endocrinology

The document discusses various aspects of endocrinology, focusing on homeostatic control mechanisms, hormone characteristics, classification, processing, release, and the roles of specific endocrine glands and hormones.

Introduction to Endocrinology

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Endocrinology is the study of the endocrine system, which uses hormones for communication and coordination within the body.12

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The endocrine system, along with the nervous system, helps maintain homeostasis by regulating processes like metabolism, fluid balance, temperature, reproduction, and growth.2

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Hormones are chemical messengers that activate target sites at a distance.3

Hormone Characteristics

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Multiple hormones can be produced by a single endocrine gland, and the same hormone can be secreted by multiple tissues.4

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A single hormone can act on multiple target cell types, and a single target cell can be influenced by multiple hormones.4

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Hormone secretion varies over time and is affected by environmental changes.4

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Hormones can be transported in the blood or derived from neurons.34

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Some hormones are produced by tissues that also have other functions.4

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Negative feedback control is the predominant mechanism regulating hormone release, where the output counteracts the input.56

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Example: Thyroid hormone regulation5

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Positive feedback is less common but also occurs.

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Example: Oxytocin release during childbirth5

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Hormone release is also influenced by modulation and rhythms, such as changes throughout the day or in response to environmental cycles.57

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Carrier proteins, either specific or general like albumin, transport hormones in the blood.7

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Specific carriers protect hormones from degradation and filtration.7

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Hormone activation can occur through metabolism of the precursor or release from the carrier protein.8

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Hormones have a specific half-life in the blood, which varies depending on their chemical structure.8

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Amino acid derivatives: minutes8

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Peptide hormones: minutes to hours8

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Steroid hormones: hours8

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Hormone inactivation occurs through enzyme degradation, endocytosis of the hormone-receptor complex, and conjugation.8

Hormone Classification and Processing

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Chemical classification of hormones includes:91011

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Peptides: chains of amino acids, hydrophilic, secreted by exocytosis.910

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Examples: pituitary hormones, pancreatic hormones, GI tract hormones10

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Amino acid derivatives: derived from single amino acids, hydrophilic, secreted by exocytosis.10

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Examples: catecholamines, thyroid hormone10

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Steroids: cholesterol derivatives, hydrophobic, secreted by diffusion.10

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Examples: adrenal and sex steroids10

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Hormones undergo post-translational modification, including:11

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Peptide cleavage11

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Glycosylation11

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Phosphorylation11

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Sulfation11

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Amidation11

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Acetylation11

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Subunit aggregation11

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Hormone processing involves secretion, binding to carrier proteins, activation, inactivation, metabolism, and excretion.12

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Peptide hormones are often synthesized as pre-prohormones, which are then cleaved and modified to form the active hormone.11

Endocrine Dysfunction

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Hyposecretion: reduced hormone secretion, can be primary (gland dysfunction) or secondary (dysfunction in regulatory mechanisms).68

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Treated with replacement therapy68

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Hypersecretion: excessive hormone secretion, can be primary (e.g., tumor) or secondary.6

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Treated with inhibition or removal of the source6

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Target cell dysfunction: lack of receptors or downstream signaling machinery.13

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Example: Hyperinsulinemia (insulin resistance)13

Hormone Receptors and Target Cell Responses

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Hormone receptors can be:1415

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Membrane-bound: located on the cell surface14

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Types: ligand-gated, enzyme-linked, guanylyl cyclase, G-protein linked receptors14

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Utilize second messenger systems like adenylate cyclase, guanylate cyclase, inositol phosphate, and diacylglycerol.15

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Nuclear receptors: located inside the cell, bind to lipophilic hormones (e.g., steroids).15

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Regulate gene transcription15

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Target cell responsiveness is influenced by:131617

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Up and down regulation: changes in receptor abundance and affinity in response to hormone levels1316

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Permissiveness: one hormone requires the presence of another to exert its full effect.1718

