Endocrine System

• Endocrine system

  Made up of endocrine glands that secrete hormones
• Exocrine glands


  • Secrete into a duct that carries the secretion to the body surface or body cavities
  • Examples: Sweat glands, mucous glands, salivary glands, glands of the alimentary canal
• Endocrine glands


  • Secrete hormones into the extracellular fluid surrounding the cells of the gland
  • Secretion usually passes into capillaries for transport by the blood
  • Sometimes called ductless glands
• Hormones

  • Chemicals secreted by endocrine glands, transported throughout the body in the blood
  • Change the functioning of cells by altering type, activities, or quantities of proteins produced
  • Not enzymes, but can influence enzymes or their concentration
• Hormones may

  1. Activate certain genes in the nucleus for specific protein production

  2. Change the shape or structure of an enzyme to turn it 'on' or 'off'

  3. Change the rate of production of an enzyme or structural protein by altering transcription or translation

  4. Influence cells with the correct receptor for the hormone
• Cells affected by hormones

  • All cells of the body
  • Only particular groups of cells (target cells)
  • Only particular organs (target organs)
• Hormone types

  • Steroids
  • Proteins
  • Amines
• Steroid Hormones

  • Lipid-soluble hormones like oestrogen, progesterone, cortisol, and aldosterone
  • Released into blood, bind to transport proteins for travel, then diffuse across cell membrane to bind with receptor protein in cytoplasm or nucleus
  • Activate genes controlling protein formation, leading to long-lasting effects
• Protein and Amine Hormones

  • Water-soluble hormones that attach to receptor proteins in the membrane of target cells
  • Cause secondary messenger substances to activate enzymes, leading to quick but short-lasting effects
• Hormone Receptors


  • Specific receptor proteins bind with only one specific molecule
  • Limited number of receptor proteins in cell membrane, saturation can occur
  • Variation in sensitivities of cells to hormones due to different types and numbers of receptor proteins
• Enzyme Amplification

  1. One hormone molecule triggers the production of thousands of enzyme molecules through cascading effects along metabolic pathways

  2. Small stimulus can lead to the production of a large number of enzyme molecules
• Enzyme amplification


  A process where a molecule activates thousands of molecules of an enzyme
• Hormone Clearance


  Breaking down hormone molecules after they have produced the required effect, often in the liver and kidneys, then excreting them in bile or urine
• Control of Hormone Secretions

  Regulation of hormone production by endocrine glands to maintain homeostasis, generally through negative feedback systems
• The endocrine system is made up of endocrine glands that secrete steroid, protein, or amine hormones affecting cell functioning
• Exocrine glands secrete into a duct that carries the secretion to the body surface or body cavities
• Activation of genes in the nucleus

  By hormones to produce specific enzymes or structural proteins
• Shape or structure change of enzymes

  By hormones to turn them 'on' or 'off'
• Rate of production change of enzymes or structural proteins

  By hormones altering the rate of transcription or translation during protein production
• Protein and amine hormone action

  Attach to receptor proteins in the membrane of the target cell, causing a secondary messenger to diffuse through the cytoplasm and activate particular enzymes
• Steroid hormones are lipid-soluble, allowing them to diffuse across the cell membrane and enter the cytoplasm, hence their receptors are found inside the cell
• If hormone clearance fails, the hormones won't be turned off, leading to continuous stimulation of target cells
• The hypothalamus, pituitary, thyroid, parathyroid, pancreas, thymus, gonads, pineal, and adrenal glands are endocrine glands in the human body
• Hormones secreted by specific glands affect specific target organs, contributing to homeostasis
• The pituitary gland secretions are controlled by the hypothalamus through hormone transport via nerve cells or vascular link
• Hormones can be lipid-soluble, crossing cell membranes to activate intracellular receptors, or water-soluble, binding with receptors on cell membranes and requiring secondary messengers for cell functioning
• Secretions from endocrine glands are hormones, which can be proteins, amines, or steroids, carried by blood to target cells/organs
• Paracrines are secreted by all cells in a tissue into extracellular fluid, while hormones are secreted by specialised cells and influence cells with the correct receptor
• Hormones need to bind with the correct receptor to work, making them specific to affect only cells with the correct receptor
• Once all receptor sites are occupied, hormones are saturated, unable to produce a greater effect
• Hormones attach to receptor proteins in the membrane of target cells, causing secondary messengers to activate specific enzymes
• Each receptor protein is specific, binding with only one type of molecule, like a lock and key mechanism
• When all receptor proteins are occupied by hormone molecules, no more hormones can bind, leading to no further increase in response
• Insulin binds to a receptor protein in the membrane of the target cell


  Causes a secondary messenger substance to diffuse through the cell and activate particular enzymes
• Receptor proteins

  • Each receptor protein is specific and binds with only one type of molecule (lock and key concept)
• When all receptor proteins in the membrane are taken by a hormone molecule


  No more increase in activity can occur
• Increasing body's insulin when each receptor is already taken

  Will not increase the cell rate of glucose uptake if activity is already at a maximum
• Different cells have different numbers of protein receptors


  They will have different sensitivities to hormones
• Steroid Hormones


  Enter the target cell and combine with receptor protein in the cell's mitochondria or nucleus
• Steroid Hormones


  • Change the type, activity, or quantity of proteins produced, not enzymes