Control Systems in the Living World - Building the Most Complex Structure on Earth - page 17

Lake

Water input Water output

Figure 1.10 A diagrammatic representation of the open-loop control system of a lake.

Living system

Controller

Error
detection

Deviation
identification

Output
signal

Effector

Set point Comparator Processor

Afferent sensory input

Sensor

Figure 1.11 A diagrammatic representation of a closed-loop control system.

the relatively constant level of water in lakes. Yes, such lakes may be considered
one-variable systems in a state of dynamic equilibrium. But there are essential differ-
ences between this and the dynamic equilibrium in living systems. First, the regula-
tion of the dynamic equilibrium state of the lake as a water system is not a function
of the lake itself, but of the level of the riverbed, which is extrinsic to the system, or
at least is not determined by the system itself. This is an open-loop control system
( Figure 1.10 ) in which the lake itself does not actively participate in maintaining a
constant water level.
Living organisms are open systems exchanging matter, energy, and information
with their environment, whose dynamic equilibrium is maintained by a closed-loop
control system. The state of dynamic equilibrium of multiple variables in living sys-
tems is a function of a built-in control system, which determines the normal levels
of variables, or their set points. While any change in the level of the riverbed may
change the shape and size of the lake, changes in the environmental factors do not
affect the size, shape, or nature of the system; the built-in regulator maintains the
system variables within the normal ranges, so long as the environmental changes
do not override the adaptive capacity of the organism. In an oversimplifed form, the
closed-loop control system in living organisms looks as shown in Figure 1.11 .
This leads us to Karl Ludwig von Bertalanffy's concept of equifinality. In a clear
distinction from inorganic systems, where the initial conditions determine the final
state of a system, in living systems, the final state may be reached even under differ-
ent initial conditions. This is a state of “flowing equilibrium” ( Fliessgleichgewicht ),