Heat regulation during exercise with controlled cooling

During heavy sustained exercise, when sweating is usually needed to dissipate the extra metabolic heat, controlled cooling caused heat loss to match total heat production with little sweating. The total heat produced and metabolic rate were varied independently by having subjects walk uphill and down. Heat loss was measured directly with a suit calorimeter; other measurements included metabolic energy from respiratory gas exchange and body temperatures. Thermoregulatory sweating was minimized by adjusting cooling in the calorimeter suit. Heat loss rose to match total heat, not metabolic rate, and there was a slow rise in rectal temperature. In the absence of major thermoregulatory response rectal temperature correlated most closely with total heat; it also correlated with the relative oxygen cost of exercise. Heat flow or heat content appeared to be the controlled variable and body temperature rise a secondary event resulting from thermal transport lag.     

Supplemental ascorbic acid does not affect inferred heat loss in broiler chickens exposed to elevated temperature

The provision of supplemental ascorbic acid has been reported to lower the body temperature of chickens maintained at elevated environmental temperatures. Since body temperature is the net effect of heat production and heat loss, it is not known if the reductions in body temperature were due to a lower heat production or an increase in heat loss. The purpose of this work was to determine if supplemental ascorbic acid facilitates heat loss in chickens exposed to an elevated temperature. On day 12 post-hatch broiler chickens were implanted intra-abdominally with a thermo-sensitive radio transmitter. The following day, birds were placed inside an indirect calorimeter maintained at 34 C for 24 h and provided water containing 0 or 400 ppm ascorbic acid. Oxygen consumption, carbon dioxide production, heat production, respiratory exchange ratio, and body core temperature were measured for 3 h; beginning 21 h after the birds were placed inside the calorimeter. No differences were observed in heat production or body core temperature between birds provided or not and 400 ppm ascorbic acid. This suggests that ascorbic acid has no effect on heat loss. Birds provided ascorbic acid did exhibit a significantly lower respiratory exchange ratio suggesting a greater utilization of lipid for energy production. Although lipid has a lower heat increment compared with protein and carbohydrate, the significance of this finding to birds exposed to elevated temperature is not known. In conclusion, under the conditions of this study the provision of supplemental ascorbic acid to broiler chickens maintained at an elevated temperature did not affect heat loss as inferred from measured heat production and body core temperature.     

Thermal equilibrium of goats

The effects of air temperature and relative humidity on thermal equilibrium of goats in a tropical region was evaluated. Nine non-pregnant Anglo Nubian nanny goats were used in the study. An indirect calorimeter was designed and developed to measure oxygen consumption, carbon dioxide production, methane production and water vapour pressure of the air exhaled from goats. Physiological parameters: rectal temperature, skin temperature, hair-coat temperature, expired air temperature and respiratory rate and volume as well as environmental parameters: air temperature, relative humidity and mean radiant temperature were measured. The results show that respiratory and volume rates and latent heat loss did not change significantly for air temperature between 22 and 26 °C. In this temperature range, metabolic heat was lost mainly by convection and long-wave radiation. For temperature greater than 30 °C, the goats maintained thermal equilibrium mainly by evaporative heat loss. At the higher air temperature, the respiratory and ventilation rates as well as body temperatures were significantly elevated. It can be concluded that for Anglo Nubian goats, the upper limit of air temperature for comfort is around 26 °C when the goats are protected from direct solar radiation.     

Human heat balance during postexercise recovery: separating metabolic and nonthermal effects

Previous studies report greater postexercise heat loss responses during active recovery relative to inactive recovery despite similar core temperatures between conditions. Differences have been ascribed to nonthermal factors influencing heat loss response control since elevations in metabolism during active recovery are assumed to be insufficient to change core temperature and modify heat loss responses. However, from a heat balance perspective, different rates of total heat loss with corresponding rates of metabolism are possible at any core temperature. Seven male volunteers cycled at 75% of Vo(2peak) in the Snellen whole body air calorimeter regulated at 25.0 degrees C, 30% relative humidity (RH), for 15 min followed by 30 min of active (AR) or inactive (IR) recovery. Relative to IR, a greater rate of metabolic heat production (M - W) during AR was paralleled by a greater rate of total heat loss (H(L)) and a greater local sweat rate, despite similar esophageal temperatures between conditions. At end-recovery, rate of body heat storage, that is, [(M - W) - H(L)] approached zero similarly in both conditions, with M - W and H(L) elevated during AR by 91 +/- 26 W and 93 +/- 25 W, respectively. Despite a higher M - W during AR, change in body heat content from calorimetry was similar between conditions due to a slower relative decrease in H(L) during AR, suggesting an influence of nonthermal factors. In conclusion, different levels of heat loss are possible at similar core temperatures during recovery modes of different metabolic rates. Evidence for nonthermal influences upon heat loss responses must therefore be sought after accounting for differences in heat production.     

