Blunted cutaneous vasoconstriction and increased frequency of presyncope during an orthostatic challenge under moderate heat stress in the morning

Purpose

In normothermia, the tolerance time to presyncope during an orthostatic challenge is shortened during the early morning. Heat stress reduces tolerance to presyncope and the degree of cutaneous vasoconstriction prior to presyncope. However, whether these changes show diurnal variations remains unknown. Therefore, we examined diurnal changes in orthostatic tolerance and cutaneous vascular conductance (CVC) during an orthostatic challenge under moderate heat stress.

Methods

Each lower body negative pressure (LBNP) under normothermia and whole body heat stress was applied for 7 min or until the appearance of presyncopal symptoms in 16 males at both 08:00 (a.m.) and 17:00 hours (p.m.). Measurements included internal and skin temperatures, forearm skin blood flow, arterial pressure, and heart rate. CVC was calculated as skin blood flow/mean arterial pressure, normalized to CVC prior to LBNP and expressed as %CVC.

Results

The average tolerance time in eight subjects exhibiting presyncopal symptoms due to LBNP and moderate heat stress was significantly shorter in the a.m. than in the p.m. (3.7 ± 0.8 versus 6.7 ± 0.3 min, respectively; P = 0.005). Neither %CVC during LBNP in these subjects under moderate heat stress nor normothermia were significantly decreased in the a.m. (P > 0.05, respectively).

Conclusions

These findings indicate an orthostatic challenge even during moderate heat stress that led to an increase in the frequency of presyncope, especially in the morning. The reduction in tolerance was accompanied by blunted cutaneous vasoconstriction prior to presyncope. Hence, diurnal changes in cutaneous vascular responses during combined orthostatic and heat stresses should contribute, at least partly, to heat-induced orthostatic intolerance in the morning.     

Changes in thermal homeostasis in humans due to repeated cold water immersions

The purpose of this study was to monitor changes in body and skin temperatures, heat production, subjective shivering, cold sensation and body fat content in humans after intermittent cold water immersion. Repeated exposures of young sportsmen to cold water (head out, 14 °C, 1 h, 3 times per week for 4–6 weeks) induced changes in regulation of thermal homeostasis. ‘‘Cold acclimated’’ subjects exhibited an hypothermic type of adaptation. Central and peripheral body temperatures at rest and during cold immersion were lowered. The metabolic response to cold was delayed and subjective shivering was attenuated. The observed hypothermia was due to the shift of the threshold for induction of cold thermogenesis to lower body temperatures. ‘‘Cold acclimated’’ subjects also showed a lowered cold sensation. Because of the observed physiological changes, about 20% of the total heat production was saved during one cold water immersion of ‘‘cold acclimated’’ subjects. Maximal aerobic and anaerobic performances were not altered. No change in the thermosensitivity of the body temperature controller, as assessed from the unchanged slope of the relation between the deep body temperature and total heat production, was observed. Changes in cold sensation and regulation of cold thermogenesis were noticed first after four cold water immersions and persisted for at least 2 weeks after termination of the adaptation procedure. A trend towards a small increase in the body fat content was also observed. This findll as the increased vasoconstriction, evidenced by the lowered skin temperature, indicate that slight changes in body insulation may also occur after ‘‘cold acclimation’’ in humans. Received: 30 December 1995/Received after revision and accepted: 25 March 1996     

Effects of voluntary and forced exercise on thermoregulation and survival in aged C57BL/6J mice

