THERMOREGULATION

THERMOREGULATION, the ability of animals to maintain body temperature at a more or less constant level. According to their body temperature, the animal world can be divided into two large groups: cold-blooded and warm-blooded animals according to old terminology, or more correctly, animals with variable and constant body temperatures. The former received the name poikilothermic (Bergmaim, 1847), the latter homeothermic animals. The difference between these groups consists in that homeothermic animals are able to maintain body temperature within very narrow limits regardless of the temperature of their surrounding environment, whereas the temperature of poikilothermic animals stands in close dependence on it. This position inevitably presupposes the presence in the former of special thermoregulatory apparatus, whereas poikilothermic animals are either completely devoid of corresponding adaptations or possess them in very modest dimensions. Theoretically, stable equilibrium with the environment cannot be achieved even by poikilothermic animals, since the organism itself is a source of heat production. However small the heat production in an animal's body may be, it is still the factor that disrupts this equilibrium and thereby determines the constant dynamics of heat exchange between the organism and its environment. The amount of thermal energy in the body is the result of the relationship between its intake and expenditure. Heat intake in an animal consists of two components: heat of endogenous origin, produced in the animal's body by metabolic processes, mainly oxidation processes, and heat of exogenous origin, transferred to the organism by conduction and radiation from the surrounding environment. By means of the same radiation and conduction, as well as by evaporation of water, the animal gives off heat to the surrounding environment. The relationship between heat intake and expenditure determines the heat economy (Warmehaushalt) of the organism, the measure of which is body temperature for us. By the latter is meant the average daily temperature observed in a healthy animal in its natural environment. Obviously, no average body temperature of poikilothermic animals can be spoken of, whereas in homeothermic animals it represents a very constant value. Fluctuations in both individual observations and daily temperature always occur and depend on many reasons, however these fluctuations in warm-blooded animals are so insignificant that they justify considering daily temperature as one of the specific characteristics. This characteristic, however, can be considered reliable for comparison only if measurements are carried out under identical conditions, mainly in terms of the location of the thermometer. The most acceptable location for measuring instruments in animals is the rectum. The figures given below all refer specifically to rectal measurements (see Temperature). Temperature of poikilothermic animals. Due to the imperfection of measuring instruments, observations of the body temperature of cold-blooded animals for a long time were limited to a narrow circle of rather large animals in which these measurements could be carried out with a mercury thermometer. From 1831, Nobili and Melloni applied thermoelements, and although the circle of observations expanded significantly, shortcomings in the methodology itself were evident and experiments gave questionable results. Later observations, mainly thanks to Berthold (1835), who developed the methodology, acquired the value of strictly scientific research. One of the most important conditions for the correctness of such observations, Berthold considered the animal staying for a sufficiently long time before the experiment in a room with constant temperature to avoid the influence of the previous one. Richet transferred a frog from a room with temperature 15° to a room with temperature 35°. /For the animal's body temperature to reach this level, 3 hours were required. For animals living in water, pre-acclimation requires less time due to the higher thermal conductivity of water. Since any muscular work causes an increase in body temperature, measurements must be taken when the animal is completely at rest. According to Blanchard, in the butterfly Saturnia pyri, the difference in body temperature at rest and when moving wings reached 7.1°. Finally, for animals living in the air, its humidity is of great importance, since in dry air, in addition to the usual heat loss through conduction and radiation, there is also loss through evaporation of water from the body surface. In a frog in dry air with sufficient ventilation, body temperature is usually lower than the surrounding air. Precautions when measuring temperature in cold-blooded animals vary depending on the individual characteristics of the animal and when measuring, especially in small