 By homeostasis, we understand that there are certain variables which need to be maintained in a nearly constant range. For this to be possible, mechanisms should exist which can actually monitor the value of the variable. This is done by a sensor. It is also known as a receptor. Body has various receptors which detect different types of variable. For example, chemoreceptors for detecting the levels of oxygen and carbon dioxide, baroreceptors for detecting blood pressure levels. The information from sensors is conveyed to control center via afferents. The function of control center is to calculate the difference between the value of the variable with that of the required value of variable. This required value is known as set point and the difference between them, the measured value and the set point is known as error signal. After that, control center sends information via afferents to certain cells, tissues and organs to bring about a change in their function. These physical entities whose output is altered based on signal from control center are known as effectors. And their output is known as response. To give an example, a change in blood pressure is detected by baroreceptors and information is conveyed to control centers in medulla which bring about changes in other variables that is heart rate and contractility and the effector will be heart. So the monitored variable is blood pressure, sensor is baroreceptors, control center is medulla. The effector is the heart and output is heart rate and contractility. Now due to the change in the response, there is change in blood pressure towards the set point. Now if you see, change in effector output brings about the change in the monitored variable. This is the effector output, this is the monitored variable. This monitored variable is also known as regulated variable and the variables which are manipulated to keep regulated variable in a near normal range are known as controlled variable. So here heart rate and contractility are controlled variables. So in case of BP example, heart rate and contractility are controlled variables while BP is regulated variable. This is a classic example of negative feedback control system. So you see, control system has many components. One is the regulated variable, other is a sensor, then there are afferents, control center, efferent effectors and their response due to which there is change in the regulated variable. But this is a very simplistic form of negative feedback system. In body, depending on various circumstances, each of the components of control systems can be changed. So there can be change in sensitivity of the receptor so that the measured value is less change in afferent response from the receptor and also change in the set point. So all these changes will lead to change in the error signal which is generated. Also there can be change in the efferent output from the control center. This is known as change in the gain of the system which will be dealing somewhere else. One example of change in the set point of the system is change in the set point of blood pressure during exercise because in exercise we want blood pressure to be high. If this control system works normally, then it will not allow blood pressure to remain high. It will tend to bring it back towards the previous normal range, previous normal set point. So in exercise this set point is changed and this feedback control system works at a different level. So these are the basics about negative feedback control system. However, in body there are other types of control systems also. Not all are used classically to maintain homeostasis but to continue the proper functions of the body they are important so they are dealt somewhere else.