![]() Control engineering and system design are other areas that rely on system theory. For example, during labor the fetal head pushes on the cervix, resulting in oxytocin release, which results in deeper and faster contractions, increasing pressure on the cervix, until delivery ensues. Positive feedback is not only important in communication and education, it also occurs in nature. For example, decreasing blood oxygen content during exercise will result in increasing ventilation, increasing the oxygen levels again. Negative feedback has a stabilizing role and helps maintain critical variables in narrow ranges under changing environmental conditions. Many systems include feedback control mechanisms. Fixing or replacing damaged or malfunctioning “parts” is daily practice to extend life expectancy. Western medicine looks at the human body in terms of organ systems, organs, tissues, cells, and molecules. Related fundamental characteristics of many systems are their hierarchical organization (systems and subsystems can be distinguished) and modularity (parts or subsystems can be joined or replaced by equivalent parts). Using these quantities we can ask basic questions about the behavior of the system, and provide relevant explanations or predictions. The resulting evolving blood pressure is an example of a dependent variable. The amount of fluid administered to a patient is an example of an independent variable. A dependent variable is any time varying system quantity of interest that depends on the independent variables, if it influences the environment or not. An independent variable is a time varying quantity in the environment of the system that influences system behavior. The purpose of this short article is to introduce its basic concepts to the SIMZINE audience.Ī system is a structured ensemble of interacting parts. System theory offers an alternative approach to formulating and answering questions about such phenomena. Logistic (S-shaped) growth would be a better choice for modeling world population for this 100 year time interval shown.Simple causal chains of events do not suffice to grasp complex phenomena such as evolution of the climate, health and disease in the human body, or spread of infectious diseases. Also shown is an estimate of future world population which is close to the mid-range United Nation Environmental Program (UNEP) best guess for future population to stress that exponential growth is not realistic for world population although it works fairly well for the time between 19. In the graph below we show the world population predicted for a fixed 2% growth rate from 1950 to 2050. Not all positive feedbacks give exponential growth but all, left unchecked, will result in unlimited (or unstable) growth. ![]() The solution to this is P(t)=Po(exp) or exponential growth. This idea can be modeled nicely with the differential equation dP/dt=+rP, where P is population and r is the percent birth rate. ![]() In the figure below connecting population to births, large populations cause large numbers of births and large numbers of births result in larger population. Positive feedbacks will result in unlimited growth (until checked) and are sometimes referred to as vicious cycles. This same trick of multiplying the signs of the connections around a loop together to find out whether it is a positive or negative feedback loop works for more complicated loop structures with many more connections.Īn example of positive feedback is world population with a fixed percentage birth rate. Going around the loop the positive connection times the negative connection gives a negative loop feedback effect. ![]() When these two connections are combined we get a negative feedback loop as shown at left in which the coffee temperature approaches the stable equilibrium of the room temperature.
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