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Synergism: the combined effect of multiple hormones is greater than the sum of their individual effects.1718

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Example: Glucagon, epinephrine, and cortisol on blood glucose18

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Antagonism: one hormone reduces the effectiveness of another.14

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Binding kinetics of hormones to receptors are described by the law of mass action.16

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Association constant (Ka): describes the rate of hormone-receptor complex formation16

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Dissociation constant (Kd): describes the rate of hormone-receptor complex dissociation16

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High Kd: low binding affinity17

The Hypothalamic-Pituitary Axis

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The hypothalamus and pituitary gland are closely connected and control many endocrine functions.1920

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The pituitary gland has two lobes:20

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Anterior pituitary (adenohypophysis): secretes trophic hormones that stimulate other endocrine glands20

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Posterior pituitary (neurohypophysis): releases hormones synthesized in the hypothalamus20

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Hypophysiotropic hormones from the hypothalamus regulate hormone release from the anterior pituitary.21

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These hormones can be stimulatory or inhibitory21

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Negative feedback loops regulate hormone secretion in the hypothalamic-pituitary axis.2223

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Examples of hypothalamic-pituitary axes and their target hormones:2122

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TRH (thyrotropin-releasing hormone) - TSH (thyroid-stimulating hormone) - Thyroid hormones (T3, T4): regulates metabolic rate2122

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CRH (corticotropin-releasing hormone) - ACTH (adrenocorticotropic hormone) - Cortisol: regulates stress response and metabolism2122

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GnRH (gonadotropin-releasing hormone) - FSH (follicle-stimulating hormone) and LH (luteinizing hormone) - Androgens and estrogens: regulates reproduction22

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GHRH (growth hormone-releasing hormone) - GH (growth hormone) - IGFs (insulin-like growth factors): regulates growth22

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Dopamine - Prolactin: regulates milk production23

Posterior Pituitary Hormones

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The hypothalamus and posterior pituitary form a neuroendocrine system.20

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Posterior pituitary hormones are synthesized in the hypothalamus and transported to the posterior pituitary for release.20

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Neurophysins are carrier proteins that transport posterior pituitary hormones.24

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Vasopressin (antidiuretic hormone, ADH):242526

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Release stimulated by:26

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Reduced extracellular fluid volume (ECFV)26

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Increased plasma osmolality26

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Decreased arterial blood pressure26

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Actions:26

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Increases water reabsorption in the kidneys26

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Vasoconstriction of blood vessels26

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Oxytocin:2427

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Release stimulated by:27

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Birth canal distension27

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Infant suckling27

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Actions:27

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Increases uterine muscle contraction during labor27

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Promotes milk ejection from mammary glands27

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Behavioral aspects of oxytocin and vasopressin:28

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Oxytocin: maternal behavior, sexual arousal, social recognition, pair bonding28

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Vasopressin: ACTH release, social recognition, memory, aggression, courtship2829

Anterior Pituitary Hormones

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The anterior pituitary contains different cell types that produce various hormones.29

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Anterior pituitary hormones are often classified based on their staining characteristics:29

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Acidophils: stain with acidic dyes (e.g., GH, prolactin)29

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Basophils: stain with basic dyes (e.g., ACTH, TSH, FSH, LH)29

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Structural characterization of anterior pituitary hormones:2330

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Growth hormone family: GH and prolactin have similar structures.23

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GH is well conserved, while prolactin has many variants.23

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Glycoprotein family: FSH, LH, TSH share a common alpha subunit but have different beta subunits.30

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The pars intermedia of the pituitary gland produces alpha-melanocyte-stimulating hormone (α-MSH) from the precursor proopiomelanocortin (POMC).3132

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POMC is also a precursor for ACTH and other hormones.32

Growth Hormone and Bone Growth

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Normal growth involves:32

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Protein, fat, and cartilage synthesis32

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Cell proliferation (hyperplasia and hypertrophy)32

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Bone lengthening (increased extracellular matrix)32

