BIOLOGY Volume 2 - A Guide to General Biology - 2004

19. HOMEOSTASIS

Any Organism can be viewed as a complex physicochemical system existing in a steady state within its environment. It is precisely this ability of living systems to maintain a steady state in a continuously changing environment that ensures their survival. To maintain such a state, all organisms—from the simplest to the most complex—have evolved a vast array of adaptive mechanisms, including structural, physiological, and behavioral ones, designed to achieve the primary goal as effectively as possible: maintaining the constancy of the organism's internal environment, or its Homeostasis. This "purpose" of living systems was first formulated by the French physiologist Claude Bernard in 1857. Throughout his scientific career, Bernard was struck by the ability of organisms to regulate and maintain physiological parameters such as body Temperature or Water content within relatively narrow limits. He summarized this concept of self-regulation as The basis of physiological stability in what has become a classic statement: "The constancy of the internal environment is the condition for a free life."

Bernard consistently emphasized the distinction between the external environment in which organisms live and the internal environment in which their individual Cells reside (tissue or interstitial fluid in mammals), recognizing how crucial it is for the internal environment to remain unchanged. For example, mammals maintain a constant body temperature despite fluctuations in ambient temperature. If it becomes too cold, the animal may move to a warmer or more sheltered Location (a behavioral response). If this is not possible, internal self-regulation mechanisms come into play, increasing heat production and reducing heat loss (a physiological response). All metabolic systems operate most efficiently within a narrow range of near-optimal conditions. Consequently, the organism as a whole will also function most efficiently when its cells are under optimal conditions. Homeostatic mechanisms prevent significant deviations from the optimum caused by changes in both the external and internal environments.

In 1932, the American physiologist Walter Cannon introduced the term homeostasis (from the Greek hómoios meaning similar, and stasis meaning standing or stability), which denotes the "constancy of the internal environment" described by Bernard. By maintaining stable conditions for cellular life, homeostatic mechanisms provide the organism with a degree of independence from its surrounding environment. The more effective these mechanisms are, the greater this independence. The degree of independence can be used as a measure of the "success" of a particular life form. From this perspective, complex organisms such as mammals or flowering plants are highly successful groups because they can maintain a relatively constant level of activity even amid drastic fluctuations in external conditions. Such organisms are sometimes called regulators because they are capable of regulating various parameters of their internal environment. As a rule, their geographical ranges are much wider, and the habitats they occupy are more diverse, than those of non-regulators, which cannot effectively regulate their internal environment and therefore must live in more stable external conditions, such as oceans or lakes. Non-regulators include, in particular, Cnidarians and Algae, including phytoplankton.

To ensure a more or less stable level of organismal activity, regulation is required at all levels—from molecular to population. This entails The Use of various biochemical, physiological, and behavioral mechanisms best suited to the complexity and lifestyle of a given species. In all these respects, mammals are far better equipped than more simply organized animals such as cnidarians.

The regulatory mechanisms found in living organisms share many similarities with the control devices in inanimate systems, such as machines. In both cases, stability is achieved through a specific form of control. In 1948, N. Wiener introduced METABOLISM/2.html">THE CONCEPT OF cybernetics (from the Greek kybernetikē, meaning the art of governance). Cybernetics deals, in particular, with the general laws of regulation in living and non-living systems. This science is also firmly established under the name control theory. Physiologists studying plants and animals often use precise Mathematical models of control theory to explain the mechanisms of biological regulatory systems; therefore, before examining how parameters such as body temperature or Blood sugar levels are self-regulated, one should gain at least a general understanding of some principles of control theory.

19.1. Control Systems in Biology

Applying control theory to biological processes has provided a deeper understanding of the functional interactions between the components of many physiological mechanisms. For instance, living systems are now viewed as open systems because they require a continuous exchange of matter with their environment. Indeed, living systems exist in a dynamic equilibrium with their surroundings, requiring a constant influx of external energy to prevent complete equilibrium with the outside world. A simple analogy is a fountain: its operation requires a continuous supply of matter (water) and energy provided by a pump. Only under these conditions is its operation stable—in other words, it maintains a steady state relative to its environment. The basic components of any control system are shown in Fig. 19.1. Each of them can be referred to by various terms; for example, the regulator in mammals is either an endocrine gland that secretes Hormones or the Brain (or Spinal Cord).

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Fig. 19.1. Main Components of a control system.

The effectiveness of a control system can be judged by:

1) the degree of deviation of the regulated parameter from the required (optimal) level;

2) The rate of return to this level.

