 Dear students, in this module, we shall discuss the principle of homeostasis that has very basic place in physiology. This phenomena was first described by Walter Cannon in 1929. It describes the tendency of an animal to maintain a relatively stable internal environment despite of changes in its external environmental conditions. Dear students, environment of an animal has fluctuations in it. There is no constant change in the environment. Water availability changes. Oxygen concentration can also change in the air. Such fluctuations in physical and chemical properties of the environment are actually very hostile to the living cells and their systems. For animals, maintaining a relatively stable internal environment is necessary for their survival. And to maintain this, the phenomena that the animals have adopted is called homeostasis. Evolutionary trends that the species have attained over the generations have developed a specific set of internal environments. As a result, the members of the species have the ability to resist environmental changes by making adjustments in their own body systems. As a result, they can keep their internal fluctuations in response to major external fluctuations. Internal fluctuations are kept in narrow range. This is the definition of homeostasis. The ability to protect the internal environment from the harms of the external environmental fluctuations is termed as homeostasis. Homeostasis means that it is not necessary to keep the body's conditions at a fixed position. In fact, it is also necessary for living systems to make changes in a narrow range. We shall take some examples of homeostasis. First, we shall see how homeostatic systems for water work. Water fluctuates in the environment. Water has abundant supply or it may be totally, there may be totally dry conditions. However, the quantity of water in the body may vary but in a very narrow range. The body may not have dry conditions or the body may be flooded with water. This maintenance of narrow range is because of the homeostatic control systems that neither let the body flooded with water nor dehydrate it completely. After water, let us take the example of temperature. In the human body, the temperature is regulated around 37 degree centigrade. In the environment, the fluctuation in temperature is 0 degree centigrade to 50 degree centigrade. Similarly, the body maintains the pH in a very narrow range. For example, the pH of blood is maintained at 7.4 with a variation of only 0.1 pH unit. Glucose is also regulated by homeostasis. Our blood contains 90 mg per 100 ml of glucose whether we are fasting or we are with full stomach. Homeostasis is done for many variables in which the concentration of osmotic pressure, oxygen level or many iron concentrations also have homeostatic regulation. Dear students, now we shall discuss the mechanism of homeostasis. The living control systems operate just like the physical control systems. They have three basic components, receptors, control centers and effectors. Let us take an example here in which a physical control system regulates the temperature and we know it well that it is found in geysers and air conditioners in which both physical control systems maintain the temperature in a very narrow range. And then we will compare this physical control system with an endothermic animal's living control system. In a physical control system, there is a sensor, that is a thermometer, that monitors temperature change from a set point. Whenever there is a change in the temperature of this set point, this sensor immediately informs the control center, which sends a message to heating or cooling units and as a result, these heating or cooling units become active or the geyser's heater turns on or AC's cooling unit turns on. Result is the maintenance of temperature within a narrow range. Similar control system exists in endothermic animals. There is a set point of temperature. Thermoreceptors detect temperature changes and send signals to the control center, which is hypothalamus in the brain. Hypothalamus sends appropriate messages to the effector organs, which are sweat glands and muscles, which cause perspiration or shivering. And as a result, the required level of temperature is maintained.