HOMEOSTASIS
Claude Bernard, a French physiologist, first suggested in 1859 the idea of a
steady state: the maintenance of constant conditions within organisms.
Not until 1930, when an American physiologist, Walter B. Cannon, coined
the term homeostasis, the principle of the regulation of the internal
environment within narrow limits get named.
The majority of cells are not in direct contact with the external environment.
Living cells can survive and function only within very narrow range of
conditions.
Homeostasis refers to the body’s attempt to adjust to a fluctuating external
environment. The body’s internal environment is kept relatively constant or
in a dynamic equilibrium which is necessary for survival. To maintain this
dynamic equilibrium, constant monitoring, feedback and coordination
between body systems are essential.
Some of the internal STRESSES within the body’s internal environment
which must be maintained:
1. pH
2. [ ] of O2 and CO2
3. [ ] of nutrients
4. [ ] of waste
5. [ ] of water, salt, ions
6. temperature
7. volume and pressure water/fluid
The mechanisms of homeostasis do not maintain absolute and unchangeable
set-points, rather a fluctuation above and below a set-point within a range.
E.g. human body temperature set-point about 370C
over minutes, time of day, activity level etc. your temperature fluctuates from
“normal”
Theory of Communication and Control
To maintain homeostasis the body must be able to recognize stress and
respond to it.
There are three components to a homeostatic control system:
1) sensor
2) integrator
3) effector
Stress – change in environment, provides a stimulus

Stimulus – something that sets the system off, detects the stress

Sensors – specialized cells to pick up stimulus; change in the
environment; located in various organs

Afferent nerves – input message is carried along nerves

integrator – a relay point where bits of information are

pulled together i.e. brain and spinal cord
Efferent nerves – output message is carried along nerves

Effectors – signals are sent to muscles or glands which carry out
appropriate responses to restore “normal” balance; set point

response – leads to a change, change is fed back to sensor
Feedback Mechanisms
Many homeostatic controls work through feedback systems to make
adjustments and bring body back within an optimal set-point.
Negative Feedback - mechanisms are activated to restore original condition
- mechanisms trigger a response that reverses the
changed condition
- most common
e.g. thermostat
when temperature drops below the set point the furnace turns on
when temperature rises above the set point the furnace turns off
thermometer – sensor with a set point
thermostat – intergrator
furnace – effector
Positive Feedback - an increase in a substance or activity eventually leads to
more of an increase
- a small effect is amplifies the original input moving it
even further from set-point
- less common
e.g. birth of a baby
hormone oxytocin causes contractions of the uterus, this then causes
the cervix to stretch triggering more oxytocin to be released causing
more contractions and more stretching