animals, require great skill. In terms of heat economy, as mentioned above, cold-blooded animals occupy an intermediate position between inorganic bodies and warm-blooded animals. The continuously ongoing metabolism in their body does not, however, allow them to be placed in the category of 'playthings' (Spielball) of the environment; nevertheless, the intensity of these processes, as in inorganic bodies, according to the van't Hoff law, slows down with a decrease in environmental temperature and increases with its increase—a phenomenon exactly opposite to what is observed in warm-blooded animals due to their thermoregulation. Due to the stated positions, the temperature of cold-blooded animals continuously lags behind the progressive movement of the environmental temperature. Bakhteev gives the following picture of the temperature course when observing with the Sphinx ligustri butterfly: air temperature V body temperature Sphinx air temperature t" body Sphinx + 15.0 + 0.4 - 0.6 - 4.3 + 14.6 + 2.7 ± 0 - 2.3 - 7.7 - 16.2 - 14.2 + 16.8 - 6.7 - 14.9 - 18.8 + 13.0 In animals living in water, as well as in insects, this lag in temperature equalization is expressed less weakly, and data on the complete equalization of body temperature with the environment should be considered unlikely on the grounds that the same animals, when collected in a pile in a room with reliable thermal insulation, have a temperature higher than their surrounding environment and are able, due to the metabolic processes occurring in their body, to increase the temperature of this room. Fish increase the temperature of water in an aquarium. If in a single bee it is impossible to detect a difference between body temperature and air temperature, this is explained only by the imperfection of measurements, since the same bees at the same air temperature, when collected in a hive, can increase the temperature in their dwelling by 7-10° and higher (Réaumur, Kozhevnikov). If we compare the intensity of metabolism, and therefore heat production, of poikilothermic with homeothermic animals, we have, according to the data of Krehl and Soetbeer, such comparative figures of heat production: Poikilothermic animals Weight in g Temperature Cal excreted per hour per 1 kg weight Lizard (Lacerta) .... Frog (R. temporaria) . . 110 | 600 ( 1 880 | 1 260 1 110 | 600 1880 \ 1 150 1 25.3° 37° 0.8 0.5 0.3 0.26 1.5 0.05 0.47 0.4 For comparison, we have in homeothermic animals: guinea pig per 1 hour per 1 kg excretes 5.0 Cal and rabbit-3.0 Cal. In addition to weak cellular activity, cold-blooded animals lack thermal insulators that usually cover the bodies of warm-blooded animals (fat, feathers, fur), and since this weak activity also weakens according to purely chemical laws with decreasing temperature, it is understandable that cold-blooded animals cannot sufficiently protect themselves from cooling. For many of them, especially those living in the tropics, protection from overheating is more essential. In view of the fact that in addition to the radiant heat perceived by them (tropical sun) or through conduction (hot sand), their own heat production increases with increasing external temperature and in this sense is an unfavorable factor, the absence of a regulatory mechanism also places cold-blooded animals at risk of overheating, which is sometimes no less dangerous than hypothermia. In neither case can cold-blooded animals rely on their own protective means and must seek protection in their surrounding environment, adapting their behavior so as to preserve in themselves that optimum of heat which is most favorable for them in the struggle for existence. A frog placed in the sun without ventilation quickly dies from overheating; naturally, on quiet hot days it finds protection in the shade or staying near bodies of water. Mollusks and crabs left in puddles after the tide bury themselves in the colder sand of the bottom if the water temperature in the puddles rises above 22°, since according to Frenzel, a temperature of 25° is already critical for marine animals. The temperature of resting poikilothermic animals has a constant tendency to equalize with the temperature of their environment; this can be easily verified in all large animals. The constancy of body temperature in warm-blooded animals sharply distinguishes them from cold-blooded animals, and only in the process of ontogenetic and partly phylogenetic development does this difference become somewhat smoothed. Pembrey's observations showed that the temperature of a chick developing in an egg depends on the temperature of the external environment, i.e., the embryo is poikilothermic. All newborn chicks need protection from cooling, especially those born naked and blind. Sparrows on the 8th day of their life, transferred from the nest to a room with temperature 17°, lowered their body temperature to 18°.