Any deviation from the optimal level activates the control system, which then ensures a return to that level. When conditions once again become optimal, the regulatory processes are shut off via a mechanism known as negative feedback. For feedback to occur, the outcome of the system's operation must be compared against a set point, which represents the optimal value of the regulated variable (such as a temperature Setting on a thermostat scale). Control systems feature two forms of feedback: negative and positive. The former is much more widespread in the homeostatic systems of living organisms.

Negative Feedback

Negative feedback enhances system stability (Fig. 19.2). When the system's equilibrium is disturbed, a sequence of events is triggered to restore the initial state. The operational principle of negative feedback can be illustrated by temperature regulation in an electric furnace using a thermostat. The control system of an electric furnace consists of an effector (the heating element through which an electric current flows—the input), the output (the furnace temperature), and a thermostat pre-set to the desired temperature (the reference point). The thermostat acts as both a detector (or receptor) and the regulator itself. If the thermostat is set to a reference temperature of 150 °C, electric current will flow through the heating element until the furnace temperature reaches 150 °C; the thermostat then switches off, and heating stops. When the temperature drops below 150 °C, the thermostat switches on again, and the electric current raises the temperature back to the set value. In this system, the thermostat acts as an error detector. The error represents the difference between the output signal and the reference value, and it is eliminated by the effector (the heating element), which is activated upon detection of the error. THE PRINCIPLE OF negative feedback, which maintains a steady state within a system, is characteristic of many physiological processes in organisms.

Fig. 19.2. Homeostatic control system. Negative feedback required to shut off the process is indicated by dashed lines.

In particular, this is how the following are regulated:

1) blood oxygen and carbon dioxide levels through changes in breathing rate and depth (section 9.5.5);

2) Heart rate (section 14.7.4);

3) blood pressure (section 14.7.7);

4) blood hormone levels, such as thyroxine (sections 17.6.4 and Fig. 19.3) and Sex Hormones (sections 21.7.4 and 21.7.6);

5) metabolite levels, such as glucose (sections 17.6.6 and 19.2);

6) water-electrolyte balance (section 20.3.5);

7) pH (section 20.8);

8) body temperature (section 19.5).

Fig. 19.3 illustrates The Role of negative feedback in regulating thyroxine release by The Thyroid Gland. In this example, the Hypothalamus acts as the detector, the Pituitary Gland as the controller, and the thyroid gland itself as the effector.

Fig. 19.3. An example of a simple biological control system: Regulation of the secretion of thyroxine, a thyroid hormone. TRH — thyrotropin-releasing hormone; TSH — thyroid-stimulating hormone.

Positive feedback

Positive feedback is rarely found in biological systems because it leads to system instability and extreme states. In such situations, an initial disturbance triggers consequences that amplify it even further (Fig. 19.2). For instance, during the propagation of a Nerve Impulse, membrane depolarization increases the permeability of the neuron membrane to sodium ions. As sodium ions enter the axon across the membrane, they enhance depolarization, thereby further increasing their own influx into The Cell. The rate of this influx surges rapidly, resulting in the generation of an Action Potential. Here, positive feedback serves to amplify the system's response (depolarization). The magnitude of this response is limited by other mechanisms described in section 17.1.1. Positive feedback also operates during childbirth, when the hormone oxytocin stimulates uterine contractions, which in turn trigger the release of additional amounts of this hormone (section 21.8.12).

More complex mechanisms

More complex regulatory mechanisms also exist within the organism. Generally speaking, they involve additional detectors (early warning physiological systems) or additional effectors (in case the primary ones fail). For example, in homoiothermic (warm-blooded) animals, internal and surface temperature detectors maintain a nearly constant core body temperature. Skin thermoreceptors, which act as detectors of environmental temperature changes, send impulses to the hypothalamus; the hypothalamus acts as a controller and introduces adjustments before the blood temperature has even begun to change. Other Examples of such systems include The regulation of Respiration DURING PHYSICAL exercise, as well as the regulation of hunger and thirst long before actual nutrient and water deficiencies arise in the body. Similarly, multiple detectors and effectors provide redundant reliability in regulating such vital parameters as blood pressure: stretch receptors in the carotid sinus and aorta, along with baroreceptors in the Medulla Oblongata, detect changes in this parameter and elicit responses from various effectors, including The Heart, Blood Vessels, and Kidneys. Dysfunction in one of these Organs can be compensated for by The activity of the others.