The Endocrine System Textbook PDF

ZOOL 2023 Chapter 17 The Endocrine System Assigned Reading 17.1 An Overview of the Endocrine System 17.2 Hormones 17.3 The Pituitary Gland and Hypothalamus 17.4 The Thyroid Gland 17.5 The Parathyroid Glands...

ZOOL 2023 Chapter 17 The Endocrine System Assigned Reading 17.1 An Overview of the Endocrine System 17.2 Hormones 17.3 The Pituitary Gland and Hypothalamus 17.4 The Thyroid Gland 17.5 The Parathyroid Glands 17.6 The Adrenal Glands 17.7 The Pineal Gland 17.8 Gonadal and Placental Hormones 17.9 The Endocrine Pancreas 17.10 Organs with Secondary Endocrine Functions 17.11 Development and Aging of the Endocrine System Neural and Endocrine Signaling  Structures of the Endocrine System Endocrine glands and cells are located throughout the body and play an important role in homeostasis.  Endocrine Glands vs Exocrine Glands How do the endocrine and exocrine glands differ in structure and function?  Circulating Hormones vs. Local Hormones Circulating hormones Local hormones  Paracrine  Autocrine General Functions of the Endocrine System Maintaining homeostasis of blood composition and volume Regulating growth, development, & metabolism Controlling reproductive activities Controlling digestive processes 6 Types of Hormones Two major groups based on their chemical structure Lipid-soluble (e.g. steroid hormones) Water-soluble (amine, peptide, and protein hormones) Types of Hormones Pathways of Hormone Action Lipid-soluble hormones bind to intracellular hormone receptors. Mechanisms of Hormone Action Water-soluble hormones bind to cell membrane hormone receptors. Factors Affecting Target Cell Response Blood levels of the hormone Relative # receptors on the target cell Upregulation Downregulation Influence exerted by other hormones Permissive effect Synergistic effect Antagonistic effects 13 Regulation of Hormone Secretion Role of Endocrine Gland Stimuli Hormone release is regulated by reflexes to stimuli 3 types of stimuli: Hormonal Humoral Nervous Role of Feedback Loops Govern the initiation and maintenance of most hormone secretion in response to various stimuli.  Regulation of Hormone Secretion Most hormone regulation is achieved via negative feedback Figure 17.6 Negative Feedback Loop The release of adrenal glucocorticoids is stimulated by the release of hormones from the hypothalamus and pituitary gland. This signaling is inhibited when glucocorticoid levels become elevated by causing negative signals to the pituitary gland and hypothalamus.  Hypothalamus Master endocrine gland (along w/ pituitary) Location Functions  Direct release of HM  ADH and Oxytocin  Indirect control through release of regulatory hormones  Direct control by nervous system Adrenal medulla  Epinephrine & Norepinephrine 16 Anatomy of Pituitary Gland Infundibulum 2 Lobes - Anterior pituitary (Adenohypophysis) ~ 75% - Posterior pituitary (Neurohypophysis) ~25% 17  Posterior Pituitary Figure 17.8 Posterior Pituitary Neurosecretory cells in the hypothalamus release oxytocin (OT) or ADH into the posterior lobe of the pituitary gland. These hormones are stored or released into the blood via the capillary plexus. Posterior Pituitary Hormones Oxytocin (OT) Two target tissues: Both involved in neuroendocrine reflexes Uterus Breast/mammary glands Antidiuretic Hormone (ADH) aka Vasopressin Conserve body H2O Regulated by the osmotic pressure of blood Alcohol inhibits ADH release  Anterior Pituitary Figure 17.9 Anterior Pituitary The anterior pituitary manufactures seven hormones. The hypothalamus produces separate hormones that stimulate or inhibit hormone production in the anterior pituitary. Hormones from the hypothalamus reach the anterior pituitary via the hypophyseal portal system. Connection between Hypothalamus and Anterior Pituitary Hypothalamus hormonally stimulates anterior pituitary to release its hormones Regulatory hormones of the hypothalamus Releasing hormones Increase secretion of anterior pituitary hormones Include: thyrotropin-releasing hormone (TRH), prolactin-releasing hormone (PRH), gonadotropin-releasing hormone (GnRH), corticotropin-releasing hormone (CRH), and growth hormone-releasing hormone (GHRH). Inhibiting hormones Decrease secretion of anterior pituitary hormones Include: prolactin-inhibiting hormone (PIH) and growth hormone- inhibiting hormone (GHIH) Figure 17.11 Major Pituitary Hormones Major pituitary hormones and their target organs. Growth Hormone (GH) Figure 17.10 Hormonal Regulation of Growth Growth hormone (GH) directly accelerates the rate of protein synthesis in skeletal muscle and bones. Insulin-like growth factor 1 (IGF-1) is activated by growth hormone and indirectly supports the formation of new proteins in muscle cells and bone.  