Negative & Positive Feedback (Edexcel International A Level Biology)

Negative feedback

• The majority of homeostatic control mechanisms in organisms use negative feedback to maintain homeostatic balance, i.e. to keep certain physiological factors, such as internal temperature or blood glucose concentration, within certain limits
• Negative feedback control loops involve
  • A receptor detects a stimulus that is involved with a physiological factor
    • E.g. a change in temperature or blood glucose level
  • A coordination system transfers information between different parts of the body
    • This could be the nervous system or the hormonal system
  • An effector carries out a response
    • Effectors are muscles or glands
• The outcome of a negative feedback loop is
  • If there is an increase in the factor the body responds to make the factor decrease
  • If there is a decrease in the factor the body responds to make the factor increase
• Negative feedback systems work by reversing a change in the body to bring it back within normal limits, e.g.
  • If body temperature rises a negative feedback system will act to lower body temperature, bringing it back to normal
  • If blood glucose levels drop a negative feedback system will act to raise blood glucose, bringing it back to normal

Negative feedback loops involve the monitoring of physiological factors and act to reverse any changes, keeping the factors within normal limits. Information can be transferred via nerve signals, as shown here, or by hormonal signals.

Positive feedback

• In positive feedback loops the original stimulus produces a response that causes the factor to deviate even more from the normal range
  • They enhance the effect of the original stimulus
• An example of this is the dilation of the cervix during labour
  • The cervix stretches as baby pushes against it
  • Stretch receptors in the cervix are stimulated and send impulses to the brain
  • The pituitary gland is stimulated to release oxytocin which increases the intensity of uterine contractions
  • This pushes the baby further down the birth canal and stretches the cervix even further
• Positive feedback loops are useful to quickly activate a process, e.g. blood clotting to close up a wound
  • When the body is injured, platelets become activated
  • They release chemicals which will activate more platelets, which in turn, will release chemicals that will activate even more platelets etc.
  • This ensures that the wound is quickly closed up by a blood clot before too much blood is lost or too many pathogens enter the bloodstream
  • The body will revert to negative feedback mechanisms once the blood clot has formed
• Positive feedback may also kick in when homeostatic mechanisms break down
  • E.g. during prolonged exposure to extreme cold hypothermia can occur; body temperature drops, resulting in decreased metabolism which in turn causes body temperature to drop further
• Since these mechanisms do not maintain a constant internal environment, they are not involved in homeostasis

The control of negative feedback

• Negative feedback loops help maintain a normal range or balance within an organism
  • They reduce the initial effect of the stimulus
• Receptors detect any deviations in a factor from the normal range; this results in a corrective mechanism to return the factor back to its normal range
• In negative feedback loop there are usually two corrective mechanisms
  • One for when the factor becomes too low
  • One for when the factor becomes too high
• The corrective mechanisms may involve the nervous system or the endocrine system
• The magnitude of the correction required to bring a factor back within its normal range is monitored and regulated by negative feedback
  • As the factor gets closer to its normal value the level of correction reduces

Two corrective mechanisms are involved in the negative feedback loop