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Secretin is released in response to low duodenal pH andacts at the pancreas to stimulate the release of electrolytebuffer solution.D. Pancreatic proteases are released in active form.E. Pancreatic lipase digests all lipids.A. have no effectabsorption.A. formation of most lipoproteins, including very low andlow density lipoproteins.B. net breakdown of phospholipids.C. net breakdown of cholesterol.D.
formation of steroid hormones.E. None of the above.CHAPTER 25: DIGESTION AND ABSORPTION13. Select the TRUE statement about pancreatic secretions.A.B.C.D.E.have no effect on bile secretion.increase bile secretion.increase portal vein pressure.reduce hepatic blood content.digestionorA. stimulation of intestinal buffers by gastrin.B. stimulation of hepatic bile production by vagal afferents.C.
stimulation of pancreatic enzyme secretion bycholecystokinin.D. stimulation of intestinal buffer secretion by secretin.E. conversion of trypsinogen to trypsin by enterokinase.Review Questions21. In a patient with celiac sprue,A. gastric ulcerations are common.B. fibrosis decreases intestinal absorption and reduces portalblood flow.C. flattening of intestinal villi decreases enzymatic digestionand reduces the area for absorption.D. hypersecretion of mucus obstructs absorption.E. hyposecretion of mucus allows acidification of the intestinal lumen, decreasing enzymatic digestion.30322. The absorption of vitamin B12 (cobalamin) is dependenton all of the following EXCEPT:A.B.C.D.E.the integrity of the terminal ileum.the concentration of B12 in the enterocytes.the presence of R-protein in saliva.the presence of intrinsic factor in gastric juice.the presence of vitamin B12/intrinsic factor binding sitesin the ileum.This page intentionally left blankSection7ENDOCRINEPHYSIOLOGYThe endocrine system, along with the nervous system, is responsible formaintenance of the internal environment and has a central role in growth,development, and propagation of the species.
It consists of a number ofendocrine glands as well as regions of the brain and other organs not typicallythought of as endocrine glands, all of which secrete hormones into thebloodstream as a mechanism for regulation of function in target tissues. Anunderstanding of normal physiological function requires knowledge of thiscomplex system, and appreciation of the consequences of dysfunction of theendocrine system is essential to the practice of medicine. Nearly allphysiological processes are affected by hormones, and many of the diseases ordysfunctions of the endocrine system are commonly encountered by medicalpractitioners; examples of the more common endocrine disorders includediabetes, thyroid diseases, and some forms of infertility.Chapter 26General Principles of Endocrinology and Pituitary andHypothalamic HormonesChapter 27Thyroid HormonesChapter 28Adrenal HormonesChapter 29The Endocrine PancreasChapter 30Calcium-Regulating HormonesChapter 31Hormones of the Reproductive SystemReview Questions305This page intentionally left blank307CHAPTER26General Principles ofEndocrinology and Pituitary andHypothalamic HormonesHORMONE SYNTHESISHormones are substances that are secreted by a gland or tissueinto the blood and bind to receptors in other tissues, wherethey affect specific physiological processes.
The organizationof the endocrine system is illustrated in Figure 26.1. Chemically, hormones are peptides (e.g., insulin, growth hormone),steroids (e.g., estrogen, testosterone, cortisol), or amines ortheir derivatives (e.g., epinephrine, thyroxine). Neurohormones are a subclass of hormones secreted by neurons (e.g.,vasopressin, oxytocin).Peptide SynthesisGenes encoding peptide hormones are transcribed to producespecific messenger ribonucleotides (mRNAs) that are subsequently translated on ribosomes to form a preprohormone.The preprohormone is a protein hormone precursor thatincludes a “signal peptide” that directs its transport to theendoplasmic reticulum, where the signal peptide is cleaved,leaving a prohormone. The prohormone is transported to theGolgi apparatus, where it is sequestered into secretory vesicles.Within these vesicles, the prohormone is further cleaved tocreate the final active form of the hormone.
Secretion ofpeptide hormones occurs when the contents of these vesiclesare released.Steroid SynthesisSteroid hormones are synthesized from the precursor cholesterol, which is derived from dietary sources or synthesized denovo from acetyl-coA (acetyl coenzyme A).
The steroidogenicpathways involved in synthesis of steroid hormones fromcholesterol are considered in Chapters 28 and 31. The majorsteroid hormones are:■■■■■cortisolaldosteronetestosterone and other androgensestradiol and other estrogensprogesteroneCortisol and aldosterone are synthesized in the adrenal cortex(Chapter 28), which also produces adrenal androgens. Testosterone is produced by the testes, and estradiol and progesterone are synthesized in the ovaries (Chapter 31).MECHANISMS OF HORMONE ACTIONHormones are secreted into the blood by endocrine glandsor tissues.