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Factors influencing normal growth:3233

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Genetics32

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Diet and nutrient transfer32

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Disease and stress32

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Hormonal control33

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Growth rate varies throughout life and is influenced by:34

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Placental hormones during neonatal growth34

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Growth hormone (GH) levels, which increase during puberty34

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Testicular androgens in males34

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Adrenal androgens, particularly DHEA in females34

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Estrogen and testosterone, which eventually stop bone growth by closing the epiphyseal plates.3435

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Growth hormone (GH):35

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Secreted in a pulsatile manner, with a peak during sleep35

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Transported bound to carrier proteins35

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The most abundant anterior pituitary hormone35

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Stimulated by GHRH and inhibited by GHIH (somatostatin)35

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Metabolic actions of GH (not directly related to growth):36

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Increased fat breakdown36

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Decreased glucose uptake by muscle cells36

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Growth-promoting actions of GH:36

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Increased cell division36

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Increased protein synthesis36

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Increased bone growth36

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Somatomedin hypothesis: GH acts indirectly on growth through somatomedins, primarily insulin-like growth factors (IGFs) I and II.37

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IGFs are structurally similar to insulin.38

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Bone growth:383940

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Bone is a living tissue with an extracellular matrix and various cell types.38

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Bone widening: osteoblasts deposit new bone on outer edges.39

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Bone lengthening: chondrocytes in the epiphyseal plates divide and multiply, pushing the epiphysis away from the diaphysis.40

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Dual-effector theory: GH and IGFs have distinct but complementary roles in bone growth.41

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GH promotes IGF-I responsiveness and expression in the epiphyseal plate.41

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IGFs stimulate chondrocyte proliferation and maturation.41

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Abnormal growth:42

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Hypopituitary dwarfism: GH deficiency42

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Gigantism (infant) and acromegaly (adult): GH excess42

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Laron dwarfism: GH receptor insensitivity42

Other Hormones Involved in Growth

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Thyroid hormones (TH): essential for normal growth, act permissively with GH and IGFs.43

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Insulin: involved in carbohydrate metabolism, deficiency can block growth, excess can promote growth.43

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Androgens and estrogens: stop bone growth by closing the epiphyseal plates.43

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Prolactin: influences mammary gland growth and immune function.43

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Placental lactogen: promotes fetal growth and maternal glucose and amino acid supply.43

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Tumor-derived growth factors: various factors involved in angiogenesis, cell proliferation, and tissue repair.44

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Epidermal growth factors: stimulate proliferation of epithelial tissues.44

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Platelet-derived growth factors: involved in wound healing and atherosclerosis development.44

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Erythropoietin: stimulates red blood cell production.44

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Neurotrophic factors (NGFs): promote nerve growth and survival.45

Calcium Regulation

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Calcium is tightly regulated in the body because it plays vital roles in:46

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Neuromuscular excitability46

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Stimulus-secretion coupling46

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Cell-cell integrity46

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Blood clotting46

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Bone and teeth structure46

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Most calcium is stored in bones, with a small amount in the ECF and intracellular compartments.46

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Calcium homeostasis involves a balance between dietary intake, absorption, bone resorption and deposition, and excretion.4647

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Calcium and phosphate regulation are interconnected because they form hydroxyapatite crystals in bone.47

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Changes in calcium concentration affect phosphate levels and vice versa.48

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Bone remodeling is a continuous process of bone deposition and resorption, involving:48

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Osteocytes: mature bone cells48

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Osteoblasts: bone-building cells that deposit collagen matrix48

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Osteoclasts: bone-resorbing cells that dissolve bone minerals48

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Hormones involved in calcium regulation:495051

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Parathyroid hormone (PTH): the primary hypercalcemic hormone4950

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Released in response to low plasma calcium49

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Increases calcium release from bone, calcium reabsorption in the kidneys, and activates vitamin D3.50

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Vitamin D3 (cholecalciferol): converted to the active form calcitriol4952