To evaluate the effect of exercise on thermoregulation in senescent animals, three groups of male C57BL/6J mice aged 28-30 months were tested for cold tolerance, defined as the rate of decline in colonic temperature during 3 h exposure to 10 degrees C ambient temperature. Following this test, the mice were exposed to one of the following exercise conditions: forced exercise on a treadmill for 60 min daily at a rate of 5 m/min; continuous access to voluntary exercise in wheel-activity cages, which resulted in a mean rate of 1.1 m/min; or no expressed exercise with 60 min daily placement on the non-activated treadmill. After 3 weeks, assessment of cold tolerance was repeated. A combined mortality rate of 36% was observed in the exercise groups for this period, while there were no deaths in the non-exercised group. The high mortality rate among exercised animals indicated that these regimens were hazardous for aged mice. Moreover, between tests the non-exercised group exhibited a 0.6 degree C increase in body temperature and 38% improvement in cold tolerance which could be interpreted as a normal adaptation for repeated cold exposure. In contrast, no significant change in either of these variables was observed among survivors in the exercise groups. Thus, introduction of these exercise regimens in senescent mice decreased survival and did not improve the age-related impairment in thermoregulation.     

Prediction of the thermoregulatory response for clothed immersion in cold water

Summary A multi-compartmental thermoregulatory model was applied to data of ten resting clothed males immersed for 3 h in water at 10 and 15°C. Clothing consisted of a dry suit and either a light or heavy undergarment, representing a total insulation of 0.15 (0.95) or 0.20 m²°CW⁻¹ (1.28 clo), respectively. Data were grouped according to low (<14%) and high (14 to 24%) body fat individuals. Mean decreases in rectal temperature ranged from 0.79 to 1.38°C, mean decreases in the mean weighted skin temperature ranged from 6.3 to 10.2°C, and mean increases in the metabolic rate ranged from 33.9 to 80.8 W. The model consists of eight segments, each representing a specific region of the body. Each segment is comprised of compartments representing the core, muscle, fat, skin, and clothing. Each compartment is assigned thermophysical values of heat conduction and heat capacitance, and with the exception of clothing, physiological values of blood flow and metabolic heat production. During cold exposure, responses are directed towards increased heat production in the form of shivering and heat conservation in the form of vasoconstriction and convective heat exchange at the vascular level. Agreement between the model predictions and the experimental observations was obtained by adjusting the parameters governing these responses. These adjusted parameters were 1) the onset of limb shivering with an exponential half-time of 30 min, 2) the fractional value of 0.5 for the convective heat exchange between the core compartments of the limbs and the blood flowing through these compartments, 3) the fractional contribution of trunk shivering to overall shivering, which ranged from 0.77 to 0.95, and 4) the onset of vasoconstriction with exponential half-times that ranged from 3 to 25 min. Steady state was predicted to occur within 4 h and a heat balance analysis indicated that the limbs were responsible for most of the body's heat loss while acquiring most of their own heat from the trunk through convective heat exchange with the central blood.     

Metabolic and thermal responses of men wearing cold-protective clothing to various degrees of cold stress

Summary Five subjects were exposed in a climatic chamber for 1 h to air temperatures of 0, –10, and –15 C wearing cold-protective clothing. Heat production, and mean skin and rectal temperatures were studied.During the first 5 min of cold exposure, heat production attained high values and then decreased. The peak levels of this initial metabolic rise were higher in lower air temperatures, while the corresponding mean skin temperatures showed no significant differences in any air temperature. The relationship between changes of heat production and mean skin temperature values during the first 5 min differs from those later in the experiments, which showed a good linear relationship (r=0.89).As the experimental condition required that the body was covered with thick clothes, immediate stimulation to part of the face and the respiratory tract must be greater than stimulation of whole body skin. It is suggested that the metabolic rise during the first 5 min is related to abrupt cold stimulation to the face and mucous membrane of the respiratory tract, and to the subsequent appearence of thermal muscular tone or tension. In contrast, mean skin temperature became lower during the later period even with the cold-protective clothing, and heat production increased again at the onset of frank shivering. It is suggested that the heat production changes occurring during the later period showed that the stimulus to the shivering center from cold receptors in the skin is powerful enough to produce an increase in metabolism.     

