• Homeostasis is the steady internal state which the body maintains.
  • This steady state includes many physiological values, such as:
    • temperature
    • blood pressure
    • nutrient levels
  • To maintain a steady state, the body continuously monitors and adjusts itself.

Set Point & Normal Range

  • Set Point:
    • This is the ideal state
    • For example, the ideal temperature of the human body is 37°C (98.6°F)
  • Normal Range:
    • This is the range of values around the set point which are healthy
    • Body temperature can fluctuate a few degrees around 37°C without causing harm

Negative Feedback

  • Negative feedback corrects a deviation from the set point
  • It ensures that physiological values remain within the normal range
  • It has three components:
    • A sensor
    • A control center
    • An effector
  • Sensors:
    • Sensors monitor physiological values
    • they feed back to the control center
    • They are also called receptors
  • Control Centers:
    • Control centers compare feedback from sensors against the normal range
    • If the deviation from the set point is too high then an effector is activated
  • Effectors:
    • Effectors modify the internal state
    • They reverse deviation and bring the value back within the normal range
  • Example:
    • Nerve cells in the skin monitor temperature
    • It is fed back to an area of the brain called the hypothalamus
    • If the temperature is hotter than the normal range then sweat glands are used to cool the body

Positive Feedback

  • Positive feedback is the opposite of negative feedback
  • It increases the deviation from the set point
  • Positive feedback is only normal when there is an end point
  • An example of positive feedback is during childbirth:
    • The initial contraction of the smooth muscles of the uterus push the baby towards the cervix
    • The nerve cells in the cervix monitor the amount of stretching
    • They send feedback to the brain, which signals the pituitary gland to release a hormone called oxytocin into the bloodstream
    • The oxytocin causes further contraction of the smooth muscles of the uterus, pushing the baby further
    • The cycle only stops when the baby has been born
    • It is called the 'Ferguson reflex'