UNIT 4: Homeostasis
Chapter 9: Homeostasis: A Fine Balance
pg. 426 -
9.2: Homeostasis and Feedback Mechanisms
pg. 432 – 435
Feedback systems are continuously monitoring internal and external
conditions, to determine whether or not adjustments for deviations from the
norm are required, maintaining optimal functioning range.
Negative Feedback Mechanisms
Negative Feedback – is the response of a system that acts to maintain
equilibrium by compensating for any changes made to the system.
Sensor – is the element of a feedback system that detects changes in the
environment.
Integrator – is the element of a feedback system that compares existing
conditions with ideal conditions.
Set Point – is the optimal value for a given variable of a system.
Effector – is the element (or elements) of a feedback system that acts to
return the system to its optimal state.
The primary mechanism of homeostasis is the negative feedback system.
The negative feedback systems are responsible for compensating for changes
and returning things to normal ranges.
There are three elements that make up a negative feedback system; a sensor,
and integrator, and an effector.
The sensor is made up of tissues and organs that detect changes, in the form
of a stimulus. Information is transmitted to the integrator.
The integrator processes the information, comparing the environmental
change to the optimal functioning conditions, or set points. The integrator
will activate the effector.
The effector is a system that returns conditions back to normal ranges, this is
called the response.
The sensors and integrators are part of the nervous or endocrine system, and
the effectors are part of the tissues and organs. The effector brings things
back into balance. The negative feedback system uses antagonistic effectors,
producing the opposite effect to the change.
Figure 1: The components and process of negative feedback loop that maintains
homeostasis.
The Thermostat as a Negative Feedback Mechanism
The thermostat of a housed is responsible for maintaining the normal
temperature of a home. If the temperature should drop below the normal
room temperature of 20oC the thermostat senses a change and the circuit act
as an integrator, which then activates the effector, turning on the furnace,
heating the home.
Negative Feedback Mechanisms in Animals
Mammals and birds have a homeostatic mechanism to maintain body
temperature, around a narrow range around a set point. The hypothalamus
gland acts as the integrator. Nerve cells, thermoreceptors, act as sensors
collecting information throughout the body. The information is compared to
the set point, 35oC – 37.8oC, if there is a deviation, the hypothalamus
activates a set of physiological (vasoconstriction) and behavioural (shivering)
responses to re-establish the normal body temperature.
The effectors that are stimulated depend on whether the temperature is too
high or to low compared to the set point.
The set point can changes at times and the feedback systems will make
adjustments to the new settings, for example; causing a fever to fight an
infection.
Positive Feedback Systems
The positive feedback system is responsible for increasing the change. This
response is not really a homeostatic response since it increases the change
becoming more unstable, for example the fight or flight response, where
there is an increase of adrenaline and other hormones.
A positive feedback mechanism usually works within larger negative
feedback system. It allows the response to move further from the norm, only
to have the negative feedback system bring back to the normal range,
without harming the organism.