Thermoregulatory responses to cold during a 7-week acclimatisation process in rabbits

A group of six rabbits reared at +20°C ambient temperature was adapted to moderate cold by housing for seven weeks at +10°C. Rectal and skin temperatures, metabolic heat production and respiratory evaporative heat loss were recorded continuously over 1 h for each animal on 3 days per week in the climatic chamber.There was no significant change either of rectal or of ear skin temperature during the acclimatisation process. On the other hand, metabolic heat production was progressively reduced (20% in the 7th week). Slight changes of mean skin temperature and respiratory evaporative heat loss could not account for compensation. Therefore it must be concluded that both adaptive improvement of peripheral insulation and reduction of heat production were achieved during the acclimatisation process. Both processes together ensure that deviations of core temperature are minimal. The possible origin of the functional adaptive effects is discussed. The results are in full agreement both with former neurophysiological results and with system-theoretical considerations of adaptive processes.Dedicated to the 60th birthday of Professor Dr. Kurt Brück     

A case study on the induction of clinical tolerance in cold urticaria

The mechanism(s) by which repeated cold challenge in a patient with idiopathic acquired cold urticaria resulted in the induction of clinical tolerance to cold stimuli was studied. Plasma histamine levels, mast cell ultrastructure, and the cutaneous response to intradermal injections of morphine, histamine, and substance P were examined before and after the induction of tolerance. Plasma histamine levels draining cold-challenged, clinically tolerant skin were markedly diminished compared to histamine levels obtained during cold-induced angioedema. Furthermore, electronmicroscopy of skin samples taken from tolerant skin after cold challenge revealed intact, largely normal appearing mast cells. Intradermal injection of mast cell secretagogues and vasoactive agonists into normal and tolerant skin sites resulted in similar whealing responses. Thus, these studies suggest that the state of clinical tolerance to cold stimuli is due neither to mast cell-mediator depletion or tachyphylaxis of the cutaneous vasculature to vasoactive agonists. It appears likely that tolerance may be due to the induction of a specific state of unresponsiveness of mast cells to cold stimuli or possibly to depletion of a cold-induced cutaneous antigen capable of triggering mast cell degranulation.     

Human thermoregulation and the cardiovascular system

A key but little understood function of the cardiovascular system is to exchange heat between the internal body tissues, organs and the skin to maintain internal temperature within a narrow range in a variety of conditions that produce vast changes in external (exogenous) and/or internal (endogenous) thermal loads. Heat transfer via the flowing blood (i.e. vascular convective heat transfer) is the most important heat-exchange pathway inside the body. This pathway is particularly important when metabolic heat production increases many-fold during exercise. During exercise typical of many recreational and Olympic events, heat is transferred from the heat-producing contracting muscles to the skin surrounding the exercising limbs and to the normally less mobile body trunk and head via the circulating blood. Strikingly, a significant amount of heat produced by the contracting muscles is liberated from the skin of the exercising limbs. The local and central mechanisms regulating tissue temperature in the exercising limbs, body trunk and head are essential to avoid the deleterious consequences on human performance of either hyperthermia or hypothermia. This brief review focuses on recent literature addressing the following topics: (i) the dynamics of heat production in contracting skeletal muscle; (ii) the influence of exercise and environmental heat and cold stress on limb and systemic haemodynamics; and (iii) the impact of changes in muscle blood flow on heat exchange in human limbs. The paper highlights the need to investigate the responses and mechanisms of vascular convective heat exchange in exercising limbs to advance our understanding of local tissue temperature regulation during exercise and environmental stress.     

