Evaporative Heat Loss Research Articles

Ultradian oscillations in brain temperature in sheep: implications for thermoregulatory control?

We compared body temperature patterns and selective brain cooling (SBC) in eight adult female sheep in an indoor (22-25°C) and outdoor (mean ~ 21°C) environment, by measuring brain, carotid arterial, and jugular venous blood temperatures at 5-min intervals using implanted data loggers. To investigate whether ultradian oscillations in brain temperature had thermoregulatory consequences for the sheep, we determined the cranial arterio-venous (AV) temperature difference as an indicator of respiratory evaporative heat loss (REHL). The 24-h pattern of SBC was similar in both environments, despite carotid blood temperature fluctuating 0.4°C more outdoors compared to indoors. The sheep employed SBC more often during the night than during the day, but SBC was abolished at intervals of 1-3h throughout the 24-h period. The suppression of SBC appeared to be associated with events that increased sympathetic nervous system activity, including shifts between stages of sleep. Short-term changes (over 5-min) in brain temperature were positively correlated with changes in the AV temperature difference 5min later, and negatively correlated with changes in carotid temperature 10min later. These data support the idea that increases in brain temperature modulate thermoregulation by increasing REHL, which leads to a decrease in carotid blood temperature. Ultradian oscillations in core temperature of sheep, therefore, appear to arise as a consequence of frequent brain temperature changes invoked by non-thermal inputs, in animals housed both in indoor and outdoor environments.

Solar Radiation Exposure Has Diurnal Effects on Thermoregulatory Responses During High-Intensity Exercise in the Heat Outdoors.

Otani, H, Goto, T, Goto, H, Hosokawa, Y, and Shirato, M. Solar radiation exposure has diurnal effects on thermoregulatory responses during high-intensity exercise in the heat outdoors. J Strength Cond Res 33(10): 2608-2615, 2019-This study investigated the diurnal effects of variations in solar radiation associated with changing solar elevation angle on thermoregulatory responses during high-intensity exercise in the heat outdoors. Ten male high school soccer players completed two 2-hour soccer training sessions under a clear sky in the heat of summer. These sessions were commenced at 0900 hours (AM) and 1600 hours (PM) on separate days. Solar radiation and elevation angle were higher in AM (820-1,070 W·m and 45-69°) than PM (620-110 W·m and 34-10°: both p < 0.001). Neither ambient temperature (AM 29-32° C; PM 31-31° C) nor wet-bulb globe temperature was different between trials. Although mean skin temperature was not different between trials, infrared tympanic temperature was higher at the end of exercise in AM than PM (p < 0.001). Heart rate (p < 0.01) and body heat gain from the sun (p < 0.001) were greater during exercise in AM than PM. Dry heat loss was smaller, but evaporative heat loss was greater in AM than PM (both p < 0.001). Thermal sensation and rating of perceived exertion were similar between trials, but GPS measurements showed a less total distance and distance covered by walking, jogging, and running in AM than PM (p < 0.01). This study demonstrates a greater thermoregulatory strain in AM than PM during 2-hour high-intensity soccer training in the heat under a clear sky. This observation is accompanied by a progressive increase in environmental heat stress with rising solar radiation and elevation angle in AM and a greater body heat gain from the sun in AM compared with PM.

Spatiotemporal sweat evaporation and evaporative cooling in thermal environments determined from wearable sensors

This study aimed to provide a mobile experimental method for assessing spatiotemporal skin evaporative losses that influence human thermal states. The proposed method can be used to predict the total evaporation from skin and corresponding evaporative heat loss by combining typical climatic observations and data from wearable sensors placed on the subject’s skin over different anatomical regions. Wearable sensors can obtain humidity data, calculate local evaporation and local evaporative heat losses based on the theory of mass transfer, and then reflect the differences among individual physical characteristics. To validate the method, participants engaged in an experiment consisting of resting and a physical activity under three humidity levels. The results obtained from the proposed method are consistent with those from conventional whole-body weight measurements, thus demonstrating the effectiveness of the method to determine sweat evaporation. Furthermore, we evaluated the distribution of evaporation over the body during exercise and confirmed a reasonable agreement between the subjects’ perception and the obtained thermal states. Overall, these results offer insights on the effect of thermal environments in practical situations entailing human activity. Likewise, we outline the application of the proposed method for predicting evaporation levels to assess human thermal environments.