Afterdrop of body temperature during rewarming: an alternative explanation

Afterdrop, the continued fall of deep body temperatures during rewarming after hypothermia, is thought to endanger the heart by further cooling from cold blood presumed to be returning from the periphery. However, afterdrop is not always observed, depending on the circumstances. To explore this phenomenon, mild hypothermia was induced quantitatively with a suit calorimeter, using several patterns of cooling and rewarming. When cooling was rapid and followed immediately by rewarming, there were typical afterdrops in the temperatures measured in the rectum, auditory canal, and esophagus. However, when rewarming was delayed, or when cooling had been slow and prolonged, afterdrop was not seen. Afterdrops were then observed in two physical models that had no circulation: a bag of gelatin and a leg of beef. Central layers continued to give up heat as long as the surrounding layer was cooler. These results, together with recent findings by others that peripheral blood flow is low until afterdrop is complete, make this circulatory explanation of afterdrop improbable. Alternatively, afterdrop can be explained by the way heat moves through a mass of tissue.     

Dynamics of the heat exchange in rats upon getting out of the state of artificial hypothermia

With the help of thermometry and general calorimetry, changes in the heat exchange were determined in the non-hibernating mammals (rats) upon their getting out of artificial deep hypothermia (the temperature in the rectum was 22-24 degrees C) at the temperature in the calorimeter chamber 22 degrees C. An attempt was made to find out what part of the heal production during animal self-warming is used for increasing its body temperature and what part of the heat production is released to the environment by the animal. The experiments revealed a complex relationship between the body temperature, the heat loss, and the total heat production during the animal self-warming. The result was that the total heat production first increased and, after reaching the maximum, decreased gradually. The experiments showed that during two and more hours of observation the body temperature did not reach the starting level, the same was true for the total heat production, which was the sum of the heat loss and the heat production spent for warming the animal body.     

Warming rates as a function of body size in periodic endotherms

Summary Warm-up rates and cooling constants were measured in several groups of insects over a wide range of thoracic weights. Vertebrate heterotherms show an inverse dependence of warm-up rate on body weight, but in insects warm-up rate increases with increasing size over the range studied (Figures 1-4, 8). Equations are derived, based on known or estimated relations of heat loss and production to body weight, that predict warm-up rates in insects and mammals with reasonable accuracy. Both weight-specific heat production and loss increase with decreasing body size, but heat loss increases more rapidly. At the size range of insects, loss is so rapid that metabolism cannot fully compensate. Then warm-up rate is constant or decreases with diminishing size.     

Thermal insulation and body temperature wearing a thermal swimsuit during water immersion

This study evaluated the effects of a thermal swimsuit on body temperatures, thermoregulatory responses and thermal insulation during 60 min water immersion at rest. Ten healthy male subjects wearing either thermal swimsuits or normal swimsuits were immersed in water (26 degrees C or 29 degrees C). Esophageal temperature, skin temperatures and oxygen consumption were measured during the experiments. Metabolic heat production was calculated from oxygen consumption. Heat loss from skin to the water was calculated from the metabolic heat production and the change in mean body temperature during water immersion. Total insulation and tissue insulation were estimated by dividing the temperature difference between the esophagus and the water or the esophagus and the skin with heat loss from the skin. Esophageal temperature with a thermal swimsuit was higher than that with a normal swimsuit at the end of immersion in both water temperature conditions (p<0.05). Oxygen consumption, metabolic heat production and heat loss from the skin were less with the thermal swimsuit than with a normal swimsuit in both water temperatures (p<0.05). Total insulation with the thermal swimsuit was higher than that with a normal swimsuit due to insulation of the suit at both water temperatures (p<0.05). Tissue insulation was similar in all four conditions, but significantly higher with the thermal swimsuit in both water temperature conditions (p<0.05), perhaps due to of the attenuation of shivering during immersion with a thermal swimsuit. A thermal swimsuit can increase total insulation and reduce heat loss from the skin. Therefore, subjects with thermal swimsuits can maintain higher body temperatures than with a normal swimsuit and reduce shivering thermo-genesis.     