Change in sympathetic activity,cardiovascular functions and plasma hormone concentrations due to cold water immersion in men

The purpose of this study was to determine whether or not repeated short-term cold water immersions can induce a change in the activity of the sympathetic nervous system and, consequently, in cardiovascular functions in healthy young athletes. Changes in some plasma hormone concentrations were also followed. A single cold water immersion (head-out, at 14°C, for 1 h) increased sympathetic nervous system activity, as evidenced by a four-fold increase (P < 0.05) in plasma noradrenaline concentration. Plasma adrenaline and dopamine concentrations were not increased significantly. Plasma renin-angiotensin activity was reduced by half (P < 0.05) during immersion but plasma aldosterone concentration was unchanged. Stimulation of the sympathetic nervous system during immersion did not induce significant changes in heart rate, but induced peripheral vasoconstriction (as judged from a decrease in skin temperature) and a small increase (by 10%) in systolic and diastolic blood pressures. No clear change in reactivity of the sympathetic nervous system was observed due to repeated cold water immersions (three times a week, for 6 weeks). Neither the plasma renin-angiotensin activity, aldosterone concentration nor cardiovascular parameters were significantly influenced by repeated cold water immersions. A lowered diastolic pressure and an increase in peripheral vasoconstriction were observed after cold acclimation, however. Evidently, the repeated cold stimuli were not sufficient to induce significant adaptational changes in sympathetic activity and hormone production.     

Thermal responses to cold wind of thermoneutral and cooled subjects

The effects of initial thermal state on thermoregulatory responses to cold (−10°C) in a 0.2 (still air), 1.0, and 5.0 m · s⁻¹ wind speed were studied. Eight young male subjects were first preconditioned in thermoneutral (+20°C, TN) or cool (−5°C, CO) environment for 60 min. After preconditioning the subjects were exposed to wind at −10°C in a standing position, facing the wind, for 30 min. Precooling decreased mean skin temperature (Tˉ _sk) by 4.0 (SEM 0.1)°C (P < 0.001) and increased heat flux by 57 (SEM 2) W · m⁻² (P < 0.001) in comparison to TN. Cooling rate of Tˉ _sk was faster (P < 0.001) in TN than in CO at every wind speed. Even so, Tˉ _sk ended up at a lower level in CO (P < 0.001–0.01) than in TN at every wind speed. Local skin temperatures of hand, finger, foot and toe were significantly lower in CO than in TN at the end of all exposures to wind. Heat flux from the skin was 8% higher (NS) in TN at 5.0 m · s⁻¹ wind speed in comparison to CO. A 5.0 m · s⁻¹ wind speed increased oxygen consumption significantly (P < 0.001) in both CO and TN in comparison to still air. At 5.0 m · s⁻¹ wind speed the general thermal sensation was the same (cold) in both TN and CO, despite the higher Tˉ _sk in TN. In conclusion, Tˉ _sk decreased more rapidly in TN, probably due to rapid skin vasoconstriction and redistribution of circulation to the central body. Probably for the same reason, dry heat loss from the skin was at nearly the same level in both TN and CO. Although the initial thermal state did not affect the amount of heat loss, it significantly affected the peripheral temperatures and thermal sensations and should therefore be taken into consideration in the prediction of thermophysiological responses to wind. Accepted: 23 September 1999     

Thermoregulation in winter swimmers and physiological significance of human catecholamine thermogenesis

Thermoregulation in control subjects and cold-adapted winter swimmers was examined during 1 h of cold water immersion (13 C). It was found that the thermoregulatory functions of winter swimmers differ from those of non-cold-adapted subjects. As evident from the relationship between rectal temperature and the magnitude of cold thermogenesis, in controls a significant part of cold thermogenesis during the early phase of cooling was induced by changes in peripheral temperature input, while in the late phase of cooling it was the central temperature input which was mainly engaged in induction of cold thermogenesis. In winter swimmers the magnitude of cold thermogenesis was solely related to changes in rectal temperature, indicating the predominance of the central temperature input in activation of heat production mechanisms. The thermoregulatory threshold for induction of cold thermogenesis was lowered (by 0.34 C), but the apparent hypothalamic thermosensitivity was the same as in non-cold-adapted subjects. These differences are indicative of adaptation of thermoregulatory control centres. Additionally, the activity of thermoregulatory effectors was also changed. Shivering was induced later during cooling (after 40 min) in winter swimmers than in controls, which suggests an important participation of non-shivering thermogenesis in the early thermogenic response. Winter swimmers also showed bradycardia and a greater reduction in plasma volume during cooling. The data indirectly indicate restriction of heat loss from the body. Only a non-significant increase in quantity of subcutaneous fat was observed in winter swimmers. Thus, winter swimmers were able to survive a significantly greater temperature gradient between body and environment than non-cold-adapted subjects by modifying the sensory functions of hypothalamic thermoregulatory centres to lower heat loss and produce less heat during cold exposure. Additionally, the capacity of the total cold thermogenesis due to potentiation of non-shivering heat production was also increased. Heat produced due to thermogenic action of adrenaline may represent more than a quarter of the total cold thermogenesis. In conclusion, the data suggest that winter swimmers exhibit metabolic, hypothermic and insulative types of cold adaptation.     

