Homeostasis is the tendency for an organism or cell to maintain a constant internal environment within tolerance limits

• Homeostatic mechanisms operate via feedback loops that may involve either the nervous or endocrine systems (or both)
  

If a physiological condition moves outside of a narrow set of tolerance limits, disease will occur as a consequence

• A disease is a condition that disrupts the body's homeostatic processes, leading to a deterioration in the state of health (illness)
  

Internal equilibrium is maintained by adjusting a variety of physiological processes, including:

• Body temperature (normally between 36 – 38ºC)
  

• Blood pH (normally between 7.35 – 7.45)
  

• Blood glucose levels (normally 75 – 95 mg/dL)
  

• Blood osmotic concentrations (varies depending on individual body size)
  

Physiological processes are commonly moderated via two distinct feedback mechanisms – positive and negative feedback

• Homeostatic processes are controlled by negative feedback and hence these systems occur more commonly within the body

Negative Feedback

Negative feedback involves a response that is the reverse of the change detected (it functions to reduce the change)

• A change is detected by a receptor and an effector is activated to induce an antagonistic (opposite) effect – this promotes equilibrium
  

Examples of processes that utilise negative feedback loops include homeostatic systems, such as:

• Thermoregulation (if body temperature changes, mechanisms are induced to restore normal levels)
  

• Blood sugar regulation (insulin lowers blood glucose when levels are high ; glucagon raises blood glucose when levels are low)
  

• Osmoregulation (ADH is secreted to retain water when dehydrated and its release is inhibited when the body is hydrated)
  

Feedback Loop