Homeostasis is the maintenance of a stable internal environment. The conditions inside the body must be stable so that cells can function properly. For example, if the temperature or pH are too high, enzymes can become denatured. It is also important to maintain blood glucose concentration and blood water potential, which we will look at in future articles.
Homeostasis involves many organs and systems working together to control the internal environment despite a changing external environment. Many systems respond using negative feedback.
Negative feedback
Negative feedback restores systems to their original level.
- Receptors detect the stimulus that a level is too high or too low.
- The information is communicated via the nervous system or the endocrine system to the effector.
- The effector acts to counteract the change and bring the level back towards it’s original level (the set point). This is the response.
Negative feedback only works within limits. If a change is too large, the effectors are not able to counteract the change. But as long as changes are small, the internal environment is kept in a dynamic equilibrium, fluctuating around the set point.
Multiple negative feedback systems can act to control the same thing to allow levels to be controlled in both directions. Rather like having a heater and an air conditioning unit to control room temperature. It allows faster and more controlled responses.
Positive feedback
Positive feedback acts to amplify a change. Technically, this is not homeostasis, but it always comes up in the same topic in A-level biology. One example of positive feedback is during childbirth. The baby’s head pushing on the cervix causes the uterus to contract, which causes the baby’s head to push against the cervix, which causes the uterus to contract… and so it goes on until the baby is born.
Another example is hypothermia. Normally, physiological control systems maintain a constant body temperature, however if body temperature becomes too cold for those control systems to cope, a positive feedback cycle begins. Decreased body temperature decreases kinetic energy of enzymes and substrates, meaning rate of respiration decreases. Less metabolic heat is generated through respiration, causing body temperature to drop further. Therefore there is even less kinetic energy… and so the cycle continues unless there is external intervention to warm the organism up.
Summary
- Homeostasis is the maintenance of a stable internal environment.
- Systems maintain homeostasis using negative feedback, involving receptors and effectors.
- Positive feedback does not maintain homeostasis, but can be used to amplify a change.
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