Exercise Core Temperature Response with a Simulated Burn Injury: Effect of Body Size.

Using a simulated burn injury model to mimic the detrimental effect of a 40% TBSA injury on whole-body evaporative heat dissipation, core temperature response to exercise in physiologically uncompensable conditions between morphologically disparate groups were examined at (i) an absolute Hprod (W), and (ii) a mass-specific Hprod (W·kg). Healthy, young, nonburned individuals of small (SM, n = 11) or large (LG, n = 11) body size cycled for 60 min at 500 W or 5.3 W·kg of Hprod in 39°C and 20% relative humidity conditions. A 40% burn injury was simulated by affixing a highly absorbent, vapor-impermeable material across the torso (20% TBSA), arms (10% TBSA), and legs (10% TBSA) to impede evaporative heat loss in those regions. Although the elevation in core temperature was greater in SM compared with LG at an Hprod of 500 W (SM, 1.69°C ± 0.26°C; LG, 1.05°C ± 0.26°C; P < 0.01), elevations in core temperature were not different at an Hprod of 5.3 W·kg between groups (SM, 0.99°C ± 0.32°C; LG, 1.05°C ± 0.26°C; P = 0.66). These data suggest that among individuals with a 40% TBSA burn injury, a smaller body size leads to exacerbated elevations in core temperature during physical activities eliciting the same absolute Hprod (non-weight-bearing tasks) but not activities eliciting the same mass-specific Hprod (weight-bearing tasks).

Thermal-Performance Evaluation of Bicycle Helmets for Convective and Evaporative Heat Loss at Low and Moderate Cycling Speeds

The main objective of the study was to investigate the thermal performance of five (open and closed) bicycle helmets for convective and evaporative heat transfer using a nine-zone thermal manikin. The shape of the thermal manikin was obtained by averaging the 3D-point coordinates of the head over a sample of 85 head scans of human subjects, obtained through magnetic resonance imaging (MRI) and 3D-printed. Experiments were carried out in two stages, (i) a convective test and (ii) an evaporative test, with ambient temperature maintained at 20.5 ± 0.5 °C and manikin skin temperature at 30.5 ± 0.5 °C for both the tests. Results showed that the evaporative heat transfer contributed up to 51%–53% of the total heat loss from the nude head. For the convective tests, the open helmet A1 having the highest number of vents among tested helmets showed the highest cooling efficiency at 3 m/s (100.9%) and at 6 m/s (101.6%) and the closed helmet (A2) with fewer inlets and outlets and limited internal channels showed the lowest cooling efficiency at 3 m/s (75.6%) and at 6 m/s (84.4%). For the evaporative tests, the open helmet A1 showed the highest cooling efficiency at 3 m/s (97.8%), the open helmet A4 showed the highest cooling efficiency at 6 m/s (96.7%) and the closed helmet A2 showed the lowest cooling efficiency at 3 m/s (79.8%) and at 6 m/s (89.9%). Two-way analysis of variance (ANOVA) showed that the zonal heat-flux values for the two tested velocities were significantly different (p &lt; 0.05) for both the modes of heat transfer. For the convective tests, at 3 m/s, the frontal zone (256–283 W/m2) recorded the highest heat flux for open helmets, the facial zone (210–212 W/m2) recorded the highest heat flux for closed helmets and the parietal zone (54–123 W/m2) recorded the lowest heat flux values for all helmets. At 6 m/s, the frontal zone (233–310 W/m2) recorded the highest heat flux for open helmets and the closed helmet H1, the facial zone (266 W/m2) recorded the highest heat flux for the closed helmet A2 and the parietal zone (65–123 W/m2) recorded the lowest heat flux for all the helmets. For evaporative tests, at 3 m/s, the frontal zone (547–615 W/m2) recorded the highest heat flux for all open helmets and the closed helmet H1, the facial zone (469 W/m2) recorded the highest heat flux for the closed helmet A2 and the parietal zone (61–204 W/m2) recorded the lowest heat flux for all helmets. At 6 m/s, the frontal zone (564–621 W/m2) recorded highest heat flux for all the helmets and the parietal zone (97–260 W/m2) recorded the lowest heat flux for all helmets.