Physiological responses and manual performance in humans following repeated exposure to severe cold at night

We evaluated human physiological responses and the performance of manual tasks during exposure to severe cold (–25°C) at night (0300–0500 hours) and in the afternoon (1500–1700 hours). Thirteen male students wearing standard cold protective clothing occupied a severely cold room (–25°C) for 20 min, and were then transferred to a cool room (10°C) for 20 min. This pattern of exposure was repeated three times, for a total time of exposure to extreme cold of 60 min. The experiments were started either at 1500 hours or 0300 hours and measurements of rectal temperature, skin temperature, blood pressure, performance in a counting task, hand tremor, and subjective responses were made in each condition. At the end of the experiment at night the mean decrease in rectal temperature [0.68 (SEM 0.04)°C] was significantly greater than that at the end of the experiment in the afternoon [0.55 (SEM 0.08)°C, P<0.01]. After the second cold exposure at night the mean increase in diastolic blood pressure [90 (SEM 2.0) mmHg] was significantly greater than that at the end of the second cold exposure in the afternoon [82 (SEM 2.8) mmHg, P<0.01]. At the end of the second cold exposure at night, mean finger skin temperature [11.8 (SEM 0.8)°C] was significantly higher than that at the comparable time in the afternoon [9.0 (SEM 0.7)°C, P<0.01]. Similarly for the toe, mean skin temperature at the start of the second cold exposure at night [25.6 (SEM 1.5)°C] was significantly higher than in the afternoon [20.1 (SEM 0.8)°C, P<0.01]. The increased skin temperatures in the periphery resulted in increased heat loss. Since peripheral skin temperatures were highest at night, the subjects noted diminished sensations of thermal cold and pain at that time. Manual dexterity at the end of the first cold exposure at night [mean 83.7 (SEM 3.6) times·min^–1] had decreased significantly more than at the end of the first cold exposure in the afternoon [mean 89.4 (SEM 3.5) times·min^–1, P<0.01]. These findings of a lowered rectal temperature and diminished manual dexterity suggest that there is an increased risk of both hypothermia and accidents for those who work at night. Electronic Publication     

Mathematical Modeling of the Human Body During Water Replacement and Dehydration: Body Water Changes

A model of the human body that integrates the variables involved in temperature regulation and blood gas transport within the cardiovascular and respiratory systems is presented here. It expands upon previous work to describe the competition between skin and muscles when both require increased blood flows during exercise and/or heat stress. First, a detailed study of the control relations used to predict skin blood flow was undertaken. Four other control relations employed in the model were also examined and modified as indicated by empirical results found in literature. Internal responses to exercise and/or heat stress can affect both thermoregulation and the cardiorespiratory system. Dehydration was studied in addition to complete water replacement during similar environmental and exercise situations. Control relations for skin blood flow and evaporative heat loss were modified and a water balance was added to study how the loss of water through sweat can be limiting. Runoff from sweating as a function of relative humidity was introduced along with evaporation, and these results were compared to data to validate the model. © 2000 Biomedical Engineering Society. PAC00: 8719Pp, 8719Uv, 8719Ff, 8710+e     

