroid hormones - take longer to exhibit effect but are permanent, made when needed (Ex. Testosterone, Estrogen) Peptide hormones - faster, but temporary effects, stored and released on signal (Ex. Calcitonin, ADH)

Hormones Participate in Negative Feedback Loops Many hormones participate in internal homeostatic control mechanisms Negative feedback loop involving hormones includes the following: Endocrine gland serves as the control center Hormone is the pathway between the control center and the effectors Target tissues or organs are the effectors

Controlled variable Hormone Higher Set point Lower Figure 13.4 Controlled variable Higher Set point Lower (Reversal of initial change) Endocrine gland (control center) Hormone Target cells, tissues, and organs (effectors)

The Hypothalamus and the Pituitary Gland Homeostatic control center of the brain Links nervous system and endocrine system Produces two hormones of its own Monitors and controls hormone secretions of the pituitary gland Pituitary gland “Master” gland Secretes eight different hormones that regulate other endocrine organs Two lobes: posterior and anterior

Posterior Pituitary Stores ADH and Oxytocin Posterior pituitary is connected to hypothalamus by neuroendocrine cells Hormones (ADH and oxytocin) made in cell bodies in hypothalamus are transported down axons to axon endings in posterior pituitary for storage and release Posterior pituitary hormones: nonsteroidal Antidiuretic hormone (ADH) Conserves water in kidneys Regulates water balance in body Oxytocin Causes uterine contractions during labor and milk ejection through neuroendocrine reflex

https://www.youtube.com/watch?v=p8XKNx4s0rY Neuroendocrine cells Figure 13.5 Neuroendocrine cells Hypothalamus Anterior pituitary Posterior pituitary Oxytocin ADH https://www.youtube.com/watch?v=p8XKNx4s0rY

Neuroendocrine cells release oxytocin when stimulated. Figure 13.6 Neuroendocrine cell 3 Neuroendocrine cells release oxytocin when stimulated. Hypothalamus Anterior pituitary 4 Spinal cord Oxytocin is transported by blood to mammary glands. 2 Sensory and spinal nerves carry impulses to the neuroendocrine cells of hypothalamus. Posterior pituitary Milk ejected Oxytocin 1 Nursing stimulates nerve receptors in nipple.

The Anterior Pituitary Produces Six Key Hormones Controlled by hypothalamus Releasing and inhibiting hormones from hypothalamus travel to pituitary through pituitary portal system The release of each anterior pituitary hormone is controlled, at least partially, by the hypothalamus

The Anterior Pituitary Produces Six Key Hormones ACTH (adrenocorticotropic hormone) Stimulates adrenal cortex to release glucocorticoids (cortisol) TSH (thyroid-stimulating hormone) Acts on thyroid gland, promoting release of thyroid hormones FSH and LH (gonadotropins) Stimulate growth, development, and function of ovaries and testes Not produced until about age 10–13 (puberty) Increase in production initiates sexual maturation and development at puberty

The Anterior Pituitary Produces Six Key Hormones Prolactin Stimulates development of mammary glands and milk production Growth hormone Has widespread effects on body Major effects on bone, muscle Most of its growth-promoting effects occur during childhood and adolescence

Anterior pituitary hormones Figure 13.7 Neuroendocrine cells Hypothalamus 1 Neuroendocrine cells in hypothalamus produce and secrete releasing and inhibiting hormones 2 Blood flow The pituitary portal blood system carries releasing and inhibiting hormones directly to the anterior pituitary Anterior pituitary endocrine cells 3 The anterior pituitary produces six hormones that enter the general circulation Posterior pituitary Anterior pituitary hormones Adreno- corticotropic hormone (ACTH) Thyroid- stimulating hormone (TSH) Follicle- stimulating hormone (FSH) Luteinizing hormone (LH) Growth hormone (GH) Prolactin (PRL) Adrenal cortex Thyroid gland Ovaries or testes Ovaries or testes Mammary glands Skeletal muscle, bone