Using personally controlled air movement to improve comfort after simulated summer commute

People often feel uncomfortably warm and sweaty in their workspace after commuting there by walking or cycling in summer. This is because body heat stored during the commute takes a substantial time to dissipate. People complaining about this uncomfortable transition may cause operators to lower the thermostat setpoint, causing long-term overcooling and wasting energy. In addition, space cooling is slow, requiring minutes to take effect. This study addresses how to improve comfort in the transition by increasing the availability of convective cooling, where the response time is in seconds. Thirty-five subjects (17 men and 18 women) dressed in 0.6 clo entered a test room after exercising at 4.4 met for 15 min in 30 °C. The exercise emulates the commute activity in summer. The test room was controlled to 24, 26, and 28 °C, with and without the option of cooling using fan-produced horizontal airflow. Subjects were sedentary for 60 min, during which subjective thermal responses and physiological responses were measured. The enhanced convective and evaporative heat loss caused by fans significantly shortened the time needed to reach thermal comfort after the exercise-induced thermal stress and improved the final comfort level. Compared to a typical indoor condition of 24 °C and still air, 26 and 28 °C with fans provided equal or better comfort more quickly, and inherently required much less energy to do so. Our study suggests that personally controlled air movement should be available in spaces where thermal and metabolic down-steps take place.

Thermoregulatory adaptations with progressive heat acclimation are predominantly evident in uncompensable, but not compensable, conditions.

This study assessed whether, notwithstanding lower resting absolute core temperatures, alterations in time-dependent changes in thermoregulatory responses following partial and complete heat acclimation (HA) are only evident during uncompensable heat stress. Eight untrained individuals underwent 8 wk of aerobic training (i.e., partial HA) followed by 6 days of HA in 38°C/65% relative humidity (RH) (i.e., complete HA). On separate days, esophageal temperature (Tes), arm (LSRarm), and back (LSRback) sweat rate, and whole body sweat rate (WBSR) were measured during a 45-min compensable (37°C/30% RH) and 60-min uncompensable (37°C/60% RH) heat stress trial pre-training (PRE-TRN), post-training (POST-TRN), and post-heat acclimation (POST-HA). For compensable heat stress trials, resting Tes was lower POST-TRN (36.74 ± 0.27°C, P = 0.05) and POST-HA (36.60 ± 0.27°C, P = 0.001) compared with PRE-TRN (36.99 ± 0.19°C); however, ΔTes was similar in all trials (PRE-TRN:0.40 ± 0.23°C; POST-TRN:0.42 ± 0.20°C; POST-HA:0.43 ± 0.12°C, P = 0.97). While LSRback was unaltered by HA (P = 0.94), end-exercise LSRarm was higher POST-TRN (0.70 ± 0.14 mg·cm-2·min-1, P < 0.001) and POST-HA (0.75 ± 0.16 mg·cm-2·min-1, P < 0.001) compared with PRE-TRN (0.61 ± 0.15 mg·cm-2·min-1). Despite matched evaporative heat balance requirements, steady-state WBSR (31st-45th min) was greater POST-TRN (12.7 ± 1.0 g/min, P = 0.02) and POST-HA (12.9 ± 0.8 g/min, P = 0.004), compared with PRE-TRN (11.7 ± 0.9 g/min). For uncompensable heat stress trials, resting Tes was lower POST-TRN (36.77 ± 0.22°C, P = 0.05) and POST-HA (36.62 ± 0.15°C, P = 0.03) compared with PRE-TRN (36.86 ± 0.24°C). But ΔTes was smaller POST-TRN (0.77 ± 0.19°C, P = 0.05) and POST-HA (0.75 ± 0.15°C, P = 0.04) compared with PRE-TRN (1.10 ± 0.32°C). LSRback and LSRarm increased with HA (P < 0.007), supporting the greater WBSR with HA (POST-TRN:14.4 ± 2.4 g/min, P < 0.001; POST-HA:16.8 ± 2.8 g/min, P < 0.001) compared with PRE-TRN (12.7 ± 3.2 g/min). In conclusion, the thermal benefits of HA are primarily evident when conditions challenge the physiological capacity to dissipate heat.NEW & NOTEWORTHY We demonstrate that neither partial nor complete heat acclimation alters the change in core temperature during compensable heat stress compared with an unacclimated state, despite a marginally greater whole body sweat rate. However, the greater local and whole body sweat rate with partial and complete heat acclimation reduced the rise in core temperature during 60 min of uncompensable heat stress compared with an unacclimated state, suggesting the improvements in heat dissipation associated with heat acclimation are best observed when the upper physiological limits for evaporative heat loss are challenged.

