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Chapter 17 The Endocrine System Functions ◦ Differentiation of reproductive and CNS in fetus ◦ Stimulation of growth and development ◦ Coordination of the male and female reproductive system ◦ Maintenance of internal environment ◦ Adaptation to emergency demands of the body Hormones General characteristics ◦ Specific rates and rhythms of secretion Diurnal, pulsatile and cyclic and patterns depend upon circulating substances ◦ Operate within feedback systems (+/-) ◦ Affect only target cells with appropriate receptors ◦ The liver inactivates hormones, rendering the hormones more water soluble for renal excretion Regulation of Hormone Release • Hormones are released: – In response to an alteration in the cellular environment – To maintain a regulated level of certain substances or other hormones • Mechanism – release – Chemical factors (blood sugar , Ca++ levels) – Endocrine factors (FSH → ovary → E) – Neural control (CRF → pituitary → ACTH) Feedback Hormone Transport (Table 17-1) Hormones are released into the circulatory system by endocrine glands ◦ Water-soluble hormones circulate in free unbound forms Short-acting responses Bind to surface receptors ◦ Lipid-soluble hormones are primarily circulating bound to a carrier (Table 17-2) Rapid and long-lasting response Bind to cytoplasm or nucleoplasm receptor Cellular Mechanisms of Hormone Action Target cell – recognize, bind and initiate Up – regulation Down – regulation Hormone effects ◦ Direct – stimulation ◦ Permissive – facilitates maximum response/function Cellular Mechanisms of Hormone Action Hormone receptors ◦ Located in the plasma membrane or in the intracellular compartment of the target cell Water-soluble (peptides) hormones ◦ High molecular weight ◦ Cannot diffuse across the plasma membrane Cellular Mechanisms of Hormone Action Lipid-soluble hormones ◦ Easily diffuse across the plasma membrane and bind to cytoplasm or nuclear receptors Cellular Mechanisms of Hormone Action Water-soluble hormones ◦ First messenger Signal transduction ◦ Second messenger molecule (Table 17-3) Calcium Cyclic adenosine monophosphate (cAMP) Cyclic guanosine monophosphate (cGMP) Tyrosine kinase system Cellular Mechanisms of Hormone Action Lipid-soluble hormones ◦ Steroid hormones Androgens, estrogens, progestins, glucocorticoids, mineralocorticoids, thyroid hormones, vitamin D, retinoid ◦ Diffuse across the plasma membrane Bind to cytoplasmic or nuclear receptors Activate DNA transcriptn and translation Steroid Hormone Mechanism Structure and Function of the Endocrine Glands • • Hypothalamus Pituitary gland –Anterior pituitary (adenohypophysis) A) Chromophobes – non-secretory cells B) Chromophils - secretory cells 7 cell types → specific hormones Structure and Function of the Endocrine Glands Pituitary gland ◦ Posterior pituitary (neurohypophysis) Synthesized with binding proteins in the neurons of the supraoptic and paraventricular nuclei of the hypothalamus → stored Anti-diuretic hormone (ADH) Oxytocin Control of Posterior Pituitary Hormones Release (both) ◦ Stimulation of cholinergic receptors by Ach, angiotensin II, and Bendorphins Inhibition ◦ B-adrenergic receptors Posterior Pituitary • Anti-diuretic hormone – Controls plasma osmolality – ↑ permeability of distal renal tubules and collecting ducts – Pharmacological levels ADH (vasopressin) → vasoconstriction and ↑ BP – Regulation • Osmoreceptors of the hypothalamus • Baroreceptors: L atrium, carotid and aortic arches (intravascular volume) also: stress, trauma, pain, exercise, nausea, nicotine, heat, morphine → ↑ secretion, ↓ HTN, alcohol and ↑ plasma volume Oxytocin ◦ Uterine contractions and milk ejection with lactation ◦ Role in sperm motility in men ◦ ADH effect-weak Thyroid Gland “controls the rate of metabolic processes” • Bilobed either side of trachea – joined by isthmus • Follicles – follicle cells surrounding colloid • Parafollicular cells (C cells) – Secrete calcitonin (↓ serum Ca++ by # bone – resorption) • Regulation – Thyrotropin – releasing hormone and thyroid stimulating hormone Thyroid Gland Thyroid hormones ◦ Iodine – required for synthesis ◦ 90% T4 and 10% T3 ◦ Bound to thyroxine - binding mostly ◦ Regulation - negative feedback globulin - TRH - ↑ cold exposure, stress and ↓ T 4 ◦ Effects ↑ metabolism of protein, fat and glucose → rapid ↑ heat production and body temperature Normal linear growth requires TH CNS and ANS require TH Parathyroid Glands Parathyroid hormone ◦ ↑ Ca++ and↓ PO4= - bone, kidneys ◦ Antagonist of calcitonin Endocrine Pancreas The pancreas is both an endocrine and an exocrine gland Islets of Langerhans ◦ Secretion of glucagon and insulin ◦ Cells Alpha – glucagon Beta – insulin Delta – somatostatin and gastrin F cells – pancreatic polypeptide Endocrine Pancreas Insulin ◦ ◦ ◦ ◦ Synthesized from proinsulin Secretion is promoted by ↑ blood glucose Facilitates the rate of glucose uptake into the cells Anabolic hormone Synthesis of proteins, lipids and nucleic acids Endocrine Pancreas Glucagon ◦ Secretion is promoted by decreased blood glucose levels ◦ Stimulates glycogenolysis, gluconeogenesis and lipolysis Somatostatin (delta cells) ◦ Regulation alpha and beta cell secretions Adrenal Glands Adrenal cortex ◦ 80% of an adrenal gland’s total weight ◦ ◦ ◦ ◦ Zona glomerulosa – aldosterone 15% Zona fasciculata – glucocorticoids 78% Zona reticularis – androgens and estrogens (others) 7% Adrenal medulla ◦ Innervation by SNS Adrenal Cortex “all hormones derived from cholesterol” Stimulated by adrenocorticotropic hormone (ACTH) Glucocorticoid hormones ◦ ◦ ◦ ◦ Direct effect on carbohydrate metabolism Anti-inflammatory and growth suppression effects Influences awareness and sleep habits Inhibits bone matrix-protein matrix ◦ Cortisol – most potent naturally occurring (+) Diurnal rhythms (-) Hypothalamus Corticotropin-releasing factor (CRF) Stres s (+) (+) Hypoxia Hypoglycemia Hyperthermia Exercise Cortisol insufficiency (-) Anterior pituitary Adrenocorticotropic hormone (ACTH) Adrenal cortex Glucocorticoids (especially cortisol) Somatostatin Hypothalamic lesions Adrenal Cortex Mineralocorticoid hormones – Aldosterone ◦ ↑ Na+ uptake in epithelial cells – distal nephrons ◦ ↑ Na retention with loss of K+ and H+ ◦ Regulation by the renin-angiotensin system Na+ and H2O depletion ↑ K+ excreteion ↓ blood volume Adrenal Cortex Adrenal estrogens and androgens ◦ Estrogen secretion is minimal (vs. ovary) ◦ Androgens – weak Converted by peripheral tissues to stronger androgens such as testosterone Adrenal Medulla Chromaffin cells (pheochromocytes) ◦ Secrete catecholamines – epinephrine and norepinephrine ◦ “Fight or Flight Response” SNS, hypoglycemia, hypoxia, hypercapnia, acidosis, hemorrhage, glucagon, nicotine, pilocarpine, histamine and angiotensin II ◦ Epinephrine is 10x more potent than NE ◦ Promote hyperglycemia