Introduction

• 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'