Evaporative heat loss insufficient to attain heat balance at rest in individuals with a spinal cord injury at high ambient temperature.

The aim of the study was to determine whether climatic limits for achieving heat balance at rest are affected by spinal cord injury (SCI). Twenty-three men [8 able-bodied (AB), 8 with paraplegia (PP), and 7 with tetraplegia (TP)] rested in 37°C and 20% relative humidity (RH) for 20 min. With the ambient temperature held constant, RH was increased by 5% every 7 min, until gastrointestinal temperature (Tgi) showed a clear inflection or increased by >1°C. Tgi, skin temperatures, perceptual responses, and metabolic energy expenditure were measured throughout. Metabolic heat production [AB: 123 (21) W, PP: 111 (15) W, TP: 103 (29) W; means (SD)] and required rate of evaporative cooling for heat balance [Ereq; AB: 113 (20) W, PP: 107 (17) W, TP: 106 (29) W] were similar between groups (P = 0.22 and P = 0.79). Compared with AB, greater increases in Tgi were observed in TP (P = 0.01), with notable increases in mean skin temperature (Tsk) for TP and PP (P = 0.01). A Tgi inflection point was demonstrated by seven AB, only three of eight PP, and no TP. Despite metabolic heat production (and Ereq) being similar between groups, evaporative heat loss was not large enough to obtain heat balance in TP, linked to a shortfall in evaporative cooling potential. Although PP possess a greater sweating capacity, the continual increase in Tgi and Tsk in most PP, although lower than for TP, implies that latent heat loss for PP is also insufficient to attain heat balance.NEW & NOTEWORTHY In the absence of convective heat loss, at temperatures around 37°C evaporative heat loss is insufficient to attain heat balance at rest in individuals with paraplegia and tetraplegia. This finding was directly linked to a shortfall in evaporative cooling potential compared with required evaporative cooling. In this environment, individuals with both paraplegia and tetraplegia cannot subjectively determine the magnitude of their thermal strain; thus perceptual responses should not be relied upon for this population group.

375 Importance of cellular resistance to elevated temperature as a determinant of the magnitude of adverse effects of heat stress on farm animals

Abstract Heat stress can compromise most productive functions of mammals and birds raised for agricultural purposes. Many of the deleterious effects of heat stress are a result of physiological adaptations to maintain constant body temperature. These adaptations include reduction in physical activity, reduced feed intake, nutrient repartitioning, redistribution of blood flow from the body core to the periphery, and increased evaporative heat loss. When heat stress becomes greater than can be compensated for physiologically, hyperthermia results and cellular function becomes at risk because of free radical damage, membrane instability, and changes in protein structure. It is well established that effects of heat stress on fertility involve disruption of function of the oocyte and early embryo by direct effects of elevated temperature. It is likely that other physiological systems are also compromised because of cellular damage. There are a variety of molecular systems to limit cellular damage caused by hyperthermia, including antioxidant systems, the endoplasmic reticulum stress response, and heat shock proteins. There has been limited success in mitigating effects of heat stress by manipulating antioxidant status. Cellular resistance to heat stress is subject to genetic variation, and it may be possible to select genetically for cellular resistance to hyperthermia.

Heat Loss and Exergy Flow through Respiration of the Human Body under Different Meteorological Conditions

In this study, monthly total heat loss (evaporative and convective heat losses) and exergy flow that occur as a result of respiration were determined for human bodies in states of light and heavy activity levels, and comparisons of the results obtained were made according to different meteorological conditions and regions. For this purpose, seven different climate zones (CZ-1, CZ-2, CZ-3, CZ-4, CZ-5, CZ-6 and CZ-7) in Turkey were selected. Meteorological parameters such as atmospheric temperature, atmospheric pressure and relative humidity were used for energy and exergy analyses. According to the obtained results, total heat loss and exergy flow through respiration of the human body demonstrated considerable variations seasonally depending on the climate zones. The highest heat loss and exergy flow values were determined in the region of Continental climate (CZ-2), while the lowest heat loss and exergy flow values were obtained in the region of Mediterranean climate (CZ-5). For a human body engaging in light activity in a hot climate zone, the exhaled air temperature, specific humidity and relative humidity values were determined as 35.11 oC, 0.0333 kg H2O/kg dry air and 91.3%, respectively.