When they reach their target tissues, they bind tomembrane or nuclear receptors, initiating a chain of eventsthat ultimately results in the physiologic effects of thehormone.Steroid hormones (e.g., testosterone, estradiol, progesterone),thyroid hormone, and vitamin D are lipophilic and thereforereadily enter into the target cell, where they bind to nuclearreceptors and initiate gene transcription.
The mRNA produced is translated into proteins that regulate biochemicaland physiologic processes (Fig. 26.2).Most peptide hormones and catecholamines bind tomembrane receptors linked to heteromeric G proteins,initiating a cascade of events involving generation (orinhibition of production) of second messengers such ascAMP, cGMP, and IP3 that ultimately regulate cellular function. The second messengers can directly act on existingenzymes and other proteins or may activate or inducetranscription factors and more indirectly affect cellularfunction. Growth hormone binds to a membrane receptorthat associates with a monomeric G protein; subsequently,tyrosine kinase activity and various transcription factors areinvolved in its action.By definition, hormones (endocrine secretions) are substancesthat are carried by the blood and act on distal tissues but arepart of a larger group of regulatory secretions that also includesautocrine, paracrine, and neuroendocrine secretions (seeFig.
26.2). The common feature of these substances is thatthey are secreted and have effects on the same cell type (autocrine secretion) or other cell types.308Endocrine PhysiologyOrganization of the Endocrine SystemHypothalamusProduction of ADH, oxytocin,releasing hormones(TRH, CRH, GHRH, GnRH),and somatostatinPituitary glandAnterior pituitary:ACTH, TSH, GH, PRL,FSH, LH, and MSHPosterior pituitary:Release of ADH and oxytocinParathyroid glands(on posterior surfaceof thyroid gland)Parathyroid hormone (PTH)Pineal glandMelatoninThyroid glandThyroxine (T4)Triiodothyronine (T3)Calcitonin (CT)HeartAtrial natriureticpeptide (ANP)Adrenal glandsEach adrenal glandis divided into:Adrenal medulla:epinephrine (E),norepinephrine (NE)Adrenal cortex:cortisol,aldosterone,androgensKidneyErythropoietin(EPO), calcitriol,reninThymus(Undergoes atrophyduring adulthood)ThymopoietinDigestive tractNumerous hormones:gastrin, secretin, CCK,GIP, motilinPancreatic isletsInsulin, glucagon,somatostatinFatLeptinTestesAndrogens (especiallytestosterone), inhibinOvariesEstrogens, progestins,inhibin, relaxinFigure 26.1 Organization of the Endocrine System Hormones are secreted into the blood byendocrine organs throughout the body, affecting physiological function at various target sites.
ACTH, adrenocorticotropic hormone; ADH, antidiuretic hormone; CCK, cholecystokinin; CRH, corticotropin-releasinghormone; FSH, follicle-stimulating hormone; GH, growth hormone; GHRH, growth hormone–releasinghormone; GIP, gastric inhibitory peptide; GnRH, gonadotropin-releasing hormone; LH, luteinizing hormone;MSH, melanocyte-stimulating hormone; PRL, prolactin; TRH, thyrotropin-releasing hormone; TSH, thyroidstimulating hormone.GENERAL ENDOCRINE ROLES OF THEHYPOTHALAMUS AND PITUITARY GLANDThe hypothalamus and pituitary gland regulate the functionof much of the endocrine system (Figs.
26.3 and 26.4). Thehypothalamus is located in the ventral diencephalon and isconnected by the pituitary stalk to the pituitary gland (hypophysis), which resides in the sella turcica, a cavity in the sphenoid bone. The pituitary consists of two lobes, known as theanterior pituitary (adenohypophysis) and the posterior pituitary (neurohypophysis).The posterior pituitary is continuous with the hypothalamus,connected to it by the pituitary stalk, which contains axonsoriginating in hypothalamic nuclei. The posterior pituitarystores and secretes the neurohormones antidiuretic hormone(ADH; also known as vasopressin) and oxytocin. These hormones are synthesized mainly in the supraoptic nucleus andparaventricular nucleus of the hypothalamus and are carriedby axonal transport to the posterior pituitary.The anterior pituitary (unlike the posterior pituitary) is notdirectly connected with the hypothalamus but is connected toit by the vessels of the hypophyseal portal circulation.
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