Homeostatic Mechanisms
Homeostatic mechanisms control a property of all living things called homeostasis. Homeostasis is a built-in, automated, and essential property of living systems. Breathing is an example of a homeostatic property. Homeostatic mechanisms are self-regulating mechanisms that function to keep a system in the steady state needed for survival. These mechanisms counteract the influences that drive physiological properties towards a more unbalanced state.
The recognition of homeostasis and homeostatic mechanisms dates from the nineteenth century. Then, the French physiologist Claude Bernard observed the constancy of the composition of blood and other body fluids. He proposed that such constancy was vital for life. Indeed, the word homeostasis is from the Greek words for "same" and "steady". The term was coined in 1930 by physiologist Walter Cannon in a book called The Wisdom of the Body.
The mechanisms that regulate homeostasis operate by feedback mechanisms. Negative and positive feedback mechanisms operate in living things. Negative feedback mechanisms reverse the direction of the change. This maintains the constant, steady state and so represents homeostasis. Positive feedback, on the other hand, acts to change the variable even more in the direction in which it is changing. Thus, positive feedback is not a homeostatic mechanism.
Temperature control is an example of a negative feedback homeostatic mechanism. The region of the brain called the hypothalamus monitors the human body's temperature. Variation from the normal temperature of 98.6°F (37°C) triggers a response from the hypothalamus. The temperature can be lowered by activation of glands capable of sweating, or raised by signalling muscles to shiver to produce heat.
Homeostatic mechanisms are a fundamental characteristic of living things. Without these mechanisms, facets of a body that need to be kept operating in a steady state, such as temperature, salinity, acidity, hormone levels, concentration of gases such as carbon dioxide, and the concentrations of nutrients, would become so unbalanced as to threaten the life of the organism. In a healthy body, homeostatic mechanisms operate automatically at different levels; molecular, cellular, and at the level of the whole organism.
At the molecular level, the activity controlled by one gene can be under regulatory control by another gene. At the cellular level, a well-studied homeostatic mechanism is contact inhibition, in which cells stop dividing when they begin to crowd in on each other. Cancer, in which a hallmark is the rampant growth and division of cells, is a condition where the homeostatic mechanism of contact inhibition is inoperative or defective.
At the whole organism level, a homeostatic mechanism is a vital part of birth. During labor, the contraction of the uterus causes the release of a hormone called oxytocin from the hypothalamus. The hormone increases contraction frequency, which in turn stimulates the release of more oxytocin. This increasing contraction cycle propels the fetus down the birth canal and into the world. After birth, the oxytocin acts to contract the expanded uterus in order to minimize bleeding, thereby maintaining the mother's blood volume
The importance of homeostatic mechanisms to the well being of an organism is underscored by the consequences of their failure. For example, at body temperatures of 107°F (42°C), the negative feedback systems cease to function. The high temperature then acts to speed up the body's chemistry, raising temperature even more. This, in turn, further accelerates body chemistry, causing a further rise in temperature. This cycle of positive feedback is lethal if not halted.
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