Growth Hormone Disorders Hyposecretion of hGH childhood = Hypopituitary dwarfism Hypersecretion of hGH childhood = giantism adult = acromegaly http://www.chauvet-translation.com/figures/Figure027.jpg 25  Thyroid Gland Largest endocrine gland Location 2 lobes either side of trachea - connected by isthmus Functions Secretes hormones needed for growth & proper metabolism Consists of follicles  Follicular cells Thyroglobulin Triiodothyronine (T3) & Tetraiodothyronine/Thyroxine (T4)  Parafollicular cells (aka C cells) Calcitonin Thyroid Hormone Synthesis Thyroglobulin (Tg) synthesis Uptake and concentration of iodide (I-) Oxidation of iodide (I-) to iodine (I) Iodination of tyrosine residues on Tg (catalyzed by peroxidase)  Formation of MIT and DIT I + Tg  MIT (Monoiodotyrosine)  (2 x I) + Tg  DIT (Diiodotyrosine) Coupling of MIT & DIT to form T3 and T4  MIT + DIT  T3 (Triiodothyronine)  DIT + DIT  T4 (Tetraiodothyronine/ Thyroxine) Release of T3 and T4 into circulation Thyroid Hormone Synthesis Functions of Thyroid Hormones Thyroid hormones:  Increase basal metabolic rate (BMR)  Help maintain normal body temperature  Stimulate protein synthesis  Increase the use of glucose and fatty acids for ATP production  Upregulate beta (β) receptors that attach to catecholamines  Work with hGH and insulin to accelerate body growth  Regulation of TH Synthesis Figure 17.13 Classic Negative Feedback Loop A classic negative feedback loop controls the regulation of thyroid hormone levels. Thyroid Gland Disorders Parathyroid Glands  4 glands embedded in posterior surfaces of lateral lobes of the thyroid  2 types of cells:  Chief cells (principal cells)  Oxyphil cells  Hormonal Regulation of Calcium in Blood Negative feedback Rising blood Ca2+  thyroid gland release calcitonin Falling blood Ca2+  parathyroid glands to release PTH Figure 17.16 Parathyroid Hormone in Maintaining Blood Calcium Homeostasis Parathyroid hormone increases blood calcium levels when they drop too low. Conversely, calcitonin, which is released from the thyroid gland, decreases blood calcium levels when they become too high. These two mechanisms constantly maintain blood calcium concentration at homeostasis. Adrenal Glands (Suprarenal) Paired, pyramid-shaped, located on top of both kidneys Retroperitoneal 2 regions: Medulla Cortex - 3 layers Zona glomerulosa Zona fasciculata Zona reticularis  Adrenal Gland Disorders Hyperaldosteronism or Conn Syndrome Cause of primary hyperaldosteronism = adrenal adenoma in 80% of patients Sx = HTN, hypokalemia Tx= removal of adenoma Corrects HTN & hypokalemia in most patients MRI in a patient with Conn syndrome showing a left adrenal adenoma. http://www.ispub.com/journal/the_internet_journal_of_surgery/volume_24_number_1_1/article/conn-s-syndrome 38 Adrenal Gland Disorders Cushing’s Disease & Addison’s Disease Hyperadrenocorticism or Cushing’s Disease Etiology - Usually excess ACTH due to pituitary tumor Chronic exposure to excessive glucocorticoid hormones Hypocorticism or Addison’s Disease Etiology – genetic or autoimmune Chronic shortage of glucocorticoids and/or mineralocorticoids 39 Cushing Disease Pancreas Location Acinar & islet cells Endocrine & exocrine function Acinar cells = exocrine (99% of cells) Pancreatic islets (islets of Langerhans) = endocrine Alpha (α) cells Beta (β) cells Delta cells PP cells/F cells 41 Figure 17.18 Pancreas The pancreatic exocrine function involves the acinar cells secreting digestive enzymes that are transported into the small intestine by the pancreatic duct. Its endocrine function involves the secretion of insulin (produced by beta cells) and glucagon (produced by alpha cells) within the pancreatic islets. These two hormones regulate the rate of glucose metabolism in the body. The micrograph reveals pancreatic islets. LM × 760. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) Regulation of Blood Glucose Levels by Insulin and Glucagon Figure 17.19 Homeostatic Regulation of Blood Glucose Levels Blood glucose concentration is tightly maintained between 70 mg/dL and 110 mg/dL. If blood glucose concentration rises above this range, insulin is released, which stimulates body cells to remove glucose from the blood. If blood glucose concentration drops below this range, glucagon is released, which stimulates body cells to release glucose into the blood. Clinical Application: Diabetes Mellitus May be caused by too little insulin, resistance to insulin, or both 3 major types of diabetes: Type 1 diabetes – IDDM Pancreas makes little or no insulin Usually diagnosed in childhood Autoimmune condition Type 2 diabetes - IIDM Pancreas does not make enough insulin or cells exhibit insulin resistance Usually occurs in adulthood – linked to diet, obesity, lack of exercise More common than type 1 Gestational diabetes - high blood glucose during pregnancy Increases chance of later developing type 2 diabetes Hyperglycemia can cause several problems: Excessive thirst Frequent urination Excessive hunger Fatigue 44 What is diabetes insipidus? Ovaries and Testes Gonads (ovaries and testes) produce gametes (oocytes and sperm respectively)  Ovaries produce estrogens, progesterone, relaxin, and inhibin  Testes produce testosterone and inhibin  Pineal Gland Attached to roof of the 3rd ventricle of brain Secretes melatonin Regulates biological clock Released during darkness Linked to seasonal affective disorder (SAD), Jet lag Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. Aging and the Endocrine System Aging brings about changes in the levels of most hormones  Some hormone levels increase while some decrease  Levels of other hormones, like epinephrine and norepinephrine, remain the same Histologically, most endocrine glands reduce in size and contain increasingly more fibrous connective tissue with age How aging affects the endocrine system? Production of GH decreases Production of T3/T4 decreases Thymus atrophies after puberty replaced with adipose PTH levels increase, and calcitonin levels decrease Adrenal glands produce less cortisol & aldosterone Receptor sensitivity to glucose declines Ovaries no longer respond to gonadotropins