What is Homeostasis and what are the principles behind it?

Firstly, we need to consider what the term Homeostasis means:

Homeostasis is defined as the maintenance of a stable internal environment.

-The internal environment referred here in multi-cellular organisms like us is basically the fluid that bathes our cells-> i.e.: our extracellular fluid

Why do we need to ensure a stable internal environment?

In order to be able to survive we need to keep some important variables close to a set point. This is vital as we live in an external environment that is very unpredictable and variable. Thse external changes can pose a threat to our survival.

Example I: consider the effect of a temperature change on enzyme function. Enzymes function best at an optimum temperature. If the temperature is above optimum they denature i.e. lose their structure and the ability to catalyse essential chemical reactions. For instance, what will happen if an enzyme needed for ATP production denatures? The cell will be unable to produce enough energy to meet its demands and this will eventually lead to cell death.

EXAMPLE II: Glucose levels need to be tightly controlled as it is substrate for cell respiration. If the glucose levels are too low as in hypoglycaemia, there is insufficient glucose delivered to the brain so brain cells cannot meet their energy demands. This can lead to confusion, loss of consciousness or even death. On the other hand, in diabetes where blood glucose levels are too high for long periods of time, there is damage to structures such as blood vessels and nerves.   

 So how do we achieve homeostasis?

Since maintaining important variables close to a set point is vital for our survival we have evolved ways to RESIST DEVIATIONS IN THE SET POINT.

We have control systems that enable us to do that. The components of a control systems include:

1) RECEPTORS:

-First step: we need to DETECT the deviation in the set point of a variable (e.g. temperature). In other words we need to know that a change in the variable value has taken place. This is the role of our receptors which are activated and send an input signal to the controller.

2) CONTROLLER:

- Second step: we need to PROCESS the input signal and DECIDE what we need to do to correct the change in the set point. An example of a control centre is the hypothalamus.

3) EFFECTORS

-Third step: we need to MAKE THE CORRECTIVE CHANGES to bring the variable value back to the SET POINT.  This is the job of the effectors: structures that receive commands from the controller.