The mechanism of body temperature changes induced by intraventricular injections of adrenaline, noradrenaline and 5-hydroxytryptamine in the ox (bos taurus)

1. Adrenaline, noradrenaline and 5-hydroxytryptamine (5-HT) were injected into the lateral ventricle of the ox. The effect of these drugs was measured on the respiratory rate, tidal volume, heat production, skin temperature of the ear, evaporative loss from the skin and the rectal temperature at 20 and 10 degrees C ambient temperature.2. Neither adrenaline (3 mg) nor noradrenaline (3 mg) had any effect on the temperature regulating mechanisms of the ox, except to produce vasoconstriction if vasodilatation was already present due to high ambient temperature or previous injection of 5-HT.3. Injection of 5-HT (5 mg) caused a rise in respiratory rate, a fall in tidal volume and heat production, elevation of ear skin temperature and skin evaporative loss and a decrease in rectal temperature. Sedation of the animals occurred.4. In its reaction to these monoamines the ox is similar to the goat, sheep and rabbit, but is unlike the cat and dog.5. It was concluded that neither adrenaline nor noradrenaline has a role in the central control of temperature regulation in the ox, but that 5-HT may be involved in the control of heat dissipation mechanisms.     

An experimental validation of mathematical simulation of human thermoregulation

An experimental validation of Stolwijk's mathematical model of thermoregulation is presented. Although the model seems to be accepted widely, very little experimental data for validation exists in the open literature. Experimental data for transient conditions of rectal, head skin, trunk skin, arm skin, leg skin, mean skin and mean body temperature as well as cardiac output and evaporative heat loss under heat stress are presented and compared with simulation output for the model. In general, the predictions of the model are good; the difference between experimental data and the model averaged 0.2°C for mean body temperature.A version of Stolwijk's thermoregulatory model is described briefly. The controller equations are given as well as a short discussion of the rationale for each. Tables give coefficients for the controller equations, and, for the 25 compartments, heat capacitance, thermal conductance, basal metabolic heat production, basal evaporative heat loss, and basal effective blood flow.     

Simulation of human thermoregulation during water immersion: Application to an aircraft cabin water-spray-system

A model was developed of transient changes in metabolic heat production and core temperature for humans subjected to cold conditions. It was modified to predict thermal effects of the upper parts of the body being sprayed with water from a system designed to reduce the smoke effects of an airplane fire. Temperature changes were computed at 25 body segments in response to water immersion, cold-air exposure, and windy conditions. Inputs to the temperature controller were:(a) temperature change signals from skin segments and (b) an integrated signal of the product of skin and head-core (hypothalamic) temperature changes. The controller stimulated changes in blood flow to skin and muscle and heat production by shivering. Two controller parameters were adjusted to obtain good predictions of temperature and heat-production experimental data in head-out, water-immersion immersion (0°–28°C) studies in humans. A water layer on the skin whose thickness decreased transiently due to evaporation was added to describe the effects of the water-spray system. Because the layer evaporated rapibly in a very cold and windy environment, its additional cooling effect over a 60-min exposure period was minimal. The largest additional decrease in rectal temperature due to the water-layer was <1°C, which was in normal conditions where total decreases were small.     

Plasma levels of endothelin-1 and atrial natriuretic peptide in men during a 2-hour stay in a cold room.