Bio-thermal responses and heat balance of a hair coat sheep breed raised under an equatorial semi-arid environment.

Long-term assessments of bio-thermal responses in a hair coat sheep breed were performed to investigate the effect of the thermal environment on their physiological performance and thermal balance. Twelve healthy non-lactating Morada Nova ewes (3 ± 1.2 years old, body mass 32.7 ± 3.7 kg) were assigned in two 12 × 12 latin square designs (from 07:00 to 19:00 h and from 19:00 to 07:00 h, respectively) for assessments of their bio-thermal responses during 24 consecutive days. There was a monophasic pattern in the ambient temperature (TA), which ranged between 21 and 38 °C, thereby exposing the ewes to different levels of surrounding TA over the day and influencing several of their bio-thermal responses (P = 0.0001). Their body temperatures (i.e., rectal, skin, and hair coat surface temperatures) gradually increased (P = 0.0001) from 04:00 h. The mean peak for rectal temperature (39.3 °C) was recorded at 19:00 h, while for skin and hair coat surface temperatures it occurred at 13:00 and 14:00 h, respectively. The sensible heat loss by long wave radiation and surface convection exceeded the metabolism of ewes when the TA was below 24 °C, which usually occurred between 24:00 and 06:00 h. During exposure to higher ambient temperatures, the sheep increased respiratory evaporative heat loss, without panting. In conclusion, the sheep regulated rectal temperature within a relatively narrow range of 1.4 °C over 24 h, and appear to be well adapted to coping with heat. Minimum 24 h body temperature was correlated with minimum TA, indicating that heat conservation strategies are likely to be important for Morada Nova sheep in a tropical biotype at night, when rates of sensible heat loss exceed the heat generated by metabolism.

Different thermoregulatory responses of people from tropical and temperate zones: A computational study

Long-term heat acclimation in the tropics is a well-known positive response to cope with heat stress. A computational model of thermoregulatory responses from the population in the tropics has not yet been developed. This is partially because only a limited number of comparisons between the thermoregulation of people from different regions have been reported.Our group developed an integrated computational code of thermodynamics and thermoregulatory responses (blood flow and sweating rate) with anatomical human models. We explored parameters in the conventional formula of the thermoregulatory response model to consider the population from the tropics.A comparison of the computed core and local temperatures with those measured in previous studies suggests that the difference in thermoregulatory characteristics between populations from the tropical and temperate zones is mainly characterized by the difference in the countercurrent coefficient of blood, in addition to sweating rate. The estimated core temperature elevation in male adults from a tropical zone is 60% of the temperature of adults from temperate zones for low relative humidity. However, the difference becomes marginal for relatively high humidity, including the case for walking in an urban area of Tokyo, summer 2017, because the effect of the evaporative heat loss becomes marginal.The difference in thermoregulatory responses of people from tropical and temperate zones can be modeled by the core-to-limbs heat transfer and the sweating rate. The computational model developed here is useful to discuss the differences between heat-related illness and thermal comfort of the population in the tropical and temperate zones.

Effects of water droplet evaporation on initiation, propagation and extinction of premixed spherical flames

