Introduction to HOMEOSTASIS

Doc Brown's Biology Revision Notes

Suitable for GCSE/IGCSE/O level Biology/Science courses or equivalent

1. What is homeostasis? What is its function?

Negative feedback systems explained

 Homeostasis is the maintenance of a stable and constant internal environment in an organism. Homeostasis is crucial to the regulation of an internal environments and enables organisms to adapt to change, both internally and externally. Internal temperature, blood sugar levels, water level and osmotic balance are regulated by a number of organs and systems working together.

The conditions inside your body need to be kept as steady as possible even if the external conditions change. This 'steadiness' or 'regulation' of the 'right conditions' is vitally important for your cells to function properly eg the action of enzymes which control most of your bodies chemistry.

The body controls itself by means of negative feedback systems which constantly help keep conditions right for healthy sustainable life for what you might call 'normal conditions'. Basically if something in the changes beyond a certain limit, then the change is detected and the body automatically responds to balance things up again.

Cells in the body can only survive within narrow physical and chemical limits. They require a constant temperature and pH as well as a constant supply of dissolved food and water. This is why the body requires control systems that constantly monitor and adjust the composition of the blood. These control systems include receptors which sense changes and effectors that bring about changes.

2. How do negative feedback systems work?

There are many automatic control systems in your body that control and regulate your internal environment eg the nervous system, hormone molecule communication. Other control systems 'monitor' things such as your body temperature, water content and blood glucose levels.

The table below illustrates the general principles of maintaining the ideal 'level' of something e.g. temperature, blood glucose, and water via three 'components' working in conjunction with one another to keep conditions steady. These are ...

receptor cells which sense the state of the level and produce a stimulus,

coordination centre cells in the brain, spinal cord and pancreas, and

effector cells that produce the response.

The negative feedback in action for a 'level' too high The negative feedback in action for a 'level' too low
1. Receptor detects a stimulus that a level is too high 1. Receptor detects a stimulus that a level is too low
2. The coordination centre receives and processes the stimulus information and then organises a response 2. The coordination centre receives and processes the stimulus information and then organises a response
3. The effector produces a response which counteracts the change and restores the optimum level by reducing the level 3. The effector produces a response which counteracts the change and restores the optimum level by increasing the level
4. The effector will carry on producing the 'reducing' response as long as the coordination centre is stimulated by the receptor 4. The effector will carry on producing the 'increasing' response as long as the coordination centre is stimulated by the receptor
5. The effector response might be more than required, and the level becomes too low, if too far below the 'ideal', the receptors will detect this, and the negative feedback will stimulate the effectors to increase the level (1. - 3. on the right) 5. The effector response might be more than required, and the level becomes too high, if too far above the 'ideal' the receptors will detect this and the negative feedback will stimulate the effectors to decrease the level (1. - 3. on the left).
This is all automatically done by the organism's complex control systems and enables the organism e.g. your body, to maintain as near as possible the 'ideal' conditions for life!

Note: The coordination centre = control centre, just different phrases meaning the same thing!

This negative feedback detection system process is continuous so that there is always a small fluctuation from the 'NORMAL', illustrated by the graph below (green line ~normal).

The graph shows several decreases and increases as the negative feedback system clicks into action.

Our negative feedback systems work well if external or internal changes are small, i.e. within certain limits, BUT, if the environment (ambient conditions) change too much, then our body might not be able to counteract the enforced change.


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