5.1.2 The Principles of Homeostasis

• 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 (or sensor) – to detect a stimulus that is involved with a condition / physiological factor
  • A coordination system (nervous system and endocrine system) – to transfer information between different parts of the body
  • An effector (muscles and glands) – to carry out a response

• The outcome of a negative feedback loop:
  • The factor/stimulus is continuously monitored
  • 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

A negative feedback control loop

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 from the normal range (stimuli) which results in a corrective mechanism to return the factor back to its normal range
• In a 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

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

Positive feedback loop in bone repair

• The repair of broken bones is carried out via a positive feedback loop involving special cells called osteoblasts and osteoclasts
• The osteoblasts secrete a hormone called osteocalcin
  • Osteocalcin is a protein

• They secrete the osteocalcin in an inactive form
• The osteoclasts secrete acid which lowers the pH and the acidic conditions cause the inactive form of the protein osteocalcin to change into the active form of osteocalcin
  • The low pH alters the hydrogen and ionic bonds in the protein which changes the tertiary structure

• The active form of osteocalcin binds to a receptor on beta (β) cells in the pancreas which stimulates them to release insulin
• Osteoblast cells possess insulin receptors which when stimulated causes them to release more inactive osteocalcin
• The osteoblast cells enhance the effect of the original stimulus (insulin) - positive feedback

A positive feedback loop