Dynamics of the heat exchange in rats upon getting out of the state of artificial deep hypothermia

With the help of thermometry and general calorimetry the changes in the heat exchange were determined in the rats upon their getting out of artificial deep hypothermia (the temperature in the rectum was 20 degrees C) at the temperature in the calorimeter chamber 20 degrees C. An attempt was made to find out what part of the heat production during an animal self-warming is used for increasing its body temperature and what part of the heat production is released to the environment by the animal. The experiments revealed a complex relationship between the body temperature, the heat loss, and the total heat production during the animal selfwarming. The total heat production first increased and, after reaching the maximum, decreased gradually. Moreover, the experiments showed that during three and more hours of observation the body temperature did not reach the initial level, the same was true for the total heat production, which in these experiments was the sum of the heat loss and the production spent for warming the animal body.     

Thermoregulatory responses to acute heat loads in the FOK rat

FOK is an inbred rat strain with a genotypic adaptation to hot environments. The present study investigated the mechanism of the high heat tolerance of the FOK rat. Male FOK and WKAH rats were used. They were loosely restrained and placed individually in a direct calorimeter with an ambient temperature of 24°C. Their hypothalamic temperature, evaporative and nonevaporative heat loss and heat production were measured. After thermal equilibrium had been attained, the rats were warmed for 30 min with a chronically implanted intraperitoneal electric heater(internal heating). At least 90 min after the heating, the jacket water temperature surrounding the calorimeter chamber was gradually raised from 24°C to 36°C in 80 min (external warming). During the internal heating, changes in the thermoregulatory parameters did not differ between the groups. During the external warming, the evaporative heat loss of the FOK rat was significantly greater than that of the WKAH rat, while changes in nonevaporative heat loss and heat production did not differ between the groups. The results suggest that in the FOK rat, the improved heat tolerance is attributable to an enhanced evaporative heat loss response, but not to a facilitation of nonevaporative heat loss or of metabolic depression. Received: 8 March 1999 / Accepted: 14 July 1999     

Scaling of insulation in seals and whales

We describe scaling of morphological variables that influence total insulation in eight species of marine mammals ranging in average size from 35 to 30000 kg. We also calculate total heat loss and the partitioning of heat loss through the body surface and appendages. For the eight species investigated, heat loss in 0°C water is appreciably higher than the predicted basal metabolic rates for small species such as the ringed seal. Rorquals, on the other hand, will probably not need to raise their metabolic rates to keep warm. At rest, 10–30% of the heat production of a resting animal is lost through flippers, fins and flukes. This amount can increase to 70–80% during moderate exercise. Whole-body conductance scales with body size in the same way in marine as in terrestrial mammals, although conductance is higher for a given body size in a marine mammal.     

Correlation between heat tolerance during exercise and maximum aerobic work capacity

Observation of the physiological responses during exercise in a hot environment and measurement of maximal work capacity were made on eight young male subjects, ages 20--22. Exercise was performed on a bicycle ergometer at a constant work load of 450 kg . m/min at a cycling rate of 50 rpm for 30 min in a climatic chamber at 30 degree C with 70% relative humidity. The maximum work capacity was measured by bicycle ergometer exercise. Heat tolerance during exercise was assessed by the magnitude of physiological strain expressed by the combination of relative rise in rectal temperature, relative water loss and relative salt loss. Heat load during exercise was calculated using metabolic rates at rest and during exercise, assuming heat loss through the respiratory tract to be 10 percent of metabolic rate. Fairly good correlations were found between the ratio of work done to maximum work capacity and rise in rectal temperature, ratio of body weight loss to body weight and heat tolerance during exercise. Close correlations were found among relative heat load during exercise and rise in rectal temperature, relative body weight loss and heat tolerance. Heat tolerance during exercise in a hot environment correlated well to capacity of heat dissipation and maximum work capacity.     

