Maintenance of health
Health is not a static condition but represents a fluid range of physical and emotional well-being continually subjected to internal and external challenges such as worry, overwork, varying external temperatures, mechanical stresses, and infectious agents. These constantly changing conditions require the adjustment of the function of the various systems within the body. Mechanisms are continually at work to maintain a constant internal environment called by the French scientist Claude Bernard the milieu intérieur. The maintenance of this relatively constant internal environment is known as homeostasis. On a hot summer day, for example, the body is challenged to maintain its normal temperature of 98.6 °F (37 °C). Sweating represents a mechanism by which the skin is kept moist. By the evaporation of the moisture, heat is lost more rapidly. The hot day, therefore, represents a challenge to homeostasis. On a cold day gooseflesh may develop, an example of a homeostatic response that is a throwback to mechanisms in lower animals. In fur-bearing ancestors of humans, cold external environments caused the individual hair shafts to rise and, in effect, produce a heavier, thicker insulation of the body against the external chill. Humans still develop this primitive gooseflesh response but, regrettably, lack the luxuriant pelt to protect themselves.
Bacteria, viruses, and other microbiological agents are obvious challenges to health. The body is able, to a considerable extent, to protect itself and adjust to challenges, and, to the extent that it is successful, the state of health is maintained. While health is often thought of as fragile and subject to many onslaughts, it is, in fact, a ruggedly guarded state protected by a host of highly efficient internal mechanisms.
Some of the mechanisms vital to the maintenance of health include (1) the maintenance of the internal environment, or homeostasis, (2) adaptation to stress situations, (3) defense against microbiological agents, such as bacteria and viruses, (4) repair and regeneration of damaged tissue or cells, and (5) clotting of the blood to prevent excessive bleeding. Each of these areas will be discussed briefly. Despite these separate considerations, the commonality of purpose—the preservation and maintenance of health—must not be lost sight of. Insofar as each of these mechanisms works to maintain a constant internal environment, it can be considered as a homeostatic mechanism. Later, when disease is discussed, it will be apparent that to a considerable extent disease represents a failure of homeostasis and the other defensive responses listed above.
Homeostasis
As noted earlier, the term homeostasis refers to the maintenance of the internal environment of the body within narrow and rigidly controlled limits. The major functions important in the maintenance of homeostasis are fluid and electrolyte balance, acid-base regulation, thermoregulation, and metabolic control.
Fluid and electrolyte balance
This term refers to the controlled partition of water and major chemical constituents among the cells and the extracellular fluids of the body. The human body is basically a collection of cells grouped together into organ systems and bathed in fluids, most notably the blood. The intracellular fluid is the fluid contained within cells. The extracellular fluid—the fluid outside the cells—is divided into that found within the blood and that found outside the blood; the latter fluid is known as the interstitial fluid. These fluids are not simply water but contain varying amounts of solutes (electrolytes and other bioactive molecules). An electrolyte (sodium chloride, for example) is defined as any molecule that in solution separates into its ionic components and is capable of conducting an electric current. Cations are electrolytes that migrate toward the negative pole of an electric field; anions migrate toward the positive pole.
It is apparent from this table that the ionic compositions of the intracellular and extracellular fluids are significantly different. The major cation of extracellular fluid is sodium. The major anion of the extracellular fluid is chloride, while bicarbonate is the second most important. In contrast, the major cation of the intracellular fluid is potassium, and the major anions are proteins and organic phosphates. The marked differences in sodium and potassium concentrations between the intracellular and extracellular fluid of cells are not fortuitous but are due to active transport by energy-dependent ion pumps located in cell membranes. The pumps continuously move sodium ions out of the cell and potassium ions into the cell. The intracellular and extracellular compartments are thus closely integrated and interdependent: changes in one have immediate effects on the other. In clinical medicine most measurements of electrolyte concentration are performed on the extracellular fluid compartment, notably the blood serum. The concentrations remain fairly constant on a day-to-day basis, in spite of various dietary intakes of food and water.
It is the primary task of the kidneys to regulate the various ionic concentrations of the body. Any abnormality in these concentrations can produce serious disease; for instance, the normal sodium concentration in the serum (the blood minus its cells and clotting factors) ranges from 136 to 142 milliequivalents per litre, while the normal potassium level in the serum is kept within the narrow range of 3.5 to 5 milliequivalents per litre. A rise in the serum potassium to perhaps 6.2 milliequivalents per litre, as can occur when large numbers of cells are severely injured or die and potassium ions are released, could cause serious abnormalities in the performance of the heart by disturbing the regularity of the nervous impulses that maintain the heart’s rhythm.
The total amount of body water is also maintained at fairly constant levels from day to day by the combined action of the central nervous system and the kidneys. If one were to refrain from drinking any water for a few days, the thirst centre, located in the hypothalamus deep within the brain, would send out messages that would be translated into the feeling of thirst. At the same time a hormone from the posterior pituitary gland known as antidiuretic hormone (ADH; vasopressin) would be secreted. This hormone, released into the bloodstream, would reach the kidneys, where it would signal the kidneys to retain water and not excrete it. Should too much water be ingested, ADH secretion would be turned off, and the kidneys would promptly excrete the excess amount.
Acid-base equilibrium
The acidity of the body fluids is maintained within narrow limits. This acidity is expressed in terms of the pH of a solution, values exceeding 7 representing alkalinity and less than 7 acidity. The pH of a solution is an expression of the amount of hydrogen ion present. Increases in hydrogen ion concentration cause a lowering of the pH, and, conversely, decreases in the hydrogen ion concentration raise the pH. Any abnormal process raising the hydrogen ion concentration in the body fluids produces a state of disease referred to as acidosis; one that causes the concentration to be lowered results in alkalosis.
In health the blood is slightly alkaline, being kept at a pH of 7.35 to 7.45, a narrow range which must be maintained for the optimum operation of the many chemical reactions that go on constantly in the body. Alterations in the blood pH occur in many diseases, particularly of the lungs and kidneys, organs whose functions include regulation of the body pH.
Thermoregulation
As has been said above, the temperature of the body is kept nearly constant at 98.6 °F (37 °C). Fluctuations within a few tenths of a degree are perfectly compatible with health. Wider swings in temperature are usually indicative of disease, and thus body temperature is an important factor in assessing health. Body temperature is regulated by a thermostatic control centre in the hypothalamus. A rise in body temperature initiates a chain of events leading to an increase in the rate of breathing and in sweating, two processes that serve to lower the body temperature. Similarly, a decrease in body temperature, perhaps occasioned by a chilly winter walk, leads to increased heat-producing activity such as the muscular contractions of shivering—again mediated by the thermostatic control centre in the hypothalamus.