3.6.4.1 Principles of homeostasis and negative feedback

Homeostasis in mammals involves physiological control systems that maintain the internal environment within restricted limits. It is very important for many reasons, including:

• enzymes (e.g those involved in biochemical reactions within cells/other proteins such as channel proteins are sensitive to changes in pH and temperature). Any changes from the optimum reduce the rate of reaction and, if severe enough, may denature the enzyme and the reaction will cease altogether. Maintaining a fairly constant environment means that reactions take place at a suitable rate.
• Changes to the water potential of blood/tissue fluid may cause cells to shrink/expand as a result of water leaving/entering via osmosis. This means the cells cannot operate normally.
• The maintenance of a constant blood glucose concentration is essential to ensure a constant water potential. A constant blood glucose concentration also ensures a reliable source of glucose for respiration by cells.

Positive feedback

This occurs when a deviation from an optimum causes changes that result in an even greater deviation from the normal. As example occurs in neurones where a stimulus leads to a small influx of sodium ions (this increases the permeability of the neurone membrane to sodium ions and more ions enter).


Negative feedback
Negative feedback is when the change produced by the control system leads to a change in the stimulus detected y the receptor and turns the system off. This restores systems to their original level. 


Control of any system involves a series of stages:

• the optimum point
• a receptor
• a coordinator
• an effector
• a feedback mechanism

The possession of separate mechanisms involving negative feedback controls departures in different directions from the original state, giving a greater degree of homeostatic control as the return to the optimum can be brought about faster.

For example, if there is a fall in blood glucose concentration this is detected by receptors on the cell-surface membrane of alpha cells in the pancreas. These secrete glucagon which causes liver cells to convert glycogen to glucose to raise the blood glucose concentration. There is now reduced stimulus so the secretion of glucagon reduces.

If blood glucose concentration rises insulin will be produced from beta cells in the pancreas. Insulin increases the uptake of glucose by cells (it is converted to glycogen and fat). There is now reduced stimulus so the production of insulin reduces.