Homeostasis Basics

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Homeostasis is the Maintenance of a Stable Internal Environment

Changes in your external environment can affect your internal environment - the blood and tissue fluid that surrounds your cells.

Homeostasis involves control systems that keep your internal environment roughly constant (within certain limits).

Keeping your internal environment stable is vital for cells to function normally and to stop them being damaged.

It's particularly important to maintain the right core body temperature and blood pH. This is because temperature and pH affect enzyme activity, and enzymes control the rate of metabolic reactions:

TEMPERATURE

If body temperature is too high (e.g. 40 degrees) enzymes may become denatured. The enzyme's molecules vibrate too much, which breaks the hydrogen bonds that hold them in their 3D shape. The shape of the enzyme's active site is changed and it no longer works as a catalyst. This means metabolic reactions are less efficient.

If body temperature is too low enzyme activity is reduced, slowing the rate of metabolic reactions.

The highest rate of enzyme activity happens at their optimum temperature (about 37 degrees in humans).

pH

If blood pH is too high or too low (highly alkaline or acidic) enzymes become denatured. The hydrogen bonds that hold them in their 3D shape are broken, so the shape of the enzyme's active site is changed and it no longer works as a catalyst. This means metabolic reactions are less efficient.

The highest rate of enzyme activity happens at their optimum pH - usually found around pH 7 (neutral), but some enzymes work best at other pHs, e.g. enzymes found in the stomach work best at a low pH.

It's important to maintain the right concentration of glucose in the blood because cells need glucose for energy. Blood glucose concentration also afects the water potential of blood - this is the potential (likelihood) of water molecules to diffuse out of or into a solution.

GLUCOSE

If blood glucose concentration is too high the water potential of blood is reduced to a point where water molecules diffuse out of cells into the blood by osmosis. This can cause the cells to shrivel up and die.

If blood glucose concentration is too low, cells are unable to carry out normal activities because there isn't enough glucose for respiration to provide energy.

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Homeostatic Systems Detect a Change and Respond by Negative Feedback

Homeostatic systems involve receptors, a communication system and effectors (like the nervous system).

Receptors detect when a level is too high or too low, and the information's communicated via the nervous system or the hormonal system to the effectors.

The effectors respond to counteract the change - bringing the level back to normal.

The mechanism that restores the level back to normal is called a negative feedback mechanism.

Negative feedback keeps things around the normal level, e.g. body temperature is usually kept within 0.5 degrees above or below 37 degrees.

Negative feedback only works within certain limits though - if the change is too big then the effectors may not be able to counteract it, e.g. a huge drop in body temperature caused by prolonged exposure to cold weather may be too large to

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Multiple Negative Feedback Mechanisms Give More Control

Homeostasis involves multiple negative feedback mechanisms for each thing being controlled. This is because having more than one mechanism gives more control over changes in your internal environment than just having one negative feedback mechanism.

Having multiple negative feedback mechanisms means you can actively increase or decrease a level so it returns to normal, e.g. you have feedback mechanisms to reduce your body temperature and you also have mechanisms to increase it.

If you only had one negative feedback mechanism, all you could do would be turn it on or turn it off. You'd only be able to actively change a level in one direction so it returns to normal.

Only one negative feedback mechanism means a slower response and less control.

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Positive Feedback Mechanisms Amplify a Change from the Normal Level

Some changes trigger a positive feedback mechanism, which amplifies the change.

The effectors respond to further increase the level away from the normal level.

Positive feedback is useful to rapidly activate something, e.g. a blood clot after an injury:

Platelets become activated and release a chemical - this triggers more platelets to be activated, and so on.

Platelets very quickly form a blood clot at the injury site.

The process ends with negative feedback, when the body detects the blood clot has been formed.

Positive feedback can also happen when a homeostatic system breaks down, e.g. if you're too cold for too long:

Hypothermia is low body temperature (below 35 degrees).

It happens when heat's lost from the body quicker than it can be produced.

As body temperature falls the brain doesn't work properly and shivering stops - this makes body temperature fall even more.

Positive feedback takes body temperature further away from the normal level, and it continues to decrease unless action is taken.

Positive feedback isn't involved in homeostasis because it doesn't keep your internal environment stable.