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Promotes calcium absorption in the gut and bone resorption5253

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Activation of vitamin D3 is regulated by PTH52

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Calcitonin: the only hypocalcemic hormone5051

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Reduces blood calcium levels51

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May be involved in calcium regulation during the absorptive state and pregnancy.51

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Osteoblast-osteoclast communication:5455

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Osteoblasts produce RANKL (receptor activator of NFkB ligand), which promotes osteoclast formation and activity.5455

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Osteoblasts also produce osteoprotegerin (OPG), which inhibits RANKL signaling and reduces bone resorption.5455

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Estradiol stimulates OPG production, contributing to bone health in females.55

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Osteoporosis:56

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Reduced bone mineral density, prevalent in postmenopausal women due to estrogen decline56

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Treatments include exercise, calcium supplements, hormone replacement therapy (HRT), calcitonin, and medications like SERMs (selective estrogen receptor modulators) and ANGELS (activators of non-genomic estrogen signaling).56

Thyroid Gland and Thyroid Hormones

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The thyroid gland produces thyroid hormones (TH): tetraiodothyronine (T4) and triiodothyronine (T3).57

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TH are derived from thyroglobulin and synthesized in follicular cells and the colloid of the thyroid gland.5758

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TH synthesis requires tyrosine (an amino acid) and iodine (an essential dietary component).58

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TH regulate basal metabolic rate (BMR) and are crucial for development, particularly neural development.58

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Thyroid hormone classification:59

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T4 (thyroxine): contains four iodine atoms59

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T3 (triiodothyronine): contains three iodine atoms, the most bioactive form59

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Reverse T3: inactive form59

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Thyroid hormone synthesis, storage, and release:60

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Iodide is actively transported into follicular cells.60

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Tyrosine residues on thyroglobulin are iodinated.60

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Iodinated tyrosine residues are coupled to form T3 and T4.60

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Thyroglobulin containing T3 and T4 is stored in the colloid.60

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TSH stimulates the release of T3 and T4 from thyroglobulin.60

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Thyroid hormone deiodination:6162

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Deiodinases remove iodine atoms from TH, regulating their activity.61

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Different deiodinase types have specific tissue distributions and substrate preferences.6162

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Actions of thyroid hormones:626364

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Calorigenic: increase BMR62

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Sympathomimetic: enhance the effects of catecholamines62

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Cardiovascular: increase heart rate and stroke volume6364

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Growth: synergistic actions with GH and IGFs63

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Essential for normal development, particularly in infants (brown adipose tissue and non-shivering thermogenesis)63

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Thyroid hormone abnormalities:64656667

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Common endocrine disorders, particularly in young women64

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Goiter: enlargement of the thyroid gland, can occur in both hypothyroidism and hyperthyroidism646566

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Exophthalmos: bulging eyes, a characteristic of Graves' disease (an autoimmune hyperthyroid condition)64

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Hypothyroidism: decreased TH levels65

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Causes: primary thyroid failure, hypothalamic or pituitary dysfunction, iodine deficiency65

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Symptoms: low BMR, cold intolerance, weight gain, lethargy, hair loss, edema, menstrual irregularities67

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Hyperthyroidism: increased TH levels66

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Causes: Graves' disease, hypothalamic or pituitary hypersecretion, thyroid tumors66

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Symptoms: high BMR, heat intolerance, weight loss, nervousness, rapid pulse, increased appetite, muscle wasting, exophthalmos (sometimes)67

Pineal Gland and Melatonin

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The pineal gland secretes melatonin, a hormone synthesized from tryptophan.68

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Melatonin release follows a circadian rhythm, peaking at night (scotophase) and decreasing during the day (photophase).6869

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Darkness stimulates melatonin synthesis and release, indicating a connection between the pineal gland and the optic tract.68

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The suprachiasmatic nucleus (SCN) in the hypothalamus is the master biological clock, regulating melatonin release.70