In mammals, such as the dog, cat, and rabbit, whose young are also born blind and helpless, the same changes in temperature are observed as in poikilothermic animals. According to Richet's observations, a rabbit taken from the nest 24 hours after birth lowered its body temperature from 39° to 20.5° in half an hour, and after another 20 minutes to 18.1°. Further observations showed that only on the 11th day can a young rabbit be taken from the mother without risk. Thus, a kind of incubation is observed in mammals, as in birds. During this period, the animal's organism transforms from poikilothermic to homeothermic. The lower limit of the average body temperature of warm-blooded animals, which include higher mammals and birds, should be recognized as 37°. In the process of phylogenesis, this trait distinguishing warm-blooded from cold-blooded animals becomes somewhat blurred by the inclusion of the Monotremata group (monotremes) with a body temperature not exceeding 30°. According to body temperature, Richet proposes the following classification of animals: A. Animals with constant temperature 1) Adult mammals and birds Birds (around 42°) Mammals (around 38°) Humans (around 37°) B. Animals with variable temperature a) Animals that die if their body temperature falls below 20° Newborn mammals and chicks b) Animals that fall into hibernation if their body temperature falls below 10° Animals with winter hibernation c) Animals that are active and vital at body temperatures below 20° Reptiles, amphibians, fish, mollusks, insects, etc. As can be seen from the table, the highest body temperature is observed in birds. On average, it remains around 42°; among them, in canaries 42.2°, in chickens 42.5°, in pigeons 42.0°, in waterfowl 40.6°. Mammals. The largest number of observations have been made on common laboratory and domestic animals: dogs, rabbits, cows, horses, etc. Among other animals, the wolf stands out for its high temperature, in which Parry (Parru) found 40.5°; followed in sequence: pig-39.7°, ram-39.5°, cow-39.5°, rabbit-39.5°, dog-39.1°, fox-39.1°, guinea pig-39.2-38.6°, squirrel-38.8°, monkey-38.2°, white rat-37.9°, horse-37.7°. The average temperature of a healthy adult human (rectum) is generally accepted as 37.2-37.5°; thus, among homeothermic animals, excluding Monotremata and marsupials (Marsupialia), humans have the lowest body temperature. Portier found in whales 36.5-36.9°. The influence of age and sex has been studied mainly in humans. According to the observations of most authors, the average temperature of the elderly is several tenths of a degree lower than that of adults. Sex apparently has little effect on body temperature, and only during the menstrual period is some increase in temperature observed in women. According to some data, female birds have a higher temperature than males. The average daily temperature of adult warm-blooded animals differs greatly in constancy, and deviations from these averages, regularly repeating day after day, with rare exceptions, reach 4° (thrush, starling), and in higher mammals do not exceed 2°. Their heat economy is regulated in such a way that the amount of thermal energy in the body remains constant, except for normal daily fluctuations. This stability of the heat balance can naturally be ensured only by equality of heat intake and expenditure. The organism itself is a source of thermal energy. The range of heat production in warm-blooded animals is so large that it can cover heat expenses even with significant fluctuations in environmental temperature. The adaptation for producing and giving off heat according to the needs of the organism constitutes the essence of thermoregulation. The relatively high body temperature of warm-blooded animals is explained, on the one hand, by the high activity of cells in metabolism, and consequently in heat production, and on the other, by the presence of adaptations that allow for more economical expenditure of this heat into the environment. Heat production is based on active chemical processes, while protective adaptations play only a passive role. Not all tissues participate equally in heat production. Most researchers believe that muscles account for almost 2/3 of all heat production; if we consider their increased activity during work, this participation can reach 90% of total heat production. In experiments with dogs working on a wheel, Hanriot obtained 4 times greater CO2 output than at rest. Zuntz, when ascending mountains, also noted an increase in O2 consumption by 4.7 times. The need for muscular work when the body is cooled, the unconscious shivering, which is expressed by short muscle twitches, contractions of arrectores pilorum with the appearance of 'gooseflesh'—all these phenomena related to muscular work, and consequently to increased heat production, should be considered as a reaction to cooling, as acts of regulation. The question of the participation of resting muscles in thermoregulation, which remained open for a long time, has apparently been satisfactorily resolved in the affirmative by the experiments of Freund and Jensen on cats, in which gas exchange was studied according to Barcroft in the two hind legs, and it turned out that when the animal was cooled, gas exchange increased in the legs even after preliminary section of the motor nerves. If, however, on one of the paws, in addition to the disruption of motor connections, the sympathetic branches were also sectioned, then in this leg it was already impossible to detect any reaction to either warming or cooling