Principles of homeostasis and negative feedback - AL only (3.6.4.1)

Homeostasis is the maintenance of a stable internal environment by physiological control systems that detect and respond to changes in the internal or external environment.

It is important because enzymes and cells function best within narrow ranges of temperature, and water potential. If conditions move outside these limits, metabolic processes may be disrupted and cells may be damaged.

Examples of conditions regulated by homeostasis include:

• Body temperature
• Blood glucose concentration
• Water potential of the blood
• of blood and tissues

Positive feedback is a control mechanism that amplifies a change by increasing the effect of the original stimulus.

This response is usually not homeostatic, as it does not restore conditions back to the normal level.

Childbirth is an example of positive feedback.

Stretching of the cervix stimulates the hypothalamus to release more oxytocin from the pituitary gland. Oxytocin strengthens uterine contractions, causing further stretching of the cervix and the release of more oxytocin. This cycle continues until birth occurs.

Blood clotting is another example of positive feedback.

Platelets adhere to damaged vessel walls, releasing chemicals to attract more platelets. This accelerates clot formation until the wound is sealed.

Negative feedback is a control mechanism that maintains homeostasis by reversing changes in the internal environment, restoring conditions back to normal (the optimal range).

1. A change is detected (e.g., body temperature rises).
2. Receptors send signals to a coordination centre (e.g., hypothalamus).
3. The coordination centre activates effectors (e.g., sweat glands).
4. Effectors respond to reverse the change (e.g., cooling the body down).
5. Once normal conditions are restored, the system switches off.