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Light cues from the environment entrain the SCN and influence melatonin production.70

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Melatonin release is inhibited during the day by the sympathetic nervous system acting on the pineal gland.7071

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Melatonin has various functions, including regulating sleep-wake cycles, reproduction, and other physiological processes.7172

Adrenal Gland

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The adrenal gland has two distinct regions:72

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Cortex: outer layer, produces steroid hormones72

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Zona glomerulosa: secretes mineralocorticoids (aldosterone)72

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Zona fasciculata: secretes glucocorticoids (cortisol)72

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Zona reticularis: secretes adrenal androgens (DHEA)72

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Medulla: inner layer, produces catecholamines (epinephrine and norepinephrine)72

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Adrenal hormones:73

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Steroid hormones: synthesized from cholesterol7374

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Mineralocorticoids (aldosterone): regulate sodium and potassium balance7375

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Glucocorticoids (cortisol): regulate stress response, metabolism, and immune function73767778

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Sex hormones: DHEA (androgens) and estrogens7379

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Catecholamines: epinephrine (80%) and norepinephrine (20%)73

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Released from the adrenal medulla as part of the sympathetic nervous system response7380

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Cholesterol is the precursor for all steroid hormones.74

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Three main parent molecules: pregnane (C21), androstane (C19), estrane (C18)74

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Mineralocorticoids (aldosterone):7579

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Act on the kidneys to promote sodium retention and potassium excretion75

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Regulate blood pressure and fluid volume7579

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Secretion is controlled by the renin-angiotensin system and potassium levels75

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Hyperaldosteronism can lead to hypertension and electrolyte imbalances.79

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Dehydroepiandrosterone (DHEA):7681

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An adrenal androgen, precursor to testosterone and estrogens81

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Plays a role in pubertal growth, hair growth, and libido in females81

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Adrenogenital syndrome: hypersecretion of DHEA, causing masculinization in females.81

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Glucocorticoids (cortisol):7677788283

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Release is stimulated by stress and follows a diurnal rhythm.76

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Direct actions:77

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Stimulate gluconeogenesis (glucose production from non-carbohydrate sources)77

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Inhibit glucose uptake by peripheral tissues77

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Stimulate protein degradation in muscle77

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Stimulate lipolysis (fat breakdown)77

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Anti-inflammatory and immunosuppressive effects:78

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At supraphysiological levels78

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Permissive actions:78

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Support vascular function during stress78

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Hypersecretion (Cushing's syndrome):8283

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Causes: increased CRH or ACTH, adrenal tumors, ectopic ACTH release82

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Symptoms: hyperglycemia, central obesity, facial hair excess83

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Hyposecretion (Addison's disease):83

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Causes: adrenal gland damage83

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Symptoms: increased pigmentation, weakness, weight loss, hypotension, salt craving, hypoglycemia83

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General adaptation to stress:8485

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A three-stage response to stressors:8485

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Alarm response: catecholamine surge, increased BMR, blood flow redirection, glycogen breakdown84

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Resistance response: cortisol-mediated metabolic changes, mobilization of energy stores84

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Exhaustion response: muscle wasting, hyperglycemia, immune suppression, organ damage85

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Chronic stress can lead to maladaptation and health problems.85

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Adrenal medulla:80

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Releases catecholamines (epinephrine and norepinephrine) in response to sympathetic nervous system activation.80

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Catecholamines bind to adrenergic receptors (alpha and beta) on target organs.80

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Epinephrine:86

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Mobilizes energy reserves86

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Increases cardiac output and peripheral resistance86

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Dilates blood vessels in coronary and skeletal muscle86

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Reduces gut motility86

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Increases glycogenolysis86

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Enhances CNS alertness86

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Dilates pupils86

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Increases sweating86

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Epinephrine reversal: the effect of epinephrine can be reversed by blocking specific adrenergic receptors.86

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Integrated stress response:87

