Dynamic Equilibrium and
Homeostasis is the maintenance of
a stable internal state within an organism. Homeostasis is also known as steady
state. Organisms must respond and maintain
homeostasis in relation to many factors.
Organisms detect changes in
their environment and respond to these changes in a
variety of ways. These changes may occur
at the cellular or organism level.
The graphic above shows the response of a human
to being struck on the knee with a hammer. A
change in the environment is called a stimulus.
In this situation, the stimulus is the being struck with
the hammer. A response is the manner in
which the organism reacts to the stimulus. The knee
jerk reflex which is pictured at the right is the response
of this individual to being hit with this hammer.
Feedback mechanisms have
evolved in living things as a mechanism by which they maintain
homeostasis or dynamic equilibrium.
A feedback mechanism occurs when the level of one
substance influences the level of another substance or
activity of another organ.
An example of a feedback mechanism in humans would be the
increase in heart rate and respiratory rate which occurs in
response to increased exercise or other increased muscle cell
activity. Some other examples of feedback
mechanisms in living things appear below.
Humans maintain a relatively constant body
temperature of about 37° C.
- when we "heat up" we sweat if possible
- the evaporation of this perspiration returns the body
to its original temperature
Blood Sugar Regulation
The pancreas is an endocrine gland which
produces hormones which regulate blood glucose
An increase in blood sugar level
triggers the release of the hormone insulin
by the pancreas
the hormone insulin lowers blood sugar level restoring
the body to its original blood glucose level in two major
- it increases the ability of body cells to
take in glucose from the blood
- it converts blood glucose to the compound
glycogen -- this compound is also called
animal starch and is stored in our liver and
Maintenance of Water
- plants need to regulate water loss and carbon dioxide
intake for photosynthesis and other life activities
- when plants do not keep enough water in their cells,
they wilt and die
stomate: a microscopic hole in a plant leaf
which allows gases to enter and leave and water vapor to
leave as well. Stomata is the plural of stomate.
guard cells: open and close the stomate.
- the ability of the guard cell to close during periods
of limited water availability for the plant allows the
plant to maintain water homeostasis