This definition is just one of many that are possible. What constitutes “good” health in particular can vary widely. The rather fragile individual who stays “well” within the ordinary environment of his or her existence may succumb to a heart attack from heavy shovelling after a snowstorm; or a sea-level dweller may move to a new home in the mountains, where the atmosphere has a lower content of oxygen, and suffer from shortness of breath and anemia until his or her red blood cell count adjusts itself to the altitude. Thus, even by this definition, the conception of good health must involve some allowance for change in the environment.
Bad health can be defined as the presence of disease, good health as its absence—particularly the absence of continuing disease, because the person afflicted with a sudden attack of seasickness, for example, may not be thought of as having lost good health as a result of such a mishap.
Actually, there is a wide variable area between health and disease. Only a few examples are necessary to illustrate the point: (1) It is physiologically normal for an individual to have a high blood sugar content 15 to 20 minutes after eating a meal. If, however, the sugar content remains elevated two hours later, this condition is abnormal and may be indicative of disease. (2) A “healthy” individual may have developed an allergy, perhaps during early childhood, to a single specific substance. If the person never again comes in contact with the antigen that causes the allergy, all other factors remaining normal, he or she will remain in that state of health. However, should the individual come in contact with that allergen again, even 20 or 30 years later, he or she may suffer anything from a mild allergic reaction—a simple rash—to severe anaphylactic shock, coma, or even death, depending upon the circumstances. Thus it can be seen that, unlike disease, which is frequently recognizable, tangible, and rather easily defined, health is a somewhat nebulous condition and somewhat difficult to define.
Moreover, physical condition and health are not synonymous terms. A seven-foot-tall basketball player may be in excellent physical condition (although outside the range of normality for height) but may or may not be in good health—depending, for example, on whether the individual has fallen victim to an attack of influenza.
There are further problems in settling upon a definition of human health. A person may be physically strong, resistant to infection, and able to cope with physical hardship and other features of his or her physical environment and still be considered unhealthy if his or her mental state, as measured by behaviour, is deemed unsound. Mental health can itself be defined variously. Some say that a person is mentally healthy if he or she is able to function reasonably well and is emotionally and behaviorally stable. Others define it as the absence of mental disorder.
In the face of confusion about definitions of health, it is most useful, perhaps, to define health, good or bad, in terms that can be measured and interpreted with respect to the ability of the individual at the time of measurement to function in a normal manner, with respect to the likelihood of imminent disease. These measurements can be found in tables of “reference values” printed in textbooks of clinical medicine, diagnosis, and other references of this type. When an individual is given a health examination, the examination is likely to include a series of tests. Some of these tests are more descriptive than quantitative and can indicate the presence of disease in a seemingly healthy person. Such tests include the electrocardiogram to detect some kinds of heart disease; the electromyogram for primary muscle disorders; liver and gall bladder function tests; and X-ray techniques for determining disease or malfunction of internal organs.
Other tests give numerical results (or results that can be assigned numerical values—such as photometric colour determinations) that can be interpreted by the examiner. These are physical and chemical tests, including blood, urine, and cerebrospinal-fluid analyses. The results of the tests are compared with the reference values, and the physician receives clues as to the health of the patient and, if the values are abnormal, for the methods of improving the patient’s health.
A major difficulty in the interpretation of test results is that of biological variability. Almost without exception, reference values for variables are means or adjusted means of large group measurements. For these values to have significance, they must be considered as lying somewhere near the centre point of a 95 percent range—i.e., the so-called ordinary range or, with reservations, the range from normal to the upper and lower borderline limits. Thus, the 2.5 percent below the lower limit and the 2.5 percent above the upper limit of the 95 percent range are considered areas of abnormality or, perhaps, illness. Some areas have wide 95 percent ranges—blood pressure, for example, may vary considerably throughout the day (e.g., during exercise, fright, or anger) and remain within its range of normality. Other values have ranges so narrow that they are called physiological constants. An individual’s body temperature, for example, rarely varies (when taken at the same anatomical site) by more than a degree (from time of rising until bedtime) without being indicative of infection or other illness.
Any system in dynamic equilibrium tends to reach a steady state, a balance that resists outside forces of change. When such a system is disturbed, built-in regulatory devices respond to the departures to establish a new balance; such a process is one of feedback control. All processes of integration and coordination of function, whether mediated by electrical circuits or by nervous and hormonal systems, are examples of homeostatic regulation.
A familiar example of homeostatic regulation in a mechanical system is the action of a room-temperature regulator, or thermostat. The heart of the thermostat is a bimetallic strip that responds to temperature changes by completing or disrupting an electric circuit. When the room cools, the circuit is completed, the furnace operates, and the temperature rises. At a preset level the circuit breaks, the furnace stops, and the temperature drops. Biological systems, of greater complexity, however, have regulators only very roughly comparable to such mechanical devices. The two types of systems are alike, however, in their goals—to sustain activity within prescribed ranges, whether to control the thickness of rolled steel or the pressure within the circulatory system.
The control of body temperature in humans is a good example of homeostasis in a biological system. In humans, normal body temperature fluctuates around the value of 37 °C (98.6 °F), but various factors can affect this value, including exposure, hormones, metabolic rate, and disease, leading to excessively high or low temperatures. The body’s temperature regulation is controlled by a region in the brain called the hypothalamus. Feedback about body temperature is carried through the bloodstream to the brain and results in compensatory adjustments in the breathing rate, the level of blood sugar, and the metabolic rate. Heat loss in humans is aided by reduction of activity, by perspiration, and by heat-exchange mechanisms that permit larger amounts of blood to circulate near the skin surface. Heat loss is reduced by insulation, decreased circulation to the skin, and cultural modification such as the use of clothing, shelter, and external heat sources. The range between high and low body temperature levels constitutes the homeostatic plateau—the “normal” range that sustains life. As either of the two extremes is approached, corrective action (through negative feedback) returns the system to the normal range.
The concept of homeostasis has also been applied to ecological settings. First proposed by Canadian-born American ecologist Robert MacArthur in 1955, homeostasis in ecosystems is a product of the combination of biodiversity and large numbers of ecological interactions that occur between species. It was thought of as a concept that could help to explain an ecosystem’s stability—that is, its persistence as a particular ecosystem type over time (see ecological resilience). Since then, the concept has changed slightly to incorporate the ecosystem’s abiotic (nonliving) parts; the term has been used by many ecologists to describe the reciprocation that occurs between an ecosystem’s living and nonliving parts to maintain the status quo. The Gaia hypothesis—the model of Earth posited by English scientist James Lovelock that considers its various living and nonliving parts as components of a larger system or single organism—makes the assumption that the collective effort of individual organisms contributes to homeostasis at the planetary level. The single-organism aspect of the Gaia hypothesis is considered controversial because it posits that living things, at some level, are driven to work on behalf of the biosphere rather than toward the goal of their own survival.
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