T3.6 endocrine system

• Endocrine system

  One of two major control systems in body regulating homeostasis (nervous system is the other)
• Endocrine system

  • Comprised of endocrine gland located throughout body that make and secrete molecules called hormones
  • Secreted hormones circulate through bloodstream until they bind with target cell, eliciting a specific body response that has a relatively long lasting effect
• General functions of endocrine system

  • Regulating growth and metabolism
  • Maintaining homeostasis, blood composition and blood volume
  • Controlling digestive activities
  • Controlling reproductive processes
• Endocrine and nervous system work together to control homeostasis
• Hormones

  Classified as water or lipid soluble
• Water-soluble hormones


  • Polar molecule that can't cross plasma membrane so bind to cell surface receptors and activate second messenger systems
  • Produce faster effects than lipid-soluble hormones
• Lipid-soluble hormones


  • Non-polar, small molecules that diffuse across membrane and bind to intracellular receptors forming hormone-receptor complex
  • Results in changes to protein synthesis patterns, take several hours to days to take effect
• Receptor specificity

  • Hormones only interact with cells containing cell surface or intracellular receptors
  • Recognition of specific receptors based on complementary shape of hormone and its receptor
  • Cells that don't possess receptors for a certain hormone don't respond to that hormone
• Lipid soluble hormones


  • Bind to specific regions of DNA and alter protein synthesis patterns
  • May alter cell structure or metabolic activities (e.g. enzymes)
  • hydrophobic
  • penetrate plasma membrane
  • enter nucleus
• Water soluble hormones


  • Activate or inhibit enzymatic activity
  • Stimulate cell growth (mitosis)
  • Change membrane permeability and opening/closing of channels
  • Stimulate muscle contraction or relaxation
  • hydrophilic
  • cannot penetrate into target cell
• Magnitude of cellular response

  • Number of receptors on target cell
  • Affinity of receptor for hormone or how strongly hormone and receptor interact
  • Concentration of hormone in blood or surrounding tissue
• Hypothalamus


  • Known as autonomic control centre of brain
  • To achieve function has direct control over release of hormones from pituitary gland
  • Influences release of hormones from variety of other glands include the thyroid and adrenal glands
  • Hypothalamus receives input from cortex, thalamus, limbic system and internal organs
  • Stimulates pituitary gland directly or using series of releasing and inhibitory hormones
• Pituitary gland


  • Sits just inferior to hypothalamus and connected via thin stalk called infundibulum
  • Consists of posterior and anterior lobe which differ both structurally and functionally
• Posterior pituitary


  • Composed of neurons
  • Hormones produced in cell body of neurons in hypothalamus
  • Axons on neurons extend through infundibulum into posterior pituitary
  • Hormones stored and released from axon terminals
• Anterior pituitary


  • Consists of glandular epithelium that directly produces and stores its own hormones
  • Hypothalamus connected to anterior pituitary via two capillary plexuses connected by portal veins
• Thyroid gland


  • Located just inferior to larynx, anterior to trachea
  • Composed of right and left lobes connected by an isthmus
  • Thyroid gland contains thyroid follicles which are composed of colloid-filled spaces lined with epithelial cells called follicular cells
• Thyroid hormone


  • Follicular cells produce glycoprotein called thyroglobulin which forms part of colloid found in lumen of follicles
  • Enzymes mediate attachment of iodine (taken from blood) to tyrosine within thyroglobulin and linking of two iodinated tyrosine's forming hormones: T4 (thyroxine) and T3 (triiodothyronine)
  • Both hormones transported withing blood bound to protein carriers
  • T3 ten times more active than T4
  • Liver and peripheral tissues convert most of T4 to T3 through process called deiodination. T3 and T4 major metabolic hormones responsible for body metabolic rate (increase metabolic rate, energy and heat production - calorigenic effect)
  • Play role in regulation of tissue growth, development of skeletal and nervous systems and maintenance of blood pressure
• Calcitonin

  • Produced by parafollicular cells of thyroid gland
  • Peptide hormone that stimulates calcium uptake and incorporation into bone matrix, reducing calcium level in blood
  • Inhibits activity of osteoclasts and stimulates kidneys to decrease reabsorption of calcium from filtrate and excrete it in urine
• Parathyroid gland


  • Consists of four pea-sized glans found on anterior surface of thyroid gland
  • Produce parathyroid hormone
• Parathyroid hormone


  • Stimulus for release of parathyroid hormone (PTH) is low blood calcium levels
  • PTH works synergistically with calcitriol to raise blood calcium levels
• Adrenal glands


  • Or suprarenal glands are a pair of small, pyramid-shaped glands that sit on top of each kidney
  • Divided into two distinct structural and functional regions called the cortex (outer) and medulla (internal)
• Adrenal cortex


  • Produces three different groups of hormones, all lipid soluble steroid based hormones derived from cholesterol
  • Mineralcorticoids regulate concentration of electrolytes in body fluids
  • Innermost region of adrenal cortex secretes group of sex hormones called gonadocorticoids
  • Adrenal cortex produces important glucocorticoid called cortisol
• Adrenal medulla

  • Functionally part of sympathetic nervous system (SNS)
  • Composed of modified postganglionic neurons that receive direct innervation from SNS
  • Produces mainly adrenaline (80%) secretion and some noradrenaline (20% of secretion)
• Pancreas

  • Both exocrine and endocrine gland
  • Small pancreatic islets from endocrine component
  • Islet contain two main cell types, alpha and beta cells
• Alpha or A cells

  • Produces glucagon (29 amino acids peptide)
  • Increases glucose level by stimulating: breakdown of liver glycogen into glucose (glycogenolysis), synthesis of glucose from amino acids (gluconeogenesis)
• Beta or B cells


  • Produce insulin (51 amino acids peptide)
  • Decreases blood glucose level by: stimulating transport of glucose into peripheral tissues (mainly muscle and adipose tissue, less so in brain), stimulating storage of glucose in form of glycogen in liver and lipids in adipose tissue, liver and kidneys uptake glucose without insulin