EVERYONE knows that as soon as body temperature rises even as little as 2° or 3° F. above normal that an individual is ill. For a human body to operate efficiently, the temperature of that body must be regulated within a very fine limit. We call this and other regulatory functions of all biological systems, homeostasis. Homeostasis is the tendency to regulate the internal environment, keeping it in a steady state. Physiological constancy is the first biological commandment and the great struggle in most animals' lives is to avoid change. Another example is demonstrated by the fact that if the concentration of salt in the blood of a man or animal is increased or decreased by only one half of one percent, death results. The maintenance of the constancy of the internal environment underlies almost every physiological function.
An organ of the body does not exist alone. It is a dynamic part of a dynamic system, which in turn cooperates with every other system of the body. The nervous system, the blood circulatory system, and the endocrine system of hormones are closely interrelated to maintain a fine balance in blood flow. Then the muscle system, the digestive system, and the respiratory system are all tied in together with blood circulation. One system or organ of a system cannot and does not stand alone, but is dependent upon the stimulations, inhibitions, and energy materials from a finely controlled total body.
All of this clearly illustrates the psalmist's declaration that we are "fearfully and wonderfully made" (Ps. 139:14). Another very significant point is that the more we learn about the interrelationships of these wonderful body systems, the more we become convinced that they could not possibly come about as the result of the slow, gradual process of evolutionary change.
In 1948, an MIT scientist, Norbert Weiner, wrote a book entitled Cybernetics in which he describes the phenomena of control systems applicable both to machines and living systems. Physiologists quickly recognized the importance of cybernetics to the study of body systems. Since then cybernetic analysis has been an active research topic for physio logical systems, and although there is still much to learn, progress has been made in our under standing of several body functions. We can now apply mathematical formulas to describe a few of the constant actions and reactions of the body. These regulating systems tend to prevent changes in value of a physiological variable. This is homeostasis.
If regulation of the behavior of a biological system is to be realized, the output of the system must be fed back to an error- or change-detector system. No control is possible without such feed back functions. Two basic types of feedback are noted. One is called positive feedback, in which the feedback signal increases when the system's input system increases, and decreases when the input signal decreases. This positive feedback adds to the input signal, thus resulting in instability or the opposite of regulation.
The second type is negative feedback, which causes the output to be decreased when added to the input signal, resulting in stability. Negative feedback systems, then, are the important controlling systems. The variety of regulating systems in the body is astounding. Some feedback systems take the form of nerve impulses in which the output signals are most often recorded by receptors such as pressure receptors, stretch receptors, touch receptors, heat and cold receptors, chemoreceptors and pain receptors. Error detectors are usually located in the brain stem. In chemical control, error signals may be slight differences of concentration from normal values, while the control ling system may be a whole system of complex reaction-accelerating enzymes.
Another interesting factor is the ability of these finely regulated systems to adapt to changing environmental conditions. One often refers to his not being acclimatized to a particular climate. Once acclimatized, the individual feels fine and can function normally, but changes of this nature take a few days for the body systems to ad just. We may call this built-in adaptability, but even then the changes are confined to within fairly narrow limits.
In order to describe all of the details of the various interrelated systems of the body, a whole text book of space is needed. Each system is so highly intertwined with other systems that it would be impossible for any one system to function alone. F. S. Crodins in his book, Control Theory and Biological Systems, describes in detail the cybernetics of the respiratory control of the carbon dioxide content of alveolar air in the lungs. Carbon dioxide is an end product of metabolism. It is not only a waste product, but also a very potent and necessary physiological chemical, and some must be retained in the body. Too much retention produces undesirable effects, and to control this, the body has a mechanism called the respiratory chemostat. This chemostat samples the quantities of carbon dioxide, oxygen, and acidity, and feeds back the in formation to a controlling system where the levels are compared with the command signals set by the demanded level of need for the three chemicals. The output of the controlling system is defined by actual breathing (ventilation of the lungs) where more carbon dioxide is emitted and more oxygen absorbed. The liberation of carbon dioxide causes an adjustment in the acidity and alkalinity properties of the blood. Disturbances of the system include increased carbon dioxide production in cellular metabolism, increased carbon dioxide (or decreased oxygen) in the air breathed, and the addition of acidic substances into the body.
