Homeostasis

Homeostasis almost literally means "staying the same". It describes the miraculous talent the body has for maintaining various aspects of the internal environment within very narrow limits so that life may continue unabated.

Such feats of stability include:

Maintaining a temperature range of roughly 36.1°C to 37.5°C when measured orally.
Retaining appropriate concentrations of water in and around cells.
Keeping the pH of bodily fluids balanced.
Maintaining healthy blood pressure...

... and much more.

There are a variety of systems used to achieve the necessary standards, but all can be defined as either a positive feedback mechanism or a negative feedback mechanism. All feedback mechanisms function with at least 3 components:
A receptor which senses a change in the environment (stimulus) such as reduced body temperature.
A control centre such as the hypothalamus which sets limits on what the normal temperature range should be and determines appropriate responses to a change.
An effector such as a muscle which behaves according to signals sent by the control center and begins "shivering".

Negative Feedback Mechanisms:
These are the most common and follow the pattern described above. A stimulus is detected by a receptor which notifies the control centre. The control centre then initiates a series of mechanisms (via effectors) which will eventually reduce or remove the original stimulus once homeostasis is restored.

Positive Feedback Mechanisms:
This system is slightly different and is found only in a limited number of contexts. It involves a constantly increasing response to the stimulus so that levels are temporarily pushed beyond the normal limits. The best example is that of childbirth. After an initial release of the hormone oxytocin, contractions force the baby's head against the cervix. This stimulates stretch receptors in the cervix which in turn prompt the release or more oxytocin so that contractions increase. This cycle of stimulation followed by oxytocin release and contraction continues until the baby is born at which point the source of stimulation has been removed and the cycle stops. Blood clotting is another example of a positive feedback mechanism.


The mechanisms involved in homeostasis are truly remarkable and frequently taken for granted. Minor homeostatic imbalances can give rise to a range of health problems and can be potentially life threatening. Many aspects of health care are aimed at supporting the body's efforts at maintaining homeostasis although it could also be said that many drugs place undue stress upon mechanisms such as those mentioned above causing side effects.