Homeostasis is Greek for ‘staying the same’. In the context of human physiology the term homeostasis describes the maintenance of nearly constant conditions in the internal environment – physiological self-regulation. All organs and tissues perform functions that help to maintain this constancy – it is a coordinated, dynamic process.

Disease can be conceptualised as a state of disrupted homeostasis.

Examples of homeostasis:

• Acid-base balance – maintaining physiological pH (7.35-7.45)
• Blood glucose concentration – controlled by the release of insulin
• Control of body temperature
• Control of heart rate and blood pressure

Negative Feedback

The physiological factor that is being controlled is called the ‘variable‘. All variables are tightly regulated by homeostatic mechanisms – the most common mechanism is negative feedback control.

The body determines a ‘set point’ which is the desired level of the variable. A negative feedback control system consists of:

• Detectors – receptor cells that measure the variable.
• Comparators – neural assembly in the CNS that receives information from the detectors and compare the size of the signal with the set point.
• Effectors – muscular/glandular tissue that is activated by the comparator to restore the variable to its set point.

Effectors always act to move the level of the variable in the opposite direction to the change that was originally detected (hence the term ‘negative’ feedback).

The set point is not a single optimum value – it comprises a narrow range of values which allows normal function to occur.

Here is a useful video using a great man-on-a-surfboard analogy to explain the constant adjustment-readjustment nature of negative feedback:

Negative Feedback Loop Oscillations

A common feature of all negative feedback systems is that they induce oscillations in the level of the variable that they are controlling. This is because it takes time for a system to detect and respond to a change in a variable – there is a lag.

This lag always causes the variable to overshoot the set point slightly, which in turn stimulates the opposite restorative mechanism to induce a smaller overshoot. This process continues over and over until the oscillations fall within the optimal range around the set point.

Positive Feedback

Unlike negative feedback which is a corrective physiological mechanism, positive feedback is a process of rapid amplification. Positive feedback is a far less common physiological process.

Positive feedback is inherently unstable – it can be thought of as a ‘vicious cycle’. Usually though, positive feedback are eventually overridden by negative feedback mechanisms, and the vicious cycle fails to develop.

In some situations positive feedback is very useful:

• Uterine contractions during labour
• Initiation of an action potential
• Formation of a blood clot

In all of the situations where positive feedback is useful, the process is always part of an overall negative feedback system.

References

Awesome physiology videos from Andrew Wolf

Physiology at a Glance – Ward, Linden

Textbook of Medical Physiology – Guyten, Hall