In this study, we develop a simplified theoretical model for flame initiation, propagation and extinction in premixed gas mixture containing water droplets, by considering water droplet evaporation in pre-flame or/and post-flame zones. The Eulerian droplet model with simplified evaporation sub-model is employed, while for gas phase the assumptions of constant-density, quasi-steady and large activation energy are made. Analytical correlations describing different flame regimes and transitions among flame balls, propagating spherical flames and planar flames are then derived to investigate the spherical flame initiation, propagation and extinction, with emphasis on the effects of water droplet evaporation on spherical flames at different Lewis numbers. Five different flame regimes are observed and discussed for droplets evaporation in pre-flame or/and post-flame zones. It is found that the droplets with larger heat exchange coefficient are more effective in reducing flame propagation speed and temperature but increasing the vaporization front. Moreover, the cooling effect of evaporation heat loss plays an important role on flame regimes and their transitions. At the relatively large heat exchange coefficient, the total evaporation heat loss from pre-flame and post-flame zones reaches its maximum at an intermediate flame radius. The cooling effect is strong enough to quench the flame and results in the self-extinguishing flame. In addition, the combined effects of stretch rate and Lewis number compete with the evaporation heat loss from droplet evaporation. For small Lewis number, the flammability limits can be achieved through self-extinguishing flames, whereas for large Lewis number the flames approach their flammability limits in their evolution into planar flames. For ignition of spherical flames, if the droplet-laden mixture is intrinsically non-flammable, although a propagating flame kernel can be initiated, however it would still be quenched due to evaporation heat loss at a critical radius. The minimum ignition energy increases with heat exchange coefficient and Lewis number.

Revisiting the influence of individual factors on heat exchange during exercise in dry heat using direct calorimetry.

What is the central question of this study? The aim was to identify the greatest contributor(s) to the variation in whole-body heat exchange, as assessed using direct calorimetry, among young men and women with heterogeneous characteristics during exercise at increasing metabolic heat production rates in dry heat. What is the main finding and its importance? The evaporative heat loss requirement, body morphology and aerobic fitness made the greatest contributions to the individual variation in evaporative and dry heat exchange, with the variance explained being exercise intensity dependent. These findings provide a foundation on which to build our ability to explain the individual variation in heat exchange during exercise-induced heat stress. Numerous individual factors (e.g. fitness, sex, body morphology) are known to independently modulate heat exchange during exercise in the heat. However, in our view, the individual factor(s) making the greatest contribution to the variation in heat exchange among men and women remains poorly understood, despite several studies. We therefore sought to revisit this question by assessing whole-body dry and evaporative heat exchange using direct calorimetry in a heterogeneous sample of 100 young men (n=57) and women (n=43). Participants performed three 30min bouts of cycling at very light (men/women; 300/250W), light (400/325W) and moderate (500/400W) metabolic heat production rates, separated by a 15min recovery, in dry heat (40°C, ∼12% relative humidity). Positive associations were observed between the evaporative heat loss requirement (metabolic heat production±dry heat exchange) and evaporative heat loss (all P<0.01), especially during moderate exercise (men, r=0.62; women, r=0.82), which explained 19-67% of individual variation. Peak aerobic power (in millilitres per kilogram per minute) was also positively related to evaporative heat loss in both sexes, albeit only during light and moderate exercise (r=0.33-0.43; all P<0.05), explaining a further 5-9% of individual variation. Dry heat exchange shared negative associations with body mass and surface area during all exercise bouts in both sexes (r=-0.29 to -0.55; all P<0.05), explaining 9-30% of individual variation. We therefore demonstrate that the evaporative heat loss requirement, peak aerobic power and body morphology are the greatest contributors to the variation in whole-body heat exchange among young men and women exercising in dry heat, with the strength of those relationships being heat-load dependent.

Plasma Motion inside Flaring Regions Revealed by Doppler Shift Information from SDO/EVE Observations

Abstract Plasma motions within flaring regions provide key information for us to understand the flare processes. Here, we study two X-class flares near the solar disk center, one on 2014 January 7 and the other on 2012 March 7, by using 10 extreme ultraviolet (EUV) emission lines from the Solar Dynamics Observatory/EUV Variability Experiment (EVE). The EVE plasma dynamic spectrum chart, a 2D map of Doppler shift against temperature and time, is constructed based on a spectroscopic analysis of the EUV lines. Three kinds of plasma motion are identified in the plasma dynamic spectrum charts: chromospheric evaporation (100–200 km s−1) above 1 MK, cooling inside post-flare loops (approximately 150 km s−1) between 0.3 and 1 MK, and condensation at footpoints (&lt;30 km s−1) below 0.3 MK. We find that the chromospheric evaporation and condensation at footpoints started in the impulsive phase almost simultaneously, while the cooling occurred later in the gradual phase, with a time delay of more than 10 minutes, probably implying the timescale of evaporation movement and heat loss. Atmospheric Imaging Assembly observations and differential emission measure (DEM) analyses suggest that the cooled plasma moves downward within the cold transition region (TR) loops, from top to feet, which are below the hot coronal loops. Besides, the reversal temperature between blue/redshifts is close to 1 MK, implying that the boundary of upflowing/downflowing plasma is located at the lower corona or the upper TR.

