Stability of Living Systems
In its stable state, the human body is something like a wellbalanced mobile. To make a mobile hang evenly, you need to
adjust the positions of the various objects so that they balance
one another. All systems, including mobiles and the human
body, need a way of maintaining equilibrium, or balance.
When a system is in equilibrium, it is stable, and all its
parts function smoothly.
All organisms, whether they are humans, bacteria, or plants,
must maintain internal equilibrium no matter what happens in
their environment. The process by which the body’s internal
environment is kept stable in spite of changes in the external
environment is called homeostasis (hoh mee oh STAY sis).
Homeostasis is at work when your body returns to its normal
state after a startling or frightening event, such as a nightmare.
The body’s resistance to disease also demonstrates
homeostasis. You are surrounded by microorganisms that can
make you sick, but you probably stay well most of the time.
And when people become ill, they usually recover quickly. The
immune system helps maintain homeostasis by killing diseasecausing organisms before they can do much harm.
Negative Feedback
Organisms have different ways of keeping internal stability. Negative
feedback is one way in which living systems maintain internal
equilibrium, or homeostasis. In negative feedback, a process is turned
off by the condition it produces.
To understand how negative feedback works, consider how a thermostat
regulates the temperature in a room. When the room becomes cool, the
thermostat “senses” the drop in temperature. The thermostat then turns
the heater on. Later, when the heater has made the room warmer, the
thermostat “senses” the rise in temperature and turns the heater off.
Negative feedback is a cycle, as you can see in the diagram below. In
humans, negative feedback helps regulate many functions. For example,
negative feedback helps control the temperature of your body and the
level of the sugar glucose in your blood.
Negative Feedback The operation of a thermostat is an example of negative feedback. In
negative feedback, a process, such as heating a room, is turned off by the condition it produces.
Interpreting Diagrams What happens when a room becomes too cool?
Keeping Body Temperature Constant
Whether the air is cold or hot, the internal body temperature of
birds and mammals is always about the same. A healthy
human, for example, always has a temperature that is close to
37°C. Several processes keep body temperatures stable.
Staying Cool Sweating is one thing that helps keep the
human body at a stable temperature. When you become warm,
you perspire. Perspiration, which is mostly water, then
evaporates from your skin. Heat is needed to make water
evaporate. That heat comes from your body. Therefore, the
process of evaporation cools your body by removing heat.
Some animals, such as dogs, get rid of excess heat by panting.
When an animal pants, some of the saliva in its mouth
evaporates. The evaporation of saliva, like the evaporation of
perspiration, helps cool the body.
Negative feedback is at work in both perspiration and panting.
That is because excessive body heat leads to perspiration or
panting, which in turn results in a reduction in body heat.
Shivering While perspiration helps the body cool off,
shivering helps keep it warm. If you become chilled, you
shiver. When you shiver, muscles in your body contract
involuntarily. These muscle contractions require energy, and
the production of energy generates heat. Therefore, shivering
has the effect of warming the body. This method of temperature
regulation, like perspiration and panting, is an example of
negative feedback. When the body becomes uncomfortably
cool, shivering results. Then shivering makes the body warmer
Maintaining Glucose Levels in the Blood
The energy your body needs to function comes from
the breakdown of the sugar glucose in chemical
reactions inside your cells. Glucose comes from the
food you eat. After glucose is absorbed by the
digestive system, blood transports the glucose and
delivers it to cells.
The level of glucose in the blood is regulated by
chemical reactions. The chemical reactions are
controlled by hormones of the endocrine system. A
hormone is a chemical produced by an endocrine
gland that affects the activity of a tissue or organ.
One hormone that helps regulate glucose levels is
insulin, which is produced by an organ called the
pancreas. When the level of glucose in the blood is
high, the pancreas releases insulin. Insulin stimulates
body cells to take up glucose from the blood. This
lowers the level of glucose in the blood. Low levels of
glucose in the blood, in turn, “turn off” the
production of insulin. This is a negative feedback
process because high levels of glucose eventually
lead to the reduction of glucose levels in the blood.
Maintaining Water Equilibrium in Plant Cells
To maintain homeostasis, living things need some
way of regulating the amount of water in their cells
and tissues. Like all organisms, plants need to
maintain water equilibrium in spite of changing
conditions.
Diffusion Many plants obtain water through roots.
Water enters these plants through root cells. The
cells in a plant’s roots, like all cells, are bound by a
thin, flexible structure called the cell membrane. The
cell membrane is selectively permeable—that is,
some substances can pass through, while others
cannot. Substances move back and forth through the
cell membrane by a process called diffusion. In
diffusion, molecules move from an area in which they
are highly concentrated into one in which their
concentration is lower.
Diffusion Molecules move from an area of higher concentration to an area of lower concentration.
(A) There is a lower concentration of molecules inside the cell than outside. (B) Molecules from
the
Water Equilibrium in Animals
Animals, like plants, need to maintain water balance. When
your body needs water, you become thirsty, and you drink
fluids. This action enables your body to obtain the water it
needs.
Many animals have excretory systems that help maintain water
balance in the body. Your kidneys, which are part of your
excretory system, remove wastes produced by your cells.
Another role of the kidneys is to adjust the amount of water in
your blood. They do this by changing the concentration of
water in the urine they produce.
Suppose, for example, that you drink a lot of water—too much
for your body to use right away. When that happens, the
kidneys produce urine in which the concentration of water
molecules is high. When the urine is eliminated from the body,
the excess water is eliminated too.
In contrast, on a hot day, you might lose a lot of water through
perspiration. In that case, the urine that the kidneys produce
will have relatively little water. This helps keep water inside
the body, where it is needed