Elements of a Simple Control System
A basic control system for
a regulated physiological variable consists of several
-these are receptors which respond either directly or indirectly
to a change in the controlled variable
-this is the coordinating and integrating component which
assesses input from the sensors and initiates a response
-these are the components which attempt, directly or indirectly
to change the value of the variable.
For a control system to
function effectively, there must be a closed loop. The change
due to the action of the effectors must be detected by the
sensors. This monitoring by the sensors provides feedback
to the central controller. This type of system is referred to as
a servo-control system.
Diagram to come ]
For many apparently
regulated physiological variables, it can be difficult to see
how this control model fits. In some cases this is due to the
complexity of many interacting factors and interacting control
systems which are difficult to separate and state simply.
However, a simple model for the control of water balance is easy
to construct within this framework. It should be noted that
additional mechanisms (eg local renal factors) can affect water
balance quite significantly in some circumstances - some of
these are discussed in Section 5.9.
circumstances, most water input is due to ingested water (as
fluids or in food). The sensitive osmoreceptors adjust water
balance by ADH-mediated changes in free water excretion into the
urine and thirst-mediated changes in water ingestion. The
mechanism for regulation of water balance is often referred to
as the ‘thirst-ADH mechanism’. The following sections discuss
the components of the control system in more detail.
All material © Copyright - Kerry Brandis, 2001