A Preliminary Analysis of the Inter‐Individual Determinants of Whole‐Body Heat Exchange in 100 Young Men and Women during Exercise in the Heat

It is well established that several inter‐individual factors (e.g., physical characteristics, aerobic fitness, others) independently modulate human thermoeffector responses (sweat secretion and cutaneous vasodilation) and the resulting changes in evaporative and dry heat exchange during exercise‐induced heat stress. However, less is known regarding the relative contribution of those factors to explaining inter‐individual variations in heat exchange or whether that contribution is modified by the heat load employed for exercise eliciting a matched rate of metabolic heat production. We therefore used direct calorimetry to assess whole‐body evaporative and dry heat exchange in a large, heterogeneous sample of young men (n = 57) and women (n = 43) during three, 30‐min bouts of cycling performed at light (men/women; 300/250 W), moderate (400/325 W) and heavy (500/400 W) fixed rates of metabolic heat production, each followed by a 15‐min recovery, in dry heat (40°C, ~12% relative humidity). Metabolic heat production, evaporative and dry heat exchange as well as the evaporative heat loss requirement (Ereq; metabolic heat production ± dry heat exchange) were measured throughout, with an average of the final five minutes of each exercise period used for statistical analysis. Relationships between the dependent (evaporative and dry heat exchange) and relevant independent variables (body mass, body surface area, body surface area‐to‐mass ratio, body fat, peak aerobic power, metabolic heat production, Ereq) were assessed using Pearson's correlation coefficient (r), while step‐wise, multiple‐linear regression analyses was performed to quantify the proportion (%) of variation (coefficient of determination; R2) in each dependent variable explained by the independent variables. Strong, positive associations were observed between Ereq and evaporative heat loss (all p&lt;0.01), especially during heavy exercise (men: r = 0.62; women: r = 0.82), which explained 19–67% of individual variation. Peak aerobic power was also positively related to evaporative heat loss in men and women, albeit only during moderate and heavy exercise (r = 0.33 to 0.43; all p&lt;0.05), explaining a further 5–9% of variation. Dry heat exchange shared moderate‐to‐strong, negative associations with body mass and surface area for all exercise intensities in men and women (r = −0.29 to −0.55; all p&lt;0.05), which explained 9–30% of variation. Observations from this preliminary analysis indicate that Ereq, body morphology and peak aerobic power are important determinants of inter‐individual variations in whole‐body heat exchange among men and women during exercise eliciting matched rates of metabolic heat production in dry heat, with the strength of those relationships being dependent on the exercise‐induced heat load.Support or Funding InformationFunded by the Government of Ontario and Natural Sciences and Engineering Research Council of Canada.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

On the effects of constant and variable work of equivalent average intensity on whole‐body heat exchange