Dynamic analysis of differential scanning calorimetry data

The apparent heat capacity function measured by high-sensitivity differential scanning calorimetry contains dynamic components of two different origins: (1) an intrinsic component arising from the finite instrument time response; and (2) a sample component arising from the kinetics of the thermal transition under study. The intrinsic instrumental component is always present and its effect on the shape of the experimental curve depends on the magnitude of the calorimeter response time. Usually, high-sensitivity instruments exhibit characteristic time constants varying from 10 to 100 s. This slow response introduces distortions in the shape of the heat capacity function especially at fast scanning rates. In addition to this instrumental component, dynamic effects due to sample relaxation processes also contribute to the shape of the experimental heat capacity profile. Since the nature and magnitude of these effects are a function of the kinetic parameters of the transition, they can be used to obtain kinetic information. This communication presents a dynamic deconvolution technique directed to remove artificial distortions in the shape of the heat capacity function measured at any scanning rate, and to obtain a kinetic characterization of a thermally induced transition. The kinetic characterization obtained by this method allows the researcher to obtain transition relaxation times as a continuous function of temperature. This technique has been applied to the thermal unfolding of ribonuclease A and the pretransition of dipalmitoylphosphatidylcholine (DPPC). In both systems the transition relaxation times are temperature dependent. For the protein system the relaxation time is very slow below the transition temperature (approximately 30 s) and very fast above Tm (less than 1 s) in agreement with direct kinetic measurements. For the pretransition of DPPC, the relaxation time is maximal at the transition midpoint and of the order of approx. 40 s.     

Energetics of East African Pycnonotids1

Rate of oxygen consumption was measured in five bulbuls (Family Pycnonotidae) from western Uganda to evaluate whether this group is indeed characterized by the very low basal rates of metabolism previously reported. For three of these species, body temperature and rate of metabolism were measured as a function of ambient temperature from 10°C to 35°C. In these species body temperature was highly variable, and declined with ambient temperature in Andropadus virens. Such variation, in conjunction with behavioral adjustments, may reduce heat loss at low ambient temperatures. Body mass accounted for 98 percent of the variation in the basal rates of metabolism presented here. Basal rates in these species ranged from 81 to 90 percent of values predicted by the Aschoff–Pohl relationship for passerines, whereas previous measurements ranged from 56 to 72 percent of predicted values. This difference may reflect differences in species or measurement techniques, which, if the latter, suggests that the reduction in metabolic rate in this family may be less than originally thought. These data underline the importance of continued data collection on the metabolism of tropical birds, few of which have been measured to date.     

A comparison of a mechanical sector and a linear array transducer for ultrasound-guided transvaginal oocyte retrieval (OPU) in the cow

A comparison was made between a linear array and a mechanical multiple angle sector (MAP) transducer for ultrasound-guided transvaginal oocyte retrieval (ovum pick-up, OPU) in the cow. The ovaries of five dairy cows were punctured, in a twice-weekly OPU program lasting for 4 weeks, using two different 5.0-MHz transducers equipped with an identical disposable needle-guidance system. Both ovaries were visualized using each transducer before puncture and the number of follicles with a diameter of less than 5 mm (small) and with a diameter equal to or greater than 5 mm (large) was recorded. Subsequently, one ovary of the pair was punctured guided by the MAP, while the other was punctured using the linear array transducer. During the next puncture session on a given animal, the two systems were switched and used on the alternate ovary in a crossover design. Parameters assessed for each system were: the total number of follicles visualized in each diameter class, and the total number of retrieved oocytes per cow. A significant difference was found for the ability to visualize smaller follicles in favor of the MAP transducer, with an average visualization of 71.6 +/- 30.3 small follicles per cow during the 4-week trial period, compared to 59.8 +/- 25.7 for the linear array transducer (t-test for paired samples, P = 0.007). No differences were found in the visualization of large follicles. A numerically greater number of oocytes were retrieved using the MAP transducer, compared to the linear array, (averages of 14.2 +/- 7.2 versus 7.4 +/- 6.1, respectively), although these differences were not statistically significant. In conclusion, both systems can be effectively used for oocyte retrieval in the cow, however, the MAP transducer demonstrated superior visualization of small follicles.     

