 Dear students, in this module we shall discuss the feedback control systems in animals. The feedback systems are the mechanisms of self-regulation of biological processes. The basic principle of feedback control systems is that the end product or output of a process itself regulates the process. The significance of these systems lies in the fact that homeostasis in the cells and body of a multi-cellular organism is maintained because of this process. Feedback control systems regulate a lot of variables in the body. These variables can be physical or chemical in both ways. For example, physical parameters such as temperature, salinity, pH and other chemical variables such as products of metabolism help them to regulate feedback control systems. This regulation requires two basic things. First is that the variables are continuously sampled. Second is that the proper corrective actions are immediately performed. There are two types of feedback systems. The first is called negative feedback system and the second is positive feedback system. Negative feedback systems are more common in living things while in which the accumulation of an end product works to stop or slow the process. We shall take two examples of negative feedback systems. In the first example we see that the breakdown of glucose in the cell generates ATP. When a cell makes enough ATP and concentration of ATP increases more than the required level an enzyme is blocked at the beginning of the pathway that is generating this ATP. As a result the production of ATP is stopped temporarily. This results in the regulation of level of ATP in the cell. The second example that can explain the negative feedback is of control of blood sugar by the hormone insulin. When blood sugar level rises in the blood, receptors sense a change in its level. The control center which is pancreas starts to secrete more insulin. When more insulin is produced it results in lowering the level of glucose in the blood. The lowering of glucose results in decrease in the level of insulin because the control center pancreas senses this change and stops the secretion of insulin. Dear students now we shall illustrate the mechanism of negative feedback. We shall take an example of a three-step chemical pathway that converts substance A to substance D. Every step of this chemical reaction is catalyzed by a specific enzyme. The accumulation of end product D feeds back negatively which results in the inhibition of the first enzyme in this sequence. When this first step is blocked, next steps automatically blocked. Thus a negative feedback stops or slows down the production of more substance D. Now we shall discuss positive feedback systems in which an end product speeds up its production by enhancing the effect of original stimulus. Many biological processes are regulated by positive feedback however they are less common in living systems. We shall illustrate positive feedback system by a figure that shows a biochemical pathway in which the product stimulates an enzyme in the reaction sequence which increases the production of the product. Let us take an example of positive feedback that is exhibited by clotting of blood. Clotting of blood is caused by platelets. When blood starts coming out of a site of injury, platelets are present. These platelets produce certain chemicals which attract other platelets. As a result, more platelets come to the site of injury. More platelets produce more chemicals and more platelets pile up. This process continues until a proper size clot is formed and it closes the wound on the injury. This is how blood flow is stopped. Chemicals which platelets produce are actually enhancing the process i.e. positive feedback. Another example of positive feedback is the release of oxytocin. Oxytocin intensifies and speeds up the contractions of the uterus. Increase in contractions causes more oxytocin to be released i.e. positive feedback. The positive feedback cycle goes on until the baby is born.