Page 19 - Color Atlas Physiology

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turbances. In the case of blood pressure regu- tends the settling time (! E, subject no. 3) and
lation (! C2), for example, the system can re- can lead to regulatory instability, i.e., a situa-
spond to events such as orthostasis (! p. 204) tion where the actual value oscillates back and
or sudden blood loss. forth between extremes (unstable oscillation,
The type of control circuits described above ! E, subject no. 4).
keep the controlled variables constant when Oscillation of a controlled variable in re-
disturbance variables cause the controlled sponse to a disturbance variable can be at-
variable to deviate from the set point (! D2). tenuated by either of two mechanisms. First,
Within the body, the set point is rarely invaria-
sensors with differential characteristics (D
Fundamentals and Cell Physiology In this case, it is the variation of the set point deviation of the controlled variable from the
sensors) ensure that the intensity of the sensor
ble, but can be “shifted” when requirements of
higher priority make such a change necessary.
signal increases in proportion with the rate of
set point (! p. 312 ff.). Second, feedforward
that creates the discrepancy between the
nominal and actual values, thus leading to the
control ensures that information regarding the
expected intensity of disturbance is reported
activation of regulatory elements (! D3).
to the controller before the value of the con-
Since the regulatory process is then triggered
trolled variable has changed at all. Feedfor-
by variation of the set point (and not by distur-
ward control can be explained by example of
bance variables), this is called servocontrol or
physiologic thermoregulation, a process in
servomechanism. Fever (! p. 224) and the ad-
terregulation before a change in the controlled
and γ-motor neurons (! p. 316) are examples
value (core temperature of the body) has actu-
of servocontrol.
1 justment of muscle length by muscle spindles which cold receptors on the skin trigger coun-
ally occurred (! p. 224). The disadvantage of
In addition to relatively simple variables
such as blood pressure, cellular pH, muscle having only D sensors in the control circuit can
length, body weight and the plasma glucose be demonstrated by example of arterial pres-
concentration, the body also regulates com- sosensors (= pressoreceptors) in acute blood
plex sequences of events such as fertilization, pressure regulation. Very slow but steady
pregnancy, growth and organ differentiation, changes, as observed in the development of
as well as sensory stimulus processing and the arterial hypertension, then escape regulation.
motor activity of skeletal muscles, e.g., to In fact, a rapid drop in the blood pressure of a
maintain equilibrium while running. The regu- hypertensive patient will even cause a coun-
latory process may take parts of a second (e.g., terregulatory increase in blood pressure.
purposeful movement) to several years (e.g., Therefore, other control systems are needed to
the growth process). ensure proper long-term blood pressure regu-
In the control circuits described above, the lation.
controlled variables are kept constant on aver-
age, with variably large, wave-like deviations.
The sudden emergence of a disturbance varia-
ble causes larger deviations that quickly nor-
malize in a stable control circuit (! E, test sub-
ject no. 1). The degree of deviation may be
slight in some cases but substantial in others.
The latter is true, for example, for the blood
glucose concentration, which nearly doubles
after meals. This type of regulation obviously
functions only to prevent extreme rises and
falls (e.g., hyper- or hypoglycemia) or chronic
deviation of the controlled variable. More pre-
cise maintenance of the controlled variable re-
quires a higher level of regulatory sensitivity
6 (high amplification factor). However, this ex-
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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