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Chapter 18: The Endocrine SystemThe Endocrine SystemThe endocrine system controls body activities by releasing mediator molecules called hormonesHormones released into the bloodstream travel throughout the bodyResults may take hours, but last longerHormones have powerful effects when present in very low concentrationsGeneral functions of hormonesHelp regulate:extracellular fluidmetabolismcontraction of cardiac & smooth muscleglandular secretionsome immune functionsgrowth & developmentreproductionEndocrine glands (Fig 18.1)Hormone ReceptorsAlthough hormones travel in blood throughout the body, they affect only specific target cellsTarget cells have specific protein or glycoprotein receptors to which hormones bindSynthetic hormones that block the receptors for particular naturally occurring hormones are available as drugsCirculating and Local HormonesEndocrines (circulating hormones) - hormones that travel in blood and act on distant target cells (Fig 18.2)Chemical Classes of HormonesAction of Lipid-Soluble HormonesLipid-soluble hormones bind to and activate receptors within cellsThe activated receptors then alter gene expression which results in the formation of new proteins. (Fig 18.3)The new proteins alter the cells activity and result in the physiological responses of those hormones.Action of Water-Soluble HormonesWater-soluble hormones alter cell functions by activating plasma membrane receptors, which set off a cascade of events inside the cellSome hormones exert their influence by increasing the synthesis of cAMPADH, TSH, ACTH, glucagon and epinephrineSome exert their influence by decreasing the level of cAMPgrowth hormone inhibiting hormone (somatostatin)Other substances can act as 2nd messengerscalcium ionscGMPPI3Tyrosine kinaseA hormone may use different 2nd messengers in different target cellsHypothalamus and Pituitary GlandThe hypothalamus is the major integrating link between the nervous and endocrine systemsHypothalamus receives input from cortex, thalamus, limbic system & internal organsHypothalamus controls pituitary gland with releasing & inhibiting hormonesSecretion of anterior pituitary gland hormones is regulated by hypothalamic regulating hormones and by negative feedback mechanismsThe hypothalamus and the pituitary gland (hypophysis) regulate virtually all aspects of growth, development, metabolism, and homeostasis.The pituitary gland is located in the sella turcica of the sphenoid bone and is differentiated into the anterior pituitary (adenohypophysis), the posterior pituitary (neurohypophysis). (Fig 18.5)Hormones of the anterior pituitary: (Table 18.3 & 18.4)Human growth hormone (hGH)Thyroid-stimulating hormone (TSH)Follicle-stimulating hormone (FSH)Luteinizing hormone (LH)Prolactin (PRL)Adrenocorticotrophic hormone (ACTH)Melanocyte-stimulating hormone (MSH)Human Growth Hormone and Insulin-like Growth FactorsHuman growth hormone (hGH) - the most plentiful anterior pituitary hormoneThyroid Stimulating Hormone (TSH)TSH stimulates the synthesis & secretion of T3 and T4 by the thyroid glandFollicle Stimulating Hormone (FSH)FSH functionsinitiates the formation of follicles within the ovarystimulates follicle cells to secrete estrogenstimulates sperm production in testesLuteinizing Hormone (LH)In females, LH stimulatessecretion of estrogenovulation of 2nd oocyte from ovaryformation of corpus luteumsecretion of progesteroneIn males, LH stimulates the interstitial cells of the testes to secrete testosteroneProlactinProlactin (PRL) works together with other hormones to initiate and maintains milk secretion by the mammary glandsSuckling reduces levels of hypothalamic inhibition and prolactin levels rise along with milk productionAdrenocorticotrophic HormoneAdrenocorticotrophic hormone (ACTH, corticotropin) controls the production and secretion of glucocorticoids (cortisol) by the cortex of the adrenal gland. (Fig 18.6)Melanocyte-Stimulating HormoneMelanocyte-stimulating hormone (MSH) increases skin pigmentation in animalsits exact role in humans is unknownOxytocinADHThyroid GlandThe thyroid gland is located just below the larynx and has right and left lateral lobes (Fig 18.10)HistologyThyroid follicles secrete the thyroid hormones, thyroxine (T4) and triiodothyronine (T3)Parafollicular cells secrete calcitonin (CT)Thyroid hormones are synthesized from iodine and tyrosine within a large glycoprotein molecule called thyroglobulin (TGB) and are transported in the blood by plasma proteins, mostly thyroxine-binding globulin (TBG).Parathyroid GlandsThe parathyroid glands are embedded on the posterior surfaces of the lateral lobes of the thyroidprincipal cells produce parathyroid hormone (Fig 18.13 & Table 18.7)oxyphil cells - function is unknownParathyroid hormone (PTH) regulates the homeostasis of calcium and phosphate (Fig 18.14)increase blood calcium leveldecrease blood phosphate levelincreases the number and activity of osteoclastsincreases the rate of Ca2+ and Mg2+ reabsorption from urine and inhibits the reabsorption of HPO42- so more is secreted in the urinepromotes formation of calcitriol, which increases the absorption of Ca2+, Mg2+,and HPO42- from the GI tractAdrenal GlandsThe adrenal glands are located superior to the kidneys (Fig 18.15)Consists of an outer cortex and an inner medullaCortex produces 3 different types of hormones from 3 zones of cortex (Table 18.8)The zona glomerulosa (outer zone)secretes mineralocorticoids (aldosterone)increase reabsorption of Na+ with Cl- , bicarbonate and water following itpromotes excretion of K+ and H+The zona fasciculata (middle zone)secretes glucocorticoids (cortisol)increase rate of protein catabolism & lipolysisconversion of amino acids to glucosestimulate lipolysisprovide resistance to stress by making nutrients available for ATP productionraise blood pressure by vasoconstrictionanti-inflammatoryThe zona reticularis (inner zone)secretes androgensinsignificant in malesmay contribute to sex drive in femalesis converted to estrogen in postmenopausal femalesMedulla produces epinephrine & norepinephrinePancreasThe pancreas is a flattened organ located posterior and slightly inferior to the stomach and can be classified as both an endocrine and an exocrine glandHistologyExocrine acini - clusters of digestive enzyme-producing exocrine cellsPancreatic islets (islets of Langerhans) (Fig 18.18 & Table 18.9)Alpha cells (20%) produce glucagonBeta cells (70%) produce insulinDelta cells (5%) produce somatostatinF cells produce


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MCC BIO 202 - The Endocrine System

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