Homeostasis is the property of a system in which variables are regulated so that internal conditions remain stable and relatively constant.


In 1775, Dr. Charles Blagden of the Royal Society of London tested the human body’s ability to withstand heat. 

He had a special room heated to 126°C, well above the boiling point of water. He then entered the room with a pet dog and a piece of raw meat.

After 45 minutes, he left the room with no ill effects other than a pulse rate that had increased to 144 beats per minute (about twice its normal rate). The dog, too, was fine, but the meat had been cooked.

ThermoregulationHealthy people share certain bodily constants:

  1. a blood glucose concentration that remains at about 100 mg/mL,
  2. a blood pH near 7.4,
  3. a blood pressure of about 160/106 KPa (120/80 mm Hg),
  4. a body temperature of approximately 37°C.


  1. The maintenance of a dynamic equilibrium (Dynamic equilibrium is a state of balance achieved within an environment as the result of internal control mechanisms that continuously oppose outside forces that tend to change that environment ).
  2. Most homeostatic mechanisms operate as negative feedback loops (A negative feedback loop is a process that detects and reverses deviations from normal body constants).

Feedback Systems

Negative feedback loops exist throughout the body to maintain homeostasis.

These systems prevent blood sugar, blood pressure, temperature, and other body constants from becoming too high or too low.

Negativ feedbackPositive feedback loops also exist, but they are usually associated with disease or change (for example, drug addiction).

An example of a positive feedback loop is high blood pressure. Damage to arteries due to high blood pressure results in the formation of scar tissue.

This scar tissue traps cholesterol, which impedes the flow of blood through the arteries and thereby increases blood pressure even more.