Homeostasis

• Homeostasis 


  The maintenance of a constant internal environment, within tolerance limits, despite changes in the internal and external environment
• Biochemical Pathway 


  A linked series of chemical reactions in living organisms
• Metabolism

  The total sum of chemical reactions in biochemical pathways in organisms
• Enzymes

  Reusable, biological catalysts (proteins) that lower the activation energy of a chemical reaction, enabling it to proceed faster
• Detecting Stimuli: Receptors

  • external and internal receptors allow an organism to respond to stimuli
  • chemoreceptor: internal, 02/ion levels, chemical concentrations - located in the aorta and carotid arteries
  • osmoreceptor: internal, changes in osmotic pressure in blood (water and solute) - located in the hypothalamus
  • photoreceptor: external, light - located in eyes
  • thermoreceptor: external and internal, changes in temperature - located in skin and hypothalamus
• Negative Feedback Loop

  A homeostatic process that changes the direction of a stimulus

  • the stimulus initially deviates away from normal
  • NFL will always reduce the stimulus
  • the NFL is represented as a cycle but the mechanism is only cyclic if the stimulus continuously occurs after negative feedback
• NFL (cycle/model)

  1. Stimulus
  2. Receptor
  3. Modulator
  4. Effector
  5. Response
  6. Feedback
• NFL: Stimulus

  Change in internal or external environment

  • deviation away from the normal/optimal value
• NFL: Receptor 


  Cell or tissue that detects the stimulus

  • may be external or internal
• NFL: Modulator 


  Receives messages from receptors (via sensory neurons), coordinates a response, then sends an instruction to an effector (via motor neurons)
• NFL: Effector


  Receives message from modulator and carries out response

  • muscle or gland
• NFL: Response 


  Change made by the effector

  • reaction to the stimulus
• NFL: Feedback

  • negative: counteracts the stimulus - returns value back to its normal/optimal value
  • positive: reinforces stimulus - value moves further away from normal