of the body. It should be assumed that even with motor paralysis, thermoregulation takes place and is carried out by impulses coming from the autonomic nervous system. From the examples given above, it is clear that due to their mass, muscles play a dominant role in heat production, and the participation of other organs in heat production, and consequently their role in thermoregulation, is very small. Among the factors affecting heat production, food intake should be noted. As is known, not all substances taken with food equally affect the overall metabolism. Fats and carbohydrates relatively weakly enhance it: protein food significantly increases general metabolism (see also Metabolism, protein metabolism). In cold-blooded animals, an increase in environmental temperature is reflected in increased chemical processes in their body, while cooling the animal lowers their intensity. In warm-blooded animals, the opposite is observed. Their organism, thanks to the ability developed in the process of evolution to steadfastly regulate its heat economy, responds to cooling by increasing and to warming by decreasing chemical processes, and consequently heat production. This distortion of the law of chemical reactions for the purpose of thermoregulation, Rubner proposed to call 'chemical thermoregulation.' Since heat loss occurs through the body surface, where the ratio of surface to volume (or weight) is greater, there is also greater heat loss; therefore, it is understandable that chemical thermoregulation is more strongly expressed in small animals than in large ones (Bergmann's law). Chemical thermoregulation, however, has limits. Pflüger observed O2 absorption in a rabbit immersed in a bath at 20°. The initial rectal temperature was 39.2°. As the body temperature decreased, O2 absorption increased until the temperature fell to 37.6°. The amount of O2 absorbed from the initial value of 738 cm3 per hour reached 888 (maximum). The organism energetically fights and maintains the body temperature for some time within the range of 37.3-37.6°. Further stay in the bath sharply reduces O2 consumption, and consequently the fall in temperature to 28.6°. Later, gas exchange continues to decrease; heat production is disrupted, and the rabbit behaves like a poikilothermic animal. In addition to strengthening processes that are fundamentally chemical in nature, warm-blooded animals are well protected from cold in the form of subcutaneous fat, which is especially strongly developed in animals of polar regions (whale, seal), woolly covering (polar bear, arctic fox, fur seal), and feathers (eider duck). To what extent all the listed adaptations achieve their purpose is evident from the table of observations by Parry, Lyon, and Back on inhabitants of polar countries, in whom the difference between external and body temperature reaches 73°-82°. Animal Body temperature Air temperature Difference 88.3° 15° 73.3° -35.6° -32.8° -38.8° 73.6° 13.3° 82.1° As can be seen from all that has been said, warm-blooded animals are sufficiently provided with Richhe observed a gradual increase in breathing rate to a certain limit (80 per minute), which, however, with further heating, jumps into polypnea (400 and more per minute) and becomes very superficial. The tongue is protruded outside, the mouth is open. The effect of polypnea in terms of heat dissipation is clearly evident from the data of the table by Garrellon and Langlois. A dog with its mouth closed, placed in summer in the sun or in an incubator, dies from overheating (rectal temperature 43°-44°), while a control dog with its mouth open maintains temperature within normal limits. Experiment on dogs.

Before polypnoe During polypnoe in experimental animals Body weight (in kg) Rectal t° Respiratory frequency (per min.) Air ventilation per 1 kg/hour CO₂ per 1 kg/hour CO₂ in exhaled air (in %) Rectal t° Respiratory frequency (per min.) Air ventilation per 1 kg/hour CO₂ per 1 kg/hour CO₂ in exhaled air (in %) VI...... 9.0 12.6 11.7 33.9-33.8 33.0-38.2 38.5-40.5 40-38 40 and above 32-38 12.0-22.5 16.0-12.5 11.8-12.2 0.58-0.71 0.65-0.84 0.35-0.61 1.5-1.6 1.9-2.2 1.5-2.5 40.6 41.6 41.6 228-550 240-372 152-330 67-70 61-67 71-73 0.42-0.54 0.74 0.80 0.3-0.4 0.4-0.3 0.6-0.4 protection from cooling; the same must be said about their ability to protect from overheating, which is achieved partly by reducing all heat production processes, and on the other hand by special regulatory adaptations that enhance heat dissipation. Of these latter ones, it is necessary to note: vasomotor regulation and cooling of the body through evaporation of water during sweating and respiration. Blood is a better conductor of heat than the skin, and it makes it possible not only to conduct heat from internal areas to the skin but also to make the skin variable in thermal conductivity: hyperemic skin gives off a large amount of heat, anemic skin gives off little. The fine adaptability to heat dissipation, depending on needs, is ensured by the play of skin vessels and variations in heat dissipation through radiation and conduction. If during intense muscular work or at high external t° vasomotor regulation proves insufficient, then heat dissipation through evaporation comes into play—by sweating or increased respiration (polypnoe). The possibility of heat loss through evaporation by the skin varies among different animals. While sweat glands are scattered over the entire surface of the skin in humans, horses, donkeys, and some species of monkeys, many warm-blooded animals are either completely without