Currently recommended heat exposure guidelines prescribe work‐rest allocations based on the level of heat stress originating from the environment and estimated work intensity with the primary goal of maintaining body core temperature within safe limits. Due to the nature of most occupations, these parameters are rarely, if ever, constant. For this reason, ambient conditions (e.g., temperature and humidity) and metabolic rate (endogenous heat production) are quantified as a time‐weighted average over a given work period. What is unknown, however, is whether time‐dependent alterations in work intensity influence whole‐body heat exchange and cardiovascular strain during work eliciting an equivalent time‐weighted average heat production. On four occasions, six young (27 years [SD 4]) males performed 90‐min of semi‐recumbent cycling at a time‐weighted average rate of heat production (~200 W·m−2) in dry heat (40°C, ~20% relative humidity) elicited via cycling at a constant external work rate of 40 W·m−2 (CON) or variable intensity cycling performed in 10‐min cycles: 1) 5 min at an external work rate of 15 W·m−2 followed by 5‐min at 60 W·m−2 (VAR‐LOW); 2) 6 min at 15 W·m−2 followed by 4 min at 70 W·m−2 (VAR‐MOD); and 3) 7 min at 15 W·m−2 followed by 3 min at 80 W·m−2 (VAR‐HIGH). Heat production was measured via indirect calorimetry while whole‐body evaporative heat loss and dry heat gain were evaluated with direct calorimetry. Body heat storage was calculated as the temporal‐summation of heat production and net heat exchange (evaporative heat loss minus dry heat gain). Calorimetric data were expressed as a time‐weighted average (W·m−2) except for heat storage, which was presented as a cumulative value (kJ). Rectal temperature and heart rate were monitored continuously and reported as an average of the final 10‐min of exercise (i.e., final cycle) as well as a peak 30‐s average during this period. Across conditions, there were no differences in evaporative heat loss (241 [20] W·m−2, P=0.20), dry heat gain (59 [13] W·m−2, P=0.52), net heat exchange (281 [10] W·m−2, P=0.54) or body heat storage (224 [55] kJ, P=0.87). Likewise, rectal temperature was not influenced by varying work intensity (average: 37.79 [0.23]°C; peak: 37.84 [0.22]°C; both P≥0.65). However, compared to CON (115 [8] beats·min−1), average heart rate over the final 10 min of exercise was elevated in VAR‐LOW (124 [12] beats·min−1; P=0.01), VAR‐MOD (122 [10] beats·min−1; P=0.04) and VAR‐HIGH (124 [11] beats·min−1; P=0.01), but similar between the variable work conditions (all P≥0.17). Similarly, peak heart rate was elevated in each of the variable work conditions compared to CON (120 [10] beats·min−1; all P&lt;0.01) and in VAR‐HIGH (154 [11] beats·min−1) compared to both VAR‐LOW (140 [12] beats·min−1; P&lt;0.01) and VAR‐MOD (143 [12] beats·min−1; P=0.01). These preliminary data suggest that time‐dependent alterations in work intensity eliciting equivalent time‐weighted rates of metabolic heat production do not influence whole‐body heat exchange or thermal strain, but exacerbate cardiovascular strain compared to constant‐intensity work.Support or Funding InformationThis project was funded by the Government of Ontario and the Natural Sciences and Engineering Research Council of Canada.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Sensitivity, Impact and Consequences of Changes in Respiratory Rate During Thermoregulation in Livestock – A Review

Abstract This review discusses the thermal conservative and heat dissipating roles of one of the most sensitive thermoregulatory variables (respiratory rate) with the aim of enhancing its application in evaluating both cold and heat adaptation. During cold exposure, livestock enhance the economy of body heat through reduction in respiratory rate with the extent of reduction being greater and commencing at relatively higher ambient temperature in poorly adapted phenotypes. This is accompanied by an increase in tidal volume and alveolar oxygen uptake, but a decrease in partial pressure of oxygen. On the other hand, heat stress induces increase in respiratory rate to enhance evaporative heat loss with the magnitude of such increase being greater and commencing at relatively lower ambient temperature in phenotypes that are poorly-adapted to heat. This is accompanied by a decrease in tidal volume and the development of hypocapnia. The increase in respiratory rate is observed to be greater, moderate and lesser in livestock that are mainly (pigs, rabbits and poultry), moderately (sheep, goats and Bos taurus) and less (Zebu cattle) dependent on respiratory evaporative heat loss, respectively. The changes during chronic heat stress may cause acid-base crisis in all livestock, in addition to reduction in eggshell quality in birds; due to marked decrease in partial pressure of carbon dioxide and a compensatory increase in elimination of bicarbonate. Within and between breed variations in sensitivity of respiratory rhythm to both cold and heat stress has shown high applicability in identifying phenotypes that are more susceptible to thermal stress; with some cellular and metabolic changes occurring to protect the animal from the consequences of hypo- or hyper-thermia. The information in this review may provide basis for identification of genes that support or suppress thermoregulation and may also be of great use in animal breeding, genomics and selective thermal stress mitigation to provide maximum protection and comfort to poorly-adapted phenotypes.

Scaling of total evaporative water loss and evaporative heat loss in birds at different ambient temperatures and seasons

Scaling of total evaporative water loss and evaporative heat loss in birds at different ambient temperatures and seasons

Scaling of total evaporative water loss and evaporative heat loss in birds at different ambient temperatures and seasons

Scaling of total evaporative water loss and evaporative heat loss in birds at different ambient temperatures and seasons