 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. Temperature changes, water availability changes. Here in the air, the concentration of oxygen can also change. Such fluctuations in physical and chemical properties of the environment are actually very hostile to the living cells and their systems. For animals, maintaining their internal conditions in a relatively stable environment is necessary. And to maintain this, the phenomena animals have adopted is called homeostasis. According to evolutionary trends, which the species have attained over the generations, each animal species has 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. The ability to protect the internal environment from the harms of the external environmental fluctuations is termed as homeostasis. Homeostasis means keeping the conditions of the body on a fixed position is not at all necessary. In fact, it is also necessary for living systems to have 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 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. It may not be in the body that it may be completely dry or that the body is flooded with water. This maintenance of narrow range is because of the homeostatic control systems that neither let the body flood with water nor dehydrate it completely. After water, we shall take the example of temperature. In human body, 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 milligram per 100 ml of glucose whether we are fasting or we are with full stomach. Homeostasis is done for many variables which include osmotic pressure, oxygen level and many concentrations of iron. 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 we regulate the temperature of a physical control system. We know it very well that in geysers and air conditioners these two 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 a set point, this sensor immediately informs the control center which sends messages to heating or cooling units and as a result these heating or cooling units become active. Either 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.