Pituitary Disorders: Hypersecretion and Hyposecretion Diabetes insipidus Hyposecretion of ADH results in inability to conserve water appropriately Causes excessive urination, dehydration, thirst Gigantism Hypersecretion of growth hormone in childhood Pituitary dwarfism Hyposecretion of growth hormone Treated by administration of GH throughout childhood

Figure 13.8

The Pancreas Secretes Glucagon, Insulin, and Somatostatin The pancreas has both exocrine and endocrine functions Pancreatic hormones are involved in regulating blood glucose levels Endocrine cells in islets of Langerhans within the pancreas secrete the following three hormones: Alpha cells: secrete glucagon Beta cells: secrete insulin Delta cells: secrete somatostatin

The Pancreas Secretes Glucagon, Insulin, and Somatostatin Raises blood sugar Causes breakdown of glycogen to glucose in liver Insulin Lowers blood sugar Promotes uptake of sugar by cells in liver, muscle, and adipose tissue Promotes conversion of glucose into glycogen, proteins, fat, Somatostatin Inhibits secretion of glucagon and insulin, regulates other hormones

Blood glucose concentration (mg/100 mL) Figure 13.10 Glucagon secretion is inhibited Pancreas secretes insulin 200 The liver uses glucose to produce glycogen Muscle cells use glucose to make glycogen and proteins Adipose tissues use glucose to produce fats for storage High blood glucose 160 120 Blood glucose concentration (mg/100 mL) 80 Meal The liver converts stored glycogen to glucose Low blood glucose 40 Insulin secretion is inhibited Pancreas secretes glucagon −1 1 2 3 4 5 6 Time (hours)

The Adrenal Glands Comprise the Cortex and Medulla Adrenal cortex: outer layer of adrenal gland Glucocorticoids, such as cortisol Mineralocorticoids, such as aldosterone Adrenal medulla: inner layer of adrenal gland Epinephrine (adrenaline) Norepinephrine (noradrenaline)

The Adrenal Cortex: Glucocorticoids and Mineralocorticoids Glucocorticoids (Cortisol is an example) Secretion mediated through hypothalamus-pituitary secretions Maintain blood glucose levels during prolonged fasting Suppress inflammatory responses Mineralocorticoids (Aldosterone is an example) Regulate sodium, potassium, water balance Act on kidneys, promoting sodium reabsorption and potassium excretion

The Adrenal Medulla: Epinephrine and Norepinephrine Neuroendocrine organ Secretion stimulated by Sympathetic nervous system Hormones: nonsteroidal Epinephrine and norepinephrine Enhance function of sympathetic nervous system (fight- or-flight response)

Thyroid and Parathyroid Glands Thyroid located just below larynx in neck Parathyroid glands embedded in back of thyroid Both thyroid and parathyroid are involved in calcium balance Thyroid is also involved in regulating metabolism

Anterior (front) Posterior (back) Figure 13.12 Opening to trachea Larynx Thyroid gland Parathyroids Trachea Anterior (front) Posterior (back)

The Thyroid Gland: Thyroxine Speeds Cellular Metabolism Secretion: mediated through hypothalamus-pituitary secretions Steroidal hormones Thyroxine (T4) and Triiodothyronine (T3) Both regulate production of ATP from glucose and modify the metabolic rate Calcitonin Lowers blood calcium levels Decreases rate of bone resorption by inhibiting osteoclasts Stimulates uptake of calcium by bones

Iodine Deficiency Can Cause Goiter Iodine is required for the production of active thyroid hormones Inadequate dietary iodine leads to underproduction of thyroid hormones The feedback response to inadequate thyroid hormone is for the hypothalamus and pituitary to further stimulate the thyroid gland in a vicious cycle that causes hypertrophy of the thyroid (goiter)

Figure 13.14

Calcitonin Promotes Bone Growth Calcitonin inhibits osteoclasts This decreases bone resorption Calcitonin also stimulates the uptake of calcium by bones Particularly important for bone growth and development in children Combined effects: lower blood calcium levels

