Testing the 1st Model
Figure 1 (on the previous page) adopts the format of an
equilibrium diagram, as explained in chapter 5. But a close inspection of
the flows will reveal that the system is not exactly in equilibrium at the
start of the simulation. The energy stored in both the core and the skin
is declining at a very slow rate. But given time, the model might make the
necessary adjustments and "find" a stable equilibrium condition
(if it exists). To test if this will happen, the model is simulated over
a 12 hour period with the ambient temperature held constant at 27.5 degrees.
Then the temperature is lowered 5 degrees to simulate the model's response
to colder conditions. Figure 2 shows the results.
Figure 2. Simulation results from a "cold step test" of the first
model.
The first 12 hours of simulated behavior in Figure 2 show
both the core temperature and skin temperature approximately constant, but
the body is slowly losing energy. By the 12th hour, the core temperature
is down to 36.86 degrees C (98.35 degrees F). The ambient temperature drops
5 degrees in the 12th hour to test the responsiveness of the model. A 5
degree drop places the new ambient temperature well within the range of
control expected by Riggs, but the model responds with a gradual decline
in both the core temperature and the skin temperature. The new equilibrium
is reached after around 24 hours. The approach to the new equilibrium may
seem unusually slow, but Riggs does not include a "time dimension"
that would allow us to compare results. (Milsum (1966, p. 77), on the other
hand, gives examples of slow responses to "cold step" tests. But
his text does not include a model.)
Feedback in the First Model
Clearly, the first model does not contain the control mechanisms
needed to maintain the core temperature close to 37 degrees. You might be
tempted to conclude that the model lacks negative feedback which is essential
for controllability. But this is not the case.
Figure 3 shows four loops in the model. The positive loop involving basal
heat production is not active because the core temperature never climbs
above 37 degrees.
The three negative loops are active, but their actions
do not provide the system with homeostatic control. |
Figure 3. Feedback Loops in the First Model. |