them or these glands have extremely limited distribution in most cases on hairless areas (the bare ends of paws in dogs and cats). Finally, not all parts are equally capable of secreting sweat. In humans, the greatest sweating is noted on the face, hands, feet, and armpits; in pigs—the snout, etc. Animals without sweat glands or with poorly developed sweating compensate for this deficiency by increased heat dissipation through respiration. Polypnoe represents a rapid, superficial type of breathing, which with an increase in the volume of respiratory air causes increased release of water vapor and thereby lowers body temperature. It has been thoroughly studied in dogs (Risch). With gradual warming Thermoregulation in humans.. Everything said about T. in warm-blooded animals applies completely to humans as a representative of this group of animals. The presence of chemical T. in him was proven by the classic experiments of Liebmeister (Liebormeister) as early as 1872, then by Voit, Rubner, and others. At that time, the question arose whether chemical T. is the result of muscle contractions caused by skin irritations and inevitably accompanying such experiments. The more the participation of muscles was excluded, the more insistent was the tendency to reduce chemical T. in humans to nothing. If even the experiments of some researchers (Speck, Loewy) seem to speak for this, it is still difficult to assume that, being so developed in most warm-blooded animals, it would be absent in humans. In any case, it is not so developed in humans as, for example, in small animals, which is explained first by the well-developed physical T., as well as artificial regulation through clothing, housing, and finally the possibility of always compensating for deficiencies in T. by taking food, muscle movements, etc. Both vasomotor regulation and dissipation through sweating are very powerful factors of T. in humans. Blagden remained for 20 min. without harm to himself in a thermostat heated by electric lamps at t° 120° and 6% relative humidity. Raw eggs placed here at the same time were hard-boiled during this time. Participation of the nervous system in T. If the hemispheres and corpus striatum are separated from the rest of the central nervous system, with the cut being made in front of the thalami optici, then T. is not disrupted. However, if the cut is made behind the optic thalami, between them and the anterior pair of quadrigemina, then the ability for T. is disrupted, and with respect to external t° the animal behaves as if it were poikilothermic, in which it is impossible to cause an increase in t° either by injection, infection, or any pharmacological means. More precise research showed that the place, the loss of which leads to such serious disruptions, is concentrated in the tuber cinereum, but not along its midline, but to the side of it. Grewing, after careful cytoarchitectonic research, locates it in special cellular conglomerates—nuclei tuberis, located symmetrically on both sides of the midline. Damage to one of them does not noticeably disrupt T., whereas after bilateral damage, sweating and polypnoe cease, and blood pressure falls due to vasodilation, paralysis of the arrectores pilorum muscles occurs, and other disorders. In the tuber cinereum, as most researchers believe, the main center of the autonomic nervous system is concentrated, among the diverse functions of which thermoregulation is perhaps the most important (see Autonomic nervous system). Centripetal pathways to the T. center pass through cerebrospinal fibers into the posterior horns, where most of them after decussation are directed into the anterior lateral columns, or Gowers' bundle, and further into the posterior part of the optic thalami. From here, part of the fibers is directed to the center of the tuber cinereum, part to the cerebral cortex. In addition to impulses traveling through nerve pathways, the T. center can also be irritated humorally. Hormonal influences. It was long known to surgeons that after removal of the thyroid gland body t° decreases; on the other hand, physicians noted that with its hypertrophy patients easily develop fever. According to Boldyrev's experiments, extirpation of the gland along with the epithelial bodies brings warm-blooded animals closer to cold-blooded animals. As is known, the thyroid hormone increases the overall metabolism, from which its role in T. is clear. The increase in body temperature of animals immersed in winter hibernation after injection of thyroid extract is explained by the stimulating effect of the extract on the overall metabolism.- Adrenal glands. In animals with removed adrenal glands, body t° strongly decreases before death. By repeated injections of adrenaline, it was possible to prolong the life of such an animal for months, and after injection t° increased by 2-3°. In patients with Addison's disease, vasomotor T., as well as the overall metabolism, are reduced. The importance of this organ for the life of the animal is closely connected with the question of whether its participation is a necessary condition for normal T.-The pituitary gland directly adjoins the tuber cinereum. Its removal is accompanied by a drop in body temperature. Of the three parts that make up its body, only the anterior glandular lobe or pars intermedia can be discussed, which are in any way connected with T., but this question is still very poorly developed. Regarding other endocrine glands, it must be said that their participation in thermoregulation, if it exists at all, is small.