Male volunteers were exposed to +10 degrees C ambient temperature for 2 hours while they were sitting undressed. The levels of endothelin-1 and atrial natriuretic peptide were determined by radioimmunoassays. Control samples were obtained at thermoneutrality. The cold exposure resulted in lowering of the mean skin temperature (from 31.2 +/- 0.3 degrees C-22.6 +/- 2.5 degrees C, mean +/- SEM), which indicates that a marked vasoconstriction took place, as well as a decrease of the body heat content (by 11.2 +/- 0.7 kJ kg-1). However, plasma endothelin-1 levels did not change significantly during the exposure. Thus circulating endothelin-1 does not seem to be responsible for the vasoconstriction associated with cold air exposure. The plasma atrial natriuretic peptide levels exhibited a slight increase towards the end of the cold exposure. This finding is in accord with the notion that atrial natriuretic peptide might contribute to the diuresis frequently observed in the cold.     

Hypothermic general cold adaptation induced by local cold acclimation

To study relationships between local cold adaptation of the lower limbs and general cold adaptation, eight subjects were submitted both to a cold foot test (CFT, 5°C water immersion, 5 min) and to a whole-body standard cold air test (SCAT, 1°C, 2 h, nude at rest) before and after a local cold acclimation (LCA) of the lower limbs effected by repeated cold water immersions. The LCA induced a local cold adaptation confirmed by higher skin temperatures of the lower limbs during CFT and a hypothermic insulative general cold adaptation (decreased rectal temperature and mean skin temperatureP < 0.05) without a change either in metabolic heat production or in lower limb skin temperatures during SCAT after LCA. It was concluded that local cold adaptation was related to the habituation process confirmed by decreased plasma concentrations of noradrenaline (NA) during LCA (P < 0.05). However, the hypothermic insulative general cold adaptation was unrelated either to local cold adaptation or to the habituation process, because an increased NA during SCAT after LCA (P < 0.05) was observed but was rather related to a “T₃ polar syndrome” occurring during LCA.     

Effects of repeated exercise/rest sessions at –10°C on skin and rectal temperatures in men wearing chemical protective clothing

The development of thermophysiological responses during four consecutive exercise/rest sessions in the cold was studied in men wearing chemical protective clothing and a face mask. Six men repeated four exercise/rest sessions during 8?h at –10°C. Each session consisted of step exercise (240 W?·?m^?2) for 60?min and rest for another 60?min. Rectal and skin temperatures were measured continuously and thermal sensations were obtained at 30-min intervals. Entering the cold from a warm environment and the onset of exercise resulted in a decrease in skin temperatures during the first session and the decrement in the temperatures of the extremities continued for 10–20?min during the following period of exercise. Torso skin temperature was at its lowest during the first rest period. After the first session of cold exposure the range and the level of variation in mean body temperature (T¯ _b) followed a pattern which was repeated until the end of the experiment. However, the torso skin temperatures increased gradually until the fourth session, while the temperatures of the extremities, in contrast, tended to decrease up to the third session. In conclusion, the present results indicated that although T¯ _b, reflecting the whole body heat balance, showed a typical pattern of change after the first session (2?h), the torso area was warming until the end of the cold exposure while the extremities continued to cool down up to the third session (6?h), obviously due to a prolonged redistribution of the circulation.     

Gravity and thermoregulation: metabolic changes and circadian rhythms

Gravity appears to alter thermoregulation through changes in both the regulated level of body temperature and the rhythmic organization of temperature regulation. Gravity has been hypothesized to have an associated metabolic cost. Increased resting energy expenditure and dietary intake have been observed in animals during centrifuge experiments at hypergravity. Thus far, only animals have shown a corresponding reduction in metabolism in microgravity. Altered heat loss has been proposed as a response to altered gravitational environments, but remains documented only as changes in skin temperature. Changes in circadian timing, including the body temperature rhythm, have been shown in both hypergravity and microgravity, and probably contribute to alterations in sleep and performance. Changes in body temperature regulation may result from circadian disturbance, from the direct or indirect actions of gravity on the regulated temperature, or from changes in thermoregulatory effectors (heat production and heat loss) due to altered gravitational load and convective changes. To date, however, we have little data on the underlying thermoregulatory changes in altered gravity, and thus the precise mechanisms by which gravity alters temperature regulation remain largely unknown.