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A coordinated response involving the hypothalamus, pituitary gland, adrenal cortex, adrenal medulla, and endocrine pancreas87

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Hormonal changes include increased CRH, ACTH, cortisol, epinephrine, glucagon, and decreased insulin.87

Endocrine Pancreas

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The pancreas has both exocrine (digestive enzymes) and endocrine functions.88

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Islets of Langerhans: clusters of endocrine cells in the pancreas88

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Alpha cells: secrete glucagon88

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Beta cells: secrete insulin and amylin88

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Delta cells: secrete somatostatin88

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PP cells (F cells): secrete pancreatic polypeptide88

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Pancreatic hormones regulate fuel metabolism.88

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Amylin, somatostatin, and pancreatic polypeptide:89

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Somatostatin: inhibits digestive and absorptive processes89

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Pancreatic polypeptide: suppresses somatostatin release and vice versa89

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Amylin: slows down glucose absorption89

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Insulin: the primary hypoglycemic hormone90

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Secretion is stimulated by glucose:90

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Glucose enters beta cells through GLUT transporters90

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Glucose metabolism increases ATP levels, closing potassium channels and depolarizing the cell membrane.90

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Depolarization opens calcium channels, triggering insulin release.90

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Fuel metabolism:91

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Anabolism: building up molecules, requires energy (ATP)91

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Catabolism: breaking down molecules, releases energy91

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Fuel sources: carbohydrates, fats, and proteins91

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Blood glucose regulation:92

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Insulin: promotes glucose uptake and storage, lowers blood glucose92

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Glucagon: promotes glucose production and release, raises blood glucose92

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Insulin actions:92939495...

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Carbohydrates:9293949596

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Facilitates glucose transport into cells via GLUT transporters, particularly GLUT-4 in muscle and adipose tissue.93

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Inhibits glycogenolysis (glycogen breakdown) in the liver95

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Inhibits gluconeogenesis (glucose production from non-carbohydrate sources)95

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Brain glucose uptake is insulin-independent96

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Fats:97

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Inhibits lipolysis (fat breakdown)97

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Stimulates fatty acid uptake into adipose tissue97

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Promotes triglyceride synthesis97

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Proteins:97

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Promotes amino acid uptake97

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Stimulates protein synthesis97

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Inhibits protein degradation97

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Diabetes mellitus:979899100

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A common endocrine disorder characterized by dysregulated glucose metabolism97

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Type 1 (insulin-dependent): autoimmune destruction of beta cells, leading to insulin deficiency98

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Usually diagnosed in childhood98

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Type 2 (non-insulin-dependent): insulin resistance, often associated with obesity98

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More prevalent form98

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Symptoms of uncontrolled diabetes:99

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Hyperglycemia (high blood glucose)99

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Glucosuria (glucose in urine)99

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Polyuria (excessive urination)99

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Polydipsia (excessive thirst)99

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Polyphagia (excessive hunger)99

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Weight loss99

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Increased hepatic glucose output99

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Increased lipolysis and blood fatty acids99

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Increased protein degradation and blood amino acids99

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Metabolic acidosis and ketosis99

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Dehydration99

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Diabetic coma99

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Insulin shock:100

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Severe hypoglycemia caused by excessive insulin administration100

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Glucagon: the primary hyperglycemic hormone101102

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Actions:101102

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Stimulates hepatic glycogenolysis and gluconeogenesis101

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Promotes fat breakdown (lipolysis)102

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Inhibits hepatic ketogenesis (ketone body formation)102

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Promotes hepatic protein catabolism102

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Insulin and glucagon work antagonistically to regulate blood glucose levels.103

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High protein meals stimulate the release of both insulin and glucagon.103

This summary provides a comprehensive overview of the endocrine system and its key components. However, it's essential to consult the original document for detailed information and to clarify any specific questions. This summary should not be used as a substitute for studying the original material. Remember, the content provided here is based solely on the information presented in the document and should be cross-referenced with other reliable sources.