Homeostasis

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 life-threatening. 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 go 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 than 0.5 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.

References

Blessing, William W. The Lower Brainstem and Bodily Homeostasis. New York: Oxford University Press, 1997.

Cannon, Walter B. The Wisdom of the Body. New York: W. W. Norton, 1932.

Saladin, Kenneth S. Anatomy and Physiology—The Unity of Form and Function, 2nd ed. Dubuque, IA: McGraw-Hill Higher Education, 2001.

Willmer, P., G. Stone, and I. Johnson. Environmental Physiology of Animals. Oxford: Blackwell Science, 2000.