Homeostasis is the property of a system in which variables are regulated so that internal conditions remain stable and relatively constant. Examples of homeostasis include the regulation of temperature and pH balance.

The two body systems that are responsible for regulating homeostasis are the nervous and endocrine systems.

Negative feedback

The principle mechanism by which homeostasis occurs is via negative feedback. Negative feedback occurs via receptors (often neural receptor cells), comparators (usually a neural assembly in the central nervous system) and effectors (typically muscular and/or glandular tissue) based upon a ‘set point’. The ‘set point’ is a narrow range of values within which normal function occurs.

The term negative feedback is used because the effectors always act to move the variable in the opposite direction to the change that was initially detected to promote equilibrium.

A characteristic feature of negative feedback systems is that they tend to induce oscillations in the variable that they control because it usually takes time for the system to detect and respond to the change in the variable. This is referred to as the lag time.

Because of this lag time, negative feedback systems cause the variable to overshoot the set point slightly, activating the opposite restorative mechanism to induce a smaller overshoot in that direction. This process results in small oscillations within the feedback loop. These oscillations usually have no visible effect and generally fall within a range of values that is optimal for physiological function.

An example of a negative feedback system is temperature regulation:

• If there is a rise in body temperature, peripheral and central sensors inform the temperature regulation centre within the hypothalamus that the setpoint has been deviated from.
• This triggers a sequence of changes including peripheral vasodilatation, which increases blood flow to the skin speeding up heat loss to the surroundings, heavy breathing and sweating, which causes evaporation from the respiratory tract and skin.
• These changes will result in a fall in the body temperature back to the set point.
• In contrast, if there is a fall in body temperature the temperature regulation centre then triggers responses including shivering, which causes the muscles to generate heat, and the hairs to stand on end (‘goosebumps’), which traps a layer of air near the skin. There is also a release of hormones that act to increase heat production.
• These changes will result in a rise in the body temperature back to the set point.

Positive feedback

Some physiological responses use a positive feedback system. In contrast to negative feedback systems, positive feedback loops tend to move a system away from its equilibrium state and make it more unstable.

Positive feedback loops are usually found in processes that need to be pushed to completion, not when the status quo needs to be maintained.

An example of a positive feedback system is the uterus in labour:

• Sensors detect the baby’s head pressing against the cervix in the latter stages of pregnancy, which sends signals to the pituitary gland to activate the release of oxytocin.
• Oxytocin increases uterine contractions, and thus pressure on the cervix.
• This causes the release of even more oxytocin and produces even stronger contractions. This positive feedback loop continues until the baby is born.

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