Parathyroid Hormone (PTH) Controls Blood Calcium Levels Secretion Response to lowered blood calcium levels Action Removes calcium and phosphate from bone Increases absorption of calcium by the digestive tract Causes kidneys to retain calcium and excrete phosphate Combined effects increase blood calcium levels Major regulator of blood calcium concentration in adults

Testes Produce Testosterone Functions of testosterone and other androgens (other testosterone-like steroid hormones) Before birth, responsible for development of external male genitalia At onset of puberty Regulates development and normal functioning of sperm, male reproductive organs, and male sex drive Regulates development of male secondary sex characteristics Small amounts of androgens produced by adrenal glands in both sexes

Ovaries Produce Estrogen and Progesterone Hormones: steroidal Estrogen Initiates development of secondary sex characteristics Regulates menstrual cycle Progesterone

Other Glands and Organs Also Secrete Hormones Thymus Located in the upper chest, over the heart Secretes thymosin and thymopoietin Assist maturation of T lymphocytes Most active during early development and childhood Pineal gland Located deep within the brain but receives input from optic nerve Secretes melatonin May be important in synchronizing the body’s circadian cycle

Endocrine Functions of the Heart, the Digestive System, and the Kidneys Atrial natriuretic hormone (ANH) Helps regulate blood pressure by increasing rate of sodium and water excretion in the urine Digestive system Gastrin, secretin, cholecystokinin Effects on stomach, pancreas, gall bladder Kidney Erythropoietin Stimulates production of erythrocytes Renin Stimulates aldosterone secretion and constricts blood vessels

Other Chemical Messengers Histamine Secreted by mast cells in response to injury or allergy Initiates and enhances inflammation Prostaglandins Local control of blood flow Contribute to inflammatory response Nitric oxide Regulates local blood flow in many tissues Controls penile erection Regulates smooth muscle contraction in digestive tract Antibacterial chemical made by macrophages Neurotransmitter in the brain

Disorders of the Endocrine System Diabetes mellitus Disorder of blood sugar regulation Inability to get glucose into cells where it can be used, results in high blood sugar levels Glucose and excess water appear in the urine Abnormal metabolism of carbohydrates, proteins, and lipids causes most of the complications Symptoms: dehydration, thirst, fatigue, frequent infections, blurred vision, tingling of hands/feet Two types: type 1 and type 2

Disorders of the Endocrine System Type 1 diabetes Pancreas does not produce enough insulin Also known as insulin dependent diabetes Treated with insulin injections Results from autoimmune destruction of beta cells of pancreatic islets May be a genetic component and/or environmental trigger Typical onset is during childhood or adolescence 5–10% of all cases of diabetes

Disorders of the Endocrine System Type 2 diabetes Non-insulin dependent Characterized by insulin resistance: cells fail to respond appropriately to insulin Most often seen in adults over 40 Lifestyle factors (diet, exercise) play a role in onset Treatment: lifestyle changes, variety of medications 90–95% of all cases of diabetes

Disorders of the Endocrine System Hypothyroidism: underactive thyroid gland Children: cretinism slowed body growth, altered brain development, delayed puberty Adults: myxedema edema, lethargy, weight gain, low BMR Hyperthyroidism: overactive thyroid gland Increased BMR, hyperactivity, nervousness, agitation, weight loss Graves’ disease: autoimmune form of hyperthyroidism

Disorders of the Endocrine System Adrenal gland disorders Addison’s disease Failure of adrenal cortex Hyposecretion of cortisol and aldosterone Lowers blood glucose and sodium Slow to develop, chronic fatigue, weakness, abdominal pain, weight loss, bronzing of the skin Cushing’s syndrome Excessive cortisol secretion Excessive production of glucose from glycogen Retention of too much salt and water Loss of muscle mass, change in fat distribution Similar symptoms seen with use of cortisol and cortisol-like drugs

Figure 13.16