preserving equilibrium in ongoing change.
Target levels vary as the need arises. Blood pressure is reduced during sleep, while during exercise it may double. This does not mean a deficiency, let alone a breakdown of the homeostatic mechanism, but simply that the target level has been reset, because the body's needs have changed. The same holds for fever.
Figure 2.1: Homeostatic model relating house temperature to heating system activity and vice versa: relating heating system activity to house temperature, with the set-point (target) temperature as the controlling variable.
Box 1: You, the user of this control system, consider various factors in determining the preferred temperature. The temperature preferred usually is a compromise between the degree of physical comfort you ideally wish, on the one hand, and the cost of the energy needed for heating or cooling, on the other.
Box a: The preferred temperature is set on the thermostat control; this is called the set-point variable. It is a variable, because you have the choice between an entire range of set points. If energy costs go up, you are likely to choose a different compromise between considerations of comfort and cost, and you set the desired temperature to a different level.
Box b: The thermostat control continuously compares the actual temperature reading of the thermometer with the set-point temperature; this comparison is made at a point in the regulating process that is called the comparator or summing point.
Box c: Whenever there is a discrepancy (symbolized as [a-b]) between the thermometer reading and the set point, and this discrepancy is greater than a given tolerance of, say, 2% to 5%, the generator of warm air (furnace) or cool air is activated. The purpose of this is to keep the difference between a and b close to zero and this is achieved through a temperature-sensitive switch that tells the unit to produce either warm air or cool air, or to do nothing at all.
Box d: In order to adjust the house temperature to the set point, the air being forced into the house is somewhat warmer than the set point in the case of thermostatic heating, and somewhat cooler in the case of air conditioning.
Box e: As a result of this adjustment action, the house temperature is changed in the direction of the set-point temperature.
Symbol f: Because the thermostat control is usually (and for an obvious reason) not located in the vicinity of the air vents, and because it takes some time for the altered air temperature to diffuse throughout the house and to finally reach the location of the thermometer, there is some time delay between the production of the adjusted house temperature and the reading on the thermometer. This brings the process back to Box b and starts another adjustment cycle. Hence the term "closed loop".
It will then begin to cool off, and when the temperature drops to the set temperature the furnace will cut in. However, since it still takes some time for the radiator to warm up and to produce additional heat, the air in the house will still continue to cool off. It is worth noting that, if the actual temperature never dropped below the set-point temperature, the furnace would never be activated. Similarly, if the actual temperature never rose above the set-point temperature, the furnace would never be turned off.
Consequently, as shown in Figure 2.2, the actual temperature fluctuates around the set-point temperature. In fact, these oscillations are necessary to produce the signals to the heat generator to cut in or cut out.
Stability of the average actual temperature over time is obtained by virtue of the occurrence of temperature unsteadiness! This apparent contradiction may be one of the reasons that the process of homeostasis is sometimes misunderstood, but it is part and parcel of its nature. A homeostatic process makes it possible to extract long-term steadiness from short-term fluctuations.
Figure 2.2: Various amplitudes and wavelengths of fluctuations of homeostatically controlled variable (solid curves) around a value that is stable when averaged over time (dotted line).
Other influencing factors have also been identified in Figure 2.1:
Box 2: The quality of the switch function controlling adjustment action. If the switch is slow to respond or marked by high tolerance for "error"--that is, the difference between the actual and the set-point temperature--then the temperature fluctuation will show a higher amplitude and a longer wavelength (compare curve b with c in Figure 2.2). The temperature swings can be reduced if the thermostat is equipped with an anticipator. This is a miniature electric heater inside the thermostat casing which heats the temperature-sensing element faster than the heating system heats the house. The anticipator is activated when the furnace kicks in and turns the furnace off before the set-point temperature is actually reached. Excess of the desired temperature is thus reduced.
Box 3: The heating capacity of the furnace. Both amplitude and wavelength of temperature fluctuation will be small when this capacity is high and when heat production can be turned on and shut off immediately after the temperature reaches the set point (compare curves a and c in Figure 2.2).
Box 4: The temperature fluctuations will be small and of short duration to the extent the thermometer is more sensitive and reliable.
In passing, it is of interest to note that low tolerance for "error" and high sensitivity are not necessarily desirable. Although fluctuations in the controlled variable would be reduced in magnitude and be of shorter duration, the heat-generating mechanism would then have to be activated and de-activated in rapid succession. As this would wear out the equipment more quickly, it makes sense to make the sacrifice of accepting some degree of fluctuation in the output. In practice, a heating engineer will allow the temperature to fluctuate within limits such that the temperature changes are not noticeable or at least not uncomfortable to the user of the equipment. In other words, the difference between the peaks and the troughs in the house temperature are kept below or around the "just noticeable difference", abbreviated as JND by psychologists.
In Chapter 4 it will be argued that there is a similarity between Boxes 4, 2 and 3 (in that order) and human perception, decision making and action. Greater precision in skilled performance can be achieved by making a greater mental effort, but at the cost of faster build-up of mental fatigue and thus at the risk of greater error at a later time.
Imagine you are driving on a perfectly straight road. You move the steering wheel in order to aim your car at a point in the distance. Since it is not possible to do this with mathematical precision, you discover, in a matter of seconds at the most, that your car is veering away from that target and you make a steering correction. Some time after that, you notice another deviation and you make another correction, and so on. Most of the time, your car is moving towards a point that is either on the left or the right of the target. You could, of course, try to reduce the magnitude (amplitude) or duration (wavelength) of the aiming errors to a bare minimum, but that would demand increased concentration and might prevent you from noticing something else that is relevant to the driving task. What you attempt, therefore, is not to minimize steering error, but to keep it within reasonable limits. Here, to err is better than not to err, provided error remains within those limits.
So, what does influence the time-averaged temperature? In Figure 2.1 there is only one factor outside the closed loop and that is the set-point variable, represented by Box a. Accordingly, the time-averaged temperature depends exclusively on the set-point temperature. This is the temperature you have chosen as a compromise between considerations of comfort and costs: the target temperature. Thus, the time-averaged temperature will match the set-point temperature--once again, of course, on the condition that the equipment functions.
There is, finally, one more feature to homeostasis that I would like to call to your attention, and that is the notion of "negative feedback". The adjustment action of the furnace (Box d in Figure 2.1) obviously determines the radiator temperature. However, the radiator temperature also determines the adjustment action of the furnace (by activating or de-activating it), and it does this through the feedback loop:
Box e --> Box b --> Box c --> Box d
We are dealing with a two-way process, with mutual dependency and thus with circular causation: Box d controls Box e and Box e controls Box d. They are linked together by a process of "each one feeding the other", and this feedback is called negative because the feedback reduces the error: a high radiator temperature will cause the furnace to be turned off and a low radiator temperature will lead to the furnace being activated. As a consequence, radiator temperature will be adjusted accordingly. Homeostasis is a self-correcting mechanism through its use of negative feedback.
In contrast, the causation that links Box e to Box a is one-way only and linear: the set-point temperature controls the radiator temperature. Here, we are dealing with open-loop control and thus with linear causation. The same holds for the factors that determine the desired temperature (Box 1): the causal process goes in one direction only. At these points in the control process, there is no feedback.
As we attempt to demonstrate in this book, these two features of homeostasis (the closed loop and the open loop) are crucial for understanding the process of accident causation, and equally important for the development of interventions that foster effective health and safety habits in the population.
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