Age-specific decrease in aerobic efficiency associated with increase in oxygen free radical production in Drosophila melanogaster

This study was designed to test the free radical theory of aging by using Drosophila melanogaster as a model system. Oxygen free radicals are generated by mitochondria during the process of normal oxidative metabolism. Age-specific measurements of oxygen consumption, heat production and anti-oxidant enzyme activity were obtained from two inbred lines of male flies, one selected for longevity and one normal-lived. The findings of this study demonstrate that although oxygen consumption remains relatively constant over the majority of the life span of each line of flies, aerobic efficiency declines with advancing age. This loss of aerobic efficiency manifests itself as a decline in total body metabolism as measured by heat production, and appears to be associated with an age-specific increase in damage inflicted upon mitochondria by oxygen free radicals.     

Fast titration experiments using heat conduction microcalorimeters.

Thermopile heat conduction calorimeters normally have high time constants. Multistep titration experiments involving fast processes may then require several hours to perform. It is demonstrated that such experiments can be conducted about 10 times faster, without loss of accuracy, by use of a "dynamic correction method". For a new small vessel thermopile conduction calorimeter, injections can be made at 1.5-min intervals.     

Comparison of anti-G trousers equipped with bladder and tightening system

Two different anti-G flight suits are compared. The first suit uses a bladder system to compress the lower limbs and abdomen and the second uses a tightening system to produce compression evenly for the lower body. Studies were performed using a centrifuge cabin with exposure to acceleration forces up to 2.5 G. Suits were compared for protection efficiency, comfort, complaints, and put on ease. Results of the study, including heart rate dynamics, ear vascular pulsation amplitude, and peripheral vision loss are presented.     

Heat transfer in a microvascular network: the effect of heart rate on heating and cooling in reptiles (Pogona barbata and Varanus varius)

Thermally-induced changes in heart rate and blood flow in reptiles are believed to be of selective advantage by allowing animal to exert some control over rates of heating and cooling. This notion has become one of the principal paradigms in reptilian thermal physiology. However, the functional significance of changes in heart rate is unclear, because the effect of heart rate and blood flow on total animal heat transfer is not known. I used heat transfer theory to determine the importance of heat transfer by blood flow relative to conduction. I validated theoretical predictions by comparing them with field data from two species of lizard, bearded dragons (Pogona barbata) and lace monitors (Varanus varius). Heart rates measured in free-ranging lizards in the field were significantly higher during heating than during cooling, and heart rates decreased with body mass. Convective heat transfer by blood flow increased with heart rate. Rates of heat transfer by both blood flow and conduction decreased with mass, but the mass scaling exponents were different. Hence, rate of conductive heat transfer decreased more rapidly with increasing mass than did heat transfer by blood flow, so that the relative importance of blood flow in total animal heat transfer increased with mass. The functional significance of changes in heart rate and, hence, rates of heat transfer, in response to heating and cooling in lizards was quantified. For example, by increasing heart rate when entering a heating environment in the morning, and decreasing heart rate when the environment cools in the evening a Pogona can spend up to 44 min longer per day with body temperature within its preferred range. It was concluded that changes in heart rate in response to heating and cooling confer a selective advantage at least on reptiles of mass similar to that of the study animals (0. 21-5.6 kg).     

Thermal inactivation of Listeria monocytogenes studied by differential scanning calorimetry

The effect of NaCl on the thermal inactivation of Listeria monocytogenes has been investigated by conventional microbiological techniques and by using differential scanning calorimetry (DSC). Addition of 1.5 M-NaCl to cells grown at lower NaCl concentrations significantly increases the tolerance of cells to mild heat stress (56-62 degrees C). DSC thermograms show five main peaks which are shifted to higher temperatures in the presence of 1.5 M-NaCl. Measurement of loss of viability in the calorimeter gave good correlation between cell death and the first major thermogram peak at two NaCl concentrations. The time course of the loss of this first peak when cells were heated and held at 60 degrees C in the calorimeter matched the loss of viability, whereas the peak attributable to DNA showed little change during this process. The use of DSC to investigate the mechanisms involved in thermal inactivation is discussed.