Endocrine system

• Endocrine System


  • Regulates long-term processes like growth, development, reproduction
  • Slow acting control system
  • Uses chemical messengers (hormones) to relay information and instructions between cells
  • Consists of cells (glands) in organs throughout the body which release chemical substances into blood that affect near and far cells
• Thyroid Gland


  • Thyroxine (T4)
  • Triiodothyronine (T3)
  • Calcitonin (CT)
• Thymus


  • Undergoes atrophy during adulthood
  • Secretes thymosins
• Digestive Tract


  Secretes numerous hormones involved in the coordination of system functions, glucose metabolism, and appetite
• Synaptic Communication


  • Ideal for crisis management
  • Occurs across synaptic clefts
  • Chemical message is "neurotransmitter"
  • Limited to a very specific area
• Secretion and Distribution of Hormones

  Hormones circulate freely or travel bound to special carrier proteins
• Peptide Hormones


  • Glycoproteins: Thyroid-stimulating hormone (TSH), Luteinizing hormone (LH), Follicle-stimulating hormone (FSH)
  • Short Polypeptides/Small Proteins: Antidiuretic hormone (ADH), oxytocin (OXT), Growth hormone (GH), prolactin (PRL)
• Lipid Derivatives


  • Eicosanoids - derived from arachidonic acid, a 20-carbon fatty acid
  • Paracrine factors that coordinate cellular activities and affect enzymatic processes (such as blood clotting) in extracellular fluids
  • Prostaglandins, thromboxanes, and prostacyclins
  • Steroid hormones - derived from cholesterol
• Free Hormones


  • Remain functional for less than 1 hour
  • Diffuse out of bloodstream and bind to receptors on target cells
  • Are broken down and absorbed by cells of liver or kidneys
  • Are broken down by enzymes in plasma or interstitial fluids
• Thyroid and Steroid Hormones


  Remain in circulation much longer because most are "bound" to special transport proteins
• Hormone Receptor


  • Is a protein molecule to which a particular molecule binds strongly
  • Responds to several different hormones
  • Different tissues have different combinations of receptors
  • Presence or absence of specific receptor determines hormonal sensitivity
• Hormones and Plasma Membrane Receptors

  • Catecholamines & Peptide Hormones: Unable to penetrate plasma membrane, bind to extracellular receptors
  • Eicosanoids: Lipid soluble, diffuse across plasma membrane to reach intracellular receptors
• G Protein


  • Enzyme complex coupled to membrane receptor
  • Involved in link between first messenger and second messenger
• G Proteins and Calcium Ions

  • Activated G proteins trigger opening of calcium ion channels and release of calcium ions from intracellular stores
  • Calcium ions may activate calmodulin which causes further cellular changes
• Hormones and Intracellular Receptors

  • Alter rate of DNA transcription in nucleus
  • Change patterns of protein synthesis
  • Directly affect metabolic activity and structure of target cell
• Trophic Hormone


  Hormone that activates other endocrine glands
• Cellular Hormone Binding


  • Receptor
  • Receptor
  • Target cell response
  • Alteration of cellular structure or activity
  • Translation and protein synthesis
  • Transcription and mRNA production
  • Gene activation
  • Binding of hormone–receptor complex to DNA
  • Binding of receptors at mitochondria and nucleus
  • Transport across plasma membrane
  • Increased ATP production
• Control of Hormone activity

  1. Endocrine reflexes regulate endocrine activity via negative feedback

  2. Reflexes are triggered by: Hormonal stimuli

  3. Humoral stimuli

  4. Neural stimuli
• Control of endocrine activity

  • Hormonal control
  • Humoral Stimuli
  • Neural Stimuli
• Humoral Stimuli


  Changing blood levels of certain metabolites and electrolytes stimulate hormone release
• Endocrine glands


  • Anatomy
  • Location
  • Structure
  • Histology
  • Physiology
  • Hormones produced
  • Control of hormonal release
  • Target of hormones
  • Effect of hormones at target
• Describe the location, hormones, and functions of the: pituitary gland, thyroid gland, parathyroid glands, suprarenal (adrenal) glands, pineal gland, intestines, kidneys, heart, thymus, pancreas, testes and ovaries.
• Hypothalamus


  • Regulates functions of the pituitary gland
  • Synthesizes ADH and OXT and transports them to the posterior pituitary glang for release
  • Secretes regulatory hormones that control secretory activity of anterior pituitary gland
  • Contains autonomic centers that exert direct control over adrenal medula
• Hypophyseal portal system


  Ensures that hypothalamic secretions in blood will reach the intended target cells before returning to the general circulation
• Releasing hormones (RH)


  Stimulate synthesis and secretion of one or more hormones at anterior lobe
• Inhibiting hormones (IH)


  Prevent synthesis and secretion of hormones from the anterior lobe
• Anterior Pituitary - trophic hormones

  • Prolactin (PRL)
  • Growth hormone (GH)
• Growth hormone (GH)


  • Stimulates cell growth and replication: Indirectly, via release of somatomedins / insulinlike growth factor (IGF)
  • Directly, by stimulating stem cell division and differentiation in connective tissue and epithelia
  • Promotes lipolysis
  • Promotes glycogenolysis
• Posterior lobe


  • Also known as neurohypophysis
  • Contains axons of hypothalamic nerves
  • Axons secrete hypothalamic hormones in poterior lobe
  • Neurons of the supraoptic nucleus produce antidiuretic hormone (ADH)
  • Neurons of the paraventricular nucleus produce oxytocin
• Pituitary Hormones and Their Targets

  • ACTH
  • TSH
  • GH
  • PRL
  • FSH
  • LH
  • MSH
  • OXT
  • ADH
• Oxytocin


  • Stimulates uterine smooth muscle contraction during childbirth
  • Stimulates milk ejection from mammary glands
  • During sexual activity: Stimulates smooth muscle contraction in ductus deferens and prostate gland
  • Stimulates vaginal and uterine contraction
• Thyroid gland


  • Lies anterior to thyroid cartilage of larynx
  • Consists of two lobes connected by narrow isthmus
  • Thyroid follicles: Hollow spheres lined by cuboidal epithelium
  • Cells surround follicle cavity that contains viscous colloid
  • Surrounded by network of capillaries
• Thyroglobulin


  • Globular Protein
  • Synthesized by follicle cells
  • Secreted into colloid of thyroid follicles
  • Molecules contain the amino acid tyrosine
• Triiodothyronine (T3)


  Contains three iodide ions
• Decreased T3 and T4 concentrations in blood or low body temperature

  • Homeostasis
  • Disturbed HOMEOSTASIS
  • Homeostasis Restored
  • Increased T3 and T4 concentrations in blood
• Thyroid-binding Globulins (TBGs)


  • Plasma proteins that bind about 75% of T4 and 70% of T3 entering the bloodstream
  • Transthyretin (thyroid-binding prealbumin – TBPA) and albumin
  • Binds most of the remaining thyroid hormones
  • About 0.3% of T3 and 0.03% of T4 are unbound
• Thyroid-Stimulating Hormone (TSH)


  • Absence causes thyroid follicles to become inactive
  • Neither synthesis nor secretion occurs
  • Binds to membrane receptors
  • Activates key enzymes in thyroid hormone production
• Functions of Thyroid Hormones


  • Enter target cells by transport system
  • Affect most cells in body
  • Bind to receptors in: Cytoplasm
  • Surfaces of mitochondria
  • Nucleus
  • In children, essential to normal development of: Skeletal, muscular, and nervous systems
• Effects of Thyroid Hormones on Peripheral Tissues

  • Elevates rates of oxygen consumption and energy consumption; In children, may cause a rise in body temperature
  • Increases heart rate and force of contraction; generally results in a rise in blood pressure
  • Increases sensitivity to sympathetic stimulation
  • Maintains normal sensitivity of respiratory centers to changes in oxygen and carbon dioxide concentrations
  • Stimulates red blood cell formation and thus enhances oxygen delivery
  • Stimulates activity in other endocrine tissues
  • Accelerates turnover of minerals in bone
• C (clear) cells / parafollicular cells

  • Produce calcitonin (CT)
  • Released when plasma Calcium levels are too high
  • ↓ Ca2+ levels in body fluids by: Inhibiting osteoclasts
  • Stimulating osteoblasts
  • ↑ Ca2+ excretion in the kidneys