HOMEOSTASIS
by Kenneth S. Saladin
Living cells can function only within a narrow range of such conditions as temperature, pH, ion
concentrations, and nutrient availability, yet living organisms must survive in an environment where
these and other conditions vary from hour to hour, day to day, and season to season. Organisms
therefore require mechanisms for maintaining internal stability in spite of environmental change.
American physiologist Walter Cannon (1871–1945) named this ability homeostasis (homeo means "the
same" and stasis means "standing or staying"). Homeostasis has become one of the most important
concepts of physiology, physiological ecology, and medicine. Most bodily functions are aimed at
maintaining homeostasis, and an inability to maintain it leads to disease and often death.
The human body, for example, maintains blood pH within the very narrow range of 7.35 to 7.45. A pH
below this range is called acidosis and a pH above this range is alkalosis. Either condition can be lifethreatening. One can live only a few hours with a blood pH below 7.0 or above 7.7, and a pH below 6.8
or above 8.0 is quickly fatal. Yet the body's metabolism constantly produces a variety of acidic waste
products that challenge its ability to maintain pH in a safe range.
Body temperature also requires careful homeostatic control. On a spring or fall day in a temperate
climate, the outdoor Fahrenheit temperature may range from the thirties or forties at night to the
eighties in the afternoon (a range of perhaps 4 to 27 degrees Celsius). In spite of this environmental
fluctuation, our core body temperature is normally 37.2 to 37.6 degrees Celsius (99.0 to 99.7 degrees
Fahrenheit) and fluctuates by only 1 degree or so over the course of 24 hours. Indeed, if core body
temperatures goes below 33 degrees Celsius (91 degrees Fahrenheit) a person is likely to die of
hypothermia, and if it goes above 42 degrees Celsius (108 degrees Fahrenheit), death from
hyperthermia is likely.
Internal conditions are not absolutely stable but fluctuate within a narrow range around an average
called the set point. The set point for core body temperature, for example, is about 37.4 degrees
Celsius, but the temperature fluctuates within about 0.5 degrees Celsius. Thus, it is more accurate to
say the body maintains an internal dynamic equilibrium than to say it maintains absolute stability.
Negative Feedback and Stability
The usual means of maintaining homeostasis is a general mechanism called a negative feedback loop.
The body senses an internal change and activates mechanisms that reverse, or negate, that change.
An example of negative feedback is body temperature regulation. If blood temperature rises too high,
this is sensed by specialized neurons in the hypothalamus of the brain. They signal other nerve centers,
which in turn send signals to the blood vessels of the skin. As these blood vessels dilate, more blood
flows close to the body surface and excess heat radiates from the body. If this is not enough to cool the
body back to its set point, the brain activates sweating. Evaporation of sweat from the skin has a strong
cooling effect, as we feel when we are sweaty and stand in front of a fan.
If the blood temperature falls too low, on the other hand, this is also sensed by the hypothalamus and
signals are sent to the cutaneous arteries (those supplying the skin) to constrict them. Warm blood is
then retained deeper in the body and less heat is lost from the surface. If this is inadequate, then the
brain activates shivering. Each muscle tremor in shivering releases heat energy and helps warm the
body back toward its 37 degrees Celsius set point.
In both cases, specialized neurons sense the abnormal body temperature and activate corrective
negative feedback loops that return the temperature to normal. As a result, body temperature seldom
goes more than0 .5 degrees Celsius above or below its set point. Other negative feedback loops regulate
blood sugar concentration, water balance, pH, and countless other variables. Many such loops are
regulated by the nervous system, and others by the hormones of the endocrine system.
Positive Feedback and Rapid Change
The counterpart to negative feedback is the positive feedback loop, a process in which the body senses
a change and activates mechanisms that accelerate or increase that change. This can also aid
homeostasis, but in many cases it produces the opposite effect and can be life-threatening.
An example of its beneficial effect is seen in blood clotting. Part of the complex biochemical pathway
of clotting is the production of an enzyme that forms the matrix of the blood clot, but also speeds up the
production of still more thrombin. That is, it has a self-catalytic, self-accelerating effect, so that once
the clotting process begins, it runs faster and faster until, ideally, bleeding stops. Thus, this positive
feedback loop is part of a larger negative feedback loop, one that is activated by bleeding and
ultimately works to stop the bleeding.
Another example of beneficial positive feedback is seen in childbirth, where stretching of the uterus
triggers the secretion of a hormone, oxytocin, which stimulates uterine contractions and speeds up
labor. Yet another is seen in protein digestion, where the presence of partially digested protein in the
stomach triggers the secretion of hydrochloric acid and pepsin, the enzyme that digests protein. Thus,
once digestion begins, it becomes a self-accelerating process.
Often, however, positive feedback produces the very opposite of homeostasis: rapid loss of internal
stability with potentially fatal consequences. For example, if the death of a small area of heart tissue
triggers a heart attack (myocardial infarction), the heart pumps an inadequate amount of blood. Thus,
the heart muscle itself is deprived of blood flow, and still more begins to die. This can lead to a rapid
worsening of cardiac function until a person dies. Many diseases involve dangerous positive feedback
loops.
Homeostasis, while described here with examples from human physiology, is a fundamental property
of life and a necessity for survival of all living things—not just humans but all other animals as well as
bacteria, plants, fungi, and protists. It enables all living organisms to maintain internal stability in spite
of a ceaselessly changing and challenging environment.
Assignment
1. What is homeostasis and what are some examples mentioned in the article?
2. What are the normal ranges for temperature and blood pH?
3. What do you think “dynamic equilibrium” means?
4. Briefly describe “negative feedback.”
5. Briefly describe “positive feedback.”
6. Dissect the words “hypothermia” and “hyperthermia.”