Control of internal environment of living organisms.
It is very important that the internal environment of living organisms is kept constant, as the cells inside the body need this constant environment to work efficiently, failure to do so could result in death. Therefore no matter what is happening in the external environment homeostasis keeps the environment around the cells the same. Examples of things that are controlled are respiratory gases (oxygen and carbon dioxide), pH, the concentration of nutrients and waste products, the concentration of salts and other small ions, the volume and pressure of blood plasma and temperature (humans sweat to lower body temperature). This regulation is called homeostasis (staying the same).