In addition to the chemoreceptors of the controlling system that are stimulated by increased carbon dioxide and acidity, and decreases in oxygen levels, there is a direct effect of all three chemicals on the recording cells of the respiratory center in the brain stem, or medulla oblongata. There are also nerve functions that result in increased or decreased nerve stimulation of the respiratory muscles. Breathing rate also influences heart rate, through pressure receptors located in large arteries, and the pressure in these arteries varies with the breathing cycle again under control of a complex feedback system from the brain centers in the medulla.
It is clear that each of the physiological systems has an influence on the function of all the other systems, and it is this inter action of systems that makes the study of physiology so complex. If all the systems were to function separately, uncoordinated, the end result would be chaos. There could be no functionally whole organism. Coordinating systems are essential to life. The nervous system, the endocrine system and all of its complex chemical hormone messengers, and the enzyme system that promotes chemical reactions are all important aspects of body control. The nerve system acts as a direct communicating system, and the blood circulation serves to transmit the chemical messengers. Nerve im pulses transmit quickly and are short-lived, while hormones, depending upon the slower blood stream for distribution, are slower but more sustained in their action. But the release of hormones is dependent upon nervous inhibition or stimulation, or stimulation or inhibition from other hormones, and nerve impulses are guarded by enzymes and hormones. So once again we see marvelously intricate systems of positive and negative feedback to keep all systems in carefully balanced function.
The nervous system also includes the mechanics that give power of interpretation of sensations, thought, the ability to originate ideas, and the motivation to function according to ideas conceived. Physiologists find that these associative functions are most intriguing but they are also the least understood because of their extreme complexity. The organization of the brain is known only in its genera! plan, while its inner ability of the highest level associative functions and memory are not yet comprehended.
Movements of the parts of the body can be made purposeful and coordinated only by the interaction of many control systems originating in the cerebellum, midbrain, basal ganglia, and motor cortex areas of the brain. The autonomic (or automatic) control of the nervous system is divided into two main channels: the sympathetic and parasympathetic. Each opposes the reaction of the other. For example, the sympathetic system dilates the iris of the eye, the parasympathetic constricts it; the sympathetic stimulates heart muscle, the parasympathetic inhibits it; and along with many other functions, the sympathetic system relaxes muscles of the bronchiolar tubes in the lungs whereas the parasympathetic system constricts the bronchioles.
The hypothalamus of the forebrain seems to be the master control center for the regulation of many internal body conditions. The hypothalamus has a center for controlling the flow of hormones from the pituitary or the "master" endocrine gland. The pituitary in turn sends hormones coursing through organs such as the thyroid, parathyroid, adrenals, ovaries, testes and even the placental hormones when formed. As feed back systems, some hormones are produced that trigger further action or cessation of activity in the pituitary through messages transmitted from the hypothalamus.
It has already been pointed out that the sympathetic system is stimulatory to the heart, whereas the parasympathetic is inhibitory. Sympathetic nerves going to the heart from the spinal column are called cardiac accelerator nerves. These nerves when stimulated cause a chemical called sympathin to be formed at their endings within the heart muscle. Sympathin speeds the heart and strengthens its beat.
Parasympathetic nerve fibers come from the brain by way of the vagus nerves. These cardiac inhibitor fibers end in the heart tissue and stimulation causes a chemical to form, which slows the heartbeat. Enzymes destroy the chemical soon after it is formed so that continued slowing down does not occur. The two sets of nerves at their respective centers are also closely associated with centers for vasoconstriction and vasodilation, which are the control centers that expand and contract the blood vessels, thus stimulating or inhibiting the flow of blood to the tissues. All of these systems are tied in with even more complex systems that have to do with salt balance and kidney excretion, muscle activity, cellular metabolism, oxygen demand, temperature control, breathing rate, nutritional status, blood pressure control, heart volume control, and others that physiologists know little about.
We could add the examples of digestive control, urine output, and kidney function, the buffering systems controlling blood chemistry and details of the interacting systems of temperature regulation, all of which are complex and highly integrated systems inter dependent upon the others for their homeostatic operation.
What is the point of the discussion of these intriguing systems? It is this. How can we rationally claim that such intricate systems came into function by mere chance, by the slow gradual process of evolutionary change? It seems obvious that one system would be chaotic and non-functional without the other. The whole body with all of its working systems functioning as a unit must necessarily come into existence at once in order to have a living body. This most fundamental principle, homeostasis, is perhaps the greatest evidence for instant creation by a Designer who is greater than science reveals